mikrobiologi

Mikrobiologi umum
________________________________________
GENERAL UMUM
Bacteria are unicellular microorganisms having a variety of characteristics allowing their classification. Bakteri adalah mikroorganisme uniseluler memiliki berbagai karakteristik yang memungkinkan klasifikasi mereka. One major classification scheme is based upon their staining properties using the "Gram stain" procedure. Salah satu skema klasifikasi utama didasarkan pada sifat menodai mereka menggunakan "Pewarnaan Gram" prosedur. In this procedure, heat-killed bacteria are exposed to the purple dye crystal violet and iodine. Dalam prosedur ini, panas-membunuh bakteri yang terkena pewarna ungu violet dan yodium kristal. This combination forms a dye complex in the bacterial cell wall. Kombinasi ini membentuk kompleks pewarna dalam dinding sel bakteri. Treatment of the stained bacteria with a decolorizer like ethanol will wash away the dye complex from some bacteria but not others. Perawatan dari bakteri bernoda dengan decolorizer seperti etanol akan membersihkan cairan kompleks dari beberapa bakteri tetapi tidak yang lain. Bacteria that retain the crystal violet-iodine complex appear purple and are called "Gram-positive". Bakteri yang mempertahankan yodium kristal ungu-ungu muncul kompleks dan disebut "Gram-positif". Bacteria that lose the dye complex can be counterstained with the red dye saffranin so that they appear red. Bakteri yang kehilangan pewarna kompleks dapat counterstained dengan saffranin pewarna merah sehingga muncul merah. These bacteria are called "Gram-negative". Bakteri ini disebut "Gram-negatif". The basis of the Gram reaction lies within the structure of the cell wall, described below. Dasar reaksi Gram terletak dalam struktur dinding sel, dijelaskan di bawah ini.
Bacteria also come in many different shapes. Bakteri juga datang dalam berbagai bentuk. Spherical shapes are referred to as "cocci" while elongated cylinders are called "bacilli" or "rods". Bentuk sferis disebut sebagai "cocci" sementara silinder memanjang disebut "basil" atau "batang". Some bacteria are slightly elongated cocci and these are referred to as "coccobacilli". Beberapa bakteri yang agak memanjang cocci dan ini disebut sebagai "coccobacilli". Even other bacteria have a corkscrew-like appearance; these spiral forms are often called "spirochetes". Bahkan bakteri lain memiliki pembuka botol-seperti penampilan; bentuk spiral ini sering disebut "spirochetes". Individual cells may also be arranged in pairs or clusters or chains. Sel-sel individual juga dapat diatur dalam pasangan atau kelompok atau rantai. Thus, may morphologies are possible and these can be useful for the identification of bacterial genera. Jadi, mungkin morfologi yang mungkin dan ini dapat berguna untuk identifikasi bakteri genera. ( Click here to see a bacterial classification flowchart ). (Klik di sini untuk melihat klasifikasi bakteri diagram alur).
The ability of a bacterium to cause disease is known as its virulence. Kemampuan suatu bakteri yang menyebabkan penyakit yang dikenal sebagai virulensi. Factors involved in determining virulence potential are discussed here . Faktor-faktor yang terlibat dalam menentukan potensi virulensi dibahas di sini. In terms of the medical aspects of bacterial structure , we are most interested in those features that interact with the host. Dalam hal aspek medis struktur bakteri, kita paling tertarik pada fitur-fitur tersebut yang berinteraksi dengan host. These features are found predominantly on the outer surface of the bacterial cell. Fitur-fitur ini ditemukan terutama pada permukaan luar sel bakteri. This page will describe some of these features. Halaman ini akan menjelaskan beberapa fitur tersebut.
________________________________________
SURFACE APPENDAGES PERMUKAAN pelengkap
Bacteria may or may not possess surface appendages that provide the organism with the ability to be motile or to transfer genetic material or to attach to host tissues. Bakteri mungkin atau mungkin tidak memiliki pelengkap permukaan yang menyediakan organisme dengan kemampuan untuk menjadi yg dpt mengubah tempat atau untuk mentransfer materi genetik atau untuk berikatan dengan jaringan inang. These appendages are outlined below: Pelengkap tersebut diuraikan di bawah ini:
1. Flagella : These are the organs of motility. Flagela: Ini adalah organ-organ pergerakan. Flagella are composed of flagellins (proteins) that make up the long filament. Flagela terdiri dari flagellins (protein) yang membentuk filamen panjang. This filament is connected to a hook and rings that anchor the flagella in the cell wall. Filamen ini terhubung ke sebuah kait dan cincin yang jangkar yang flagela di dinding sel. In Gram-positive bacteria, there are two rings attached to the cytoplasmic membrane; in Gram-negative cells, an additional two rings are found in the outer membrane. Pada bakteri Gram-positif, ada dua cincin yang melekat pada membran sitoplasma, dalam sel Gram-negatif, tambahan dua cincin yang ditemukan di luar membran. Flagella may be up to 20 µm in length. Flagela mungkin hingga 20 μm panjangnya. Some bacteria possess a single polar flagellum (monotrichous), others have several polar flagella (lophotrichous), others have several flagella at each end of the cell (amphitrichous), and still others have many flagella covering the entire cell surface (peritrichious). Beberapa bakteri memiliki satu flagela polar (monotrichous), yang lain memiliki beberapa flagela polar (lophotrichous), yang lain memiliki beberapa flagela di setiap ujung sel (amphitrichous), dan yang lain memiliki banyak flagela yang menutupi seluruh permukaan sel (peritrichious). Counterclockwise rotation of the flagella produces motility in a forward motion; clockwise rotation produces a tumbling motion. Berlawanan rotasi dari flagela menghasilkan motilitas dalam gerakan maju; searah jarum jam rotasi menghasilkan gerakan berjatuhan. Flagella may serve as antigenic determinants (eg the H antigens of Gram-negative enteric bacteria). Flagela dapat berfungsi sebagai penentu antigenik (misalnya H antigen dari Gram-negatif enterik bakteri).
2. Pili : These surface appendages come in two distinct forms having distinct purposes. Pili: pelengkap permukaan ini datang dalam dua bentuk yang berbeda yang memiliki tujuan yang berbeda. Pili (or fimbrae) may also provide antigenic determinants (eg the M protein of S. pyogenes ). Pili (atau fimbrae) mungkin juga menyediakan antigen determinan (misalnya protein M S. pyogenes).
1. Sex pili : This form of pilus can be relatively long but is often found in few numbers, generally 1 to 6, protruding from the cell surface. Sex pili: Bentuk pilus dapat relatif lama tetapi sering ditemukan dalam beberapa nomor, umumnya 1-6, yang menonjol dari permukaan sel. These structures are involved in conjugation, the transfer of genetic information from one cell to another. Struktur ini terlibat dalam konjugasi, transfer informasi genetik dari satu sel yang lain. These structures can also provide the receptor for certain male-specific bacteriophages. Struktur ini juga dapat memberikan reseptor bagi laki-laki tertentu bakteriofag tertentu.
2. Common pili : This form of pilus is usually relatively short and many (about 200) and can be found covering the cell surface. Common pili: Bentuk pilus biasanya relatif pendek dan banyak (sekitar 200) dan dapat ditemukan menutupi permukaan sel. These structures provide the means for attachment to host cells (eg epithelial cells) and often play an important role in colonization (eg N. gonorrhoeae ). Struktur ini menyediakan sarana untuk lampiran pada sel inang (misalnya sel-sel epitel) dan sering memainkan peran penting dalam kolonisasi (misalnya N. gonorrhoeae).
________________________________________
SURFACE LAYERS LAPIS PERMUKAAN
Bacteria possess several distinct surface layers that can enhance their pathogenicity. Bakteri memiliki beberapa lapisan permukaan yang berbeda yang dapat meningkatkan pathogenicity mereka. These layers are outlined below: Lapisan tersebut diuraikan di bawah ini:
1. Capsules : This type of surface layer is composed primary of high molecular weight polysaccharides. Kapsul: Jenis lapisan permukaan terdiri utama dari polisakarida berat molekul tinggi. If the layer is strongly adhered to the cell wall, it is called a capsule; if not, it is called a slime layer. Jika lapisan sangat melekat pada dinding sel, hal itu disebut kapsul; jika tidak, maka disebut lapisan lendir. These layers provide resistance to phagocytosis and serve as antigenic determinants. Lapisan ini memberikan ketahanan terhadap fagositosis dan berfungsi sebagai penentu antigenik. The production of capsules is genetically and phenotypically controlled. Produksi kapsul secara genetik dan fenotipik dikendalikan.
2. Cell wall : The cell wall is the basis for classification of bacteria according to the Gram stain. Dinding sel: dinding sel adalah dasar untuk klasifikasi bakteri sesuai dengan Pewarnaan Gram. Gram-positive bacteria have a thick layer of peptidoglycan external to the cytoplasmic membrane. Bakteri gram positif memiliki lapisan tebal peptidoglikan eksternal terhadap membran sitoplasma. In contrast, Gram-negative bacteria have a thin layer of peptidoglycan located between the cytoplasmic membrane and a second membrane called the outer membrane. Sebaliknya, bakteri Gram-negatif memiliki lapisan tipis peptidoglikan terletak antara membran sitoplasma dan membran kedua disebut membran luar. This region is known as the periplasmic space. Wilayah ini dikenal sebagai ruang periplasmic. Other important constituents of the cell wall include the following: Penting lainnya konstituen dari dinding sel adalah sebagai berikut:
o Peptidoglycan : This is a polymer of alternating N -acetylmuramic acid (NAM) and N -acetylglucosamine (NAG). Peptidoglikan: Ini adalah polimer bolak N-acetylmuramic asam (NAM) dan N-asetilglukosamin (NAG). Long strands of this alternating polymer may be linked by L-alanine, D-glutamic acid, L-lysine, D-alanine tetrapeptides to NAM ( Click here to see a graphic representation ). Helai panjang polimer bolak-balik ini dapat dihubungkan oleh L-alanin, D-asam glutamat, L-lisin, D-alanin tetrapeptides untuk NAM (Klik di sini untuk melihat representasi grafis). Gram-positive cells have a much more highly cross-linked peptidoglycan structure than Gram-negative cells. Gram-positif sel-sel yang jauh lebih tinggi cross-linked struktur peptidoglikan daripada sel Gram-negatif. Peptidoglycan is also the "target" of antimicrobial activity. Peptidoglikan juga merupakan "target" dari aktivitas antimikroba. For example, penicillins interfere with the enzymes involved in biosynthesis of peptidoglycan while lysozyme physically cleaves the NAM-NAG bond. Sebagai contoh, penisilin mengganggu enzim yang terlibat dalam biosintesis peptidoglikan sementara lisozim membelah secara fisik yang NAM-NAG ikatan.
o Lipoteichoic acids : Lipoteichoic acids (LTA) are found only in Gram-positive bacteria. Lipoteichoic asam: Lipoteichoic asam (LTA) hanya ditemukan di bakteri Gram positif. These polysaccharides extend though the entire peptidoglycan layer and appear on the cell surface. Polisakarida ini memperpanjang meskipun seluruh lapisan peptidoglikan dan muncul pada permukaan sel. As a consequence, these structures can serve as antigenic determinants. Sebagai akibatnya, struktur ini dapat berfungsi sebagai penentu antigenik.
o Lipopolysaccharides : Lipopolysaccharides (LPS) are found only in Gram-negative bacteria. Lipopolysaccharides: Lipopolysaccharides (LPS) hanya ditemukan di bakteri Gram-negatif. These structures are composed of lipid A, which binds the LPS in the outer membrane and is itself the endotoxic portion of the molecule. Struktur ini terdiri dari lipid A, yang mengikat LPS di membran luar dan itu sendiri yang endotoxic bagian dari molekul. The polysaccharide moiety appears on the cell surface, serving as an antigenic determinant ("O antigen"). Polisakarida separoh yang muncul pada permukaan sel, berfungsi sebagai antigen determinan ( "O antigen").
________________________________________
ENDOSPORE FORMATION ENDOSPORE PEMBENTUKAN
In order to transmit disease, pathogenic bacteria must have a means of surviving transit from one host to another. Agar dapat mengirim penyakit, bakteri patogen harus memiliki sebuah arti dari hidup berpindah dari satu host ke yang lain. Many organisms rely on human-to-human contact while others can survive in the environment some short periods of time. Banyak organisme bergantung pada manusia ke kontak manusia sedangkan yang lain dapat bertahan hidup dalam lingkungan beberapa periode waktu yang singkat. The extreme ability to survive environmental conditions is observed in organisms capable of forming endospores. Kemampuan yang ekstrem untuk bertahan hidup kondisi lingkungan yang diamati pada organisme mampu membentuk endospores. Two important pathogenic genera that are capable of this transformation are Bacillus and Clostridium . Dua genera patogenik penting yang mampu transformasi ini adalah Bacillus dan Clostridium. The process of sporulation begins when vegetative (or actively growing) cells exhaust their source of nutrients and involves seven distinct stages of differentiation. Proses sporulasi dimulai ketika vegetatif (atau aktif tumbuh) knalpot sel sumber nutrisi mereka dan melibatkan tujuh tahap yang berbeda dari diferensiasi. In the spore form, the organisms are very resistant to heat, radiation and drying and can remain dormant for hundreds of years. Dalam bentuk spora, organisme sangat tahan terhadap panas, radiasi dan pengeringan dan dapat tetap aktif selama ratusan tahun. Once conditions are again favorable for growth, the spores can germinate and return to the vegetative state. Sekali lagi kondisi yang menguntungkan bagi pertumbuhan, spora dapat berkecambah dan kembali ke negara vegetatif.
Normal Flora
________________________________________
Typically, when one says "I have an infection" they mean to say "I have a disease", however the latter is not quite so socially acceptable. In fact, we are all "infected" with a variety of microorganisms throughout our entire lives. Incredibly, our bodies are actually composed of more bacterial cells than human cells; while the human body is made up of about 1013 human cells, we harbor near 1014 bacteria. This group of organisms, traditionally referred to as "normal flora" (although they are not plants) is composed of a fairly stable set of genera, mostly anaerobes. While each person has a relatively unique set of normal flora, members of the Streptococcus and Bacteroides make up a large percentage of the inhabitants. These organisms contribute to our existence in several ways. These normal flora may:
• Help us by competing with pathogens such as Salmonella
• Help us by providing vitamins or eliminating toxins (e.g. Bacteroides)
• Harm us by promoting disease (e.g. dental caries)
• Cause neither help nor harm (e.g. "commensals").
One of the most important functions of our normal flora is to protect us from highly pathogenic organisms. For example, in a normal (bacterially inhabited animal), about 106 Salmonella must be ingested in order to cause disease. However, when an animal has been maintained in a sterile environment all of its life (a "gnotobiotic" animal), the same level of disease can be produced by as few as 10 Salmonella. This dramatic difference is simply due to competition.
To a microorganism, the human body seems very much like the planet Earth seems to us. Just like our planet, our bodies contain numerous different environments, ranging from dry deserts (e.g. the forearm) to tropical forests (e.g. the perineum) to extremely hostile regions (e.g. the intestinal tract). Each environment possesses certain advantages and disadvantages and different microorganisms have adapted to certain regions of the body for their particular needs. This page will examine these regions and describe the types of microorganisms found in each. You may review these regions by clicking on the human body "map" shown below.
________________________________________


Body Site



Predominant/Important Genera










________________________________________
Skin Flora
The surface of the skin itself comprises several distinct environments. Areas such as the axilla (armpit), the perineum (groin) and the toe webs provide typically moister regions for bacterial growth. These "tropical forest" environments often harbor the largest diversity amongst the skin flora. Typical organisms include Staphylococcus aureus, Corynebacterium and some Gram-negative bacteria. The bulk of the human skin surface, however, is much drier and is predominantly inhabited by Staphylococcus epidermidis and Propionobacterium.
Oral Cavity and Nasopharyngeal Flora
Streptococci predominate in the oral cavity and nasopharyngeal regions but one can also find other anaerobes and species of Neisseria. Many potential pathogens may also be found in the nasopharynx of a healthy individual, providing a reservoir for infection of others. These pathogens include Streptococcus pneumoniae, Neisseria meningitidis and Haemophilus influenzae.
Intestinal Flora
The intestinal tract is a rather hostile environment for microorganisms yet the bulk of our normal flora inhabit this region of the body. In fact, the colon may contain 109 to 1011 bacteria per gram of material. Most (95 - 99.9%) of these are anaerobes, represented by Bacteroides, Bifidobacterium, anaerobic streptococci and Clostridium. These organisms inhibit the growth of other pathogens but some can be opportunistic (e.g. C. difficile can produce pseudomembranous colitis).
Urogenital Flora
The urogenital tract is normally sterile with the exception of the vagina and the distal 1 cm of the urethra. Various members of the genus Lactobacillus predominate in the vagina. These organisms generally lower the pH to around 4-5, which is optimal for the lactobacilli but inhibitory for the growth of many other bacteria. Loss of this protective effect by antibiotic therapy can lead to infection by Candida ("yeast infection"). The urethra may contain predominantly skin microorganisms including staphylococci, streptococci and diphtheroids.
Antimicrobial Chemotherapy
________________________________________
CLASSIFICATION
Antimicrobials can be classified by at least three different schemes:
1. Effects on cells
2. Range of activity
3. Sites of activity
This page will examine these different classification schemes and describe several examples of each type of antimicrobial. The mechanisms by which organisms become resistant to these agents will also be discussed. Finally, toxicologic properties of antimicrobial chemotherapy will be described.
________________________________________
1. Antimicrobial Effects on Cells
Antimicrobials can be divided into two classifications based upon their effects on target cells. Drugs that actually kill microorganisms are termed bactericidal. Drugs that only inhibit the growth of microorganisms are termed bacteriostatic. The decision to use a bactericidal or bacteriostatic drug to treat infection depends entirely upon the type of infection. For example, bactericidal drugs will only kill cells that are actively growing. Bacteriostatic drugs, in comparison, will only inhibit the growth of cells; ultimate elimination of the organisms is dependent upon host phagocytic activity. Some examples of bactericidal and bacteriostatic drugs are listed below.
Bactericidal Drugs Bacteriostatic Drugs
Streptomycin
Sulfonamides

Aminoglycosides
Tetracycline

Penicillin
Chloramphenicol

________________________________________
2. Range of Activity
Antimicrobials can also be classified by their range of activity. In general, five classifications can be described. The first of these is termed narrow spectrum. Narrow spectrum drugs, as the name implies, are only active against a relatively small number of organisms. In general, narrow spectrum antibiotics are effective against Gram-positive organisms. The second classification is termed moderate spectrum. These drugs are generally effective against the Gram-positives and most systemic, enteric and urinary tract Gram-negative pathogens. The beta-lactam antibiotics (penicillin, ampicillin, cephalosporins, etc.) belong in a third classification, narrow and moderate spectrum because some members are only effective against Gram-positive organisms while other members can also kill certain Gram-negative bacteria. A fourth classification is termed broad spectrum. These drugs are effective against all prokaryotes with two exceptions: Mycobacteria (see below) and Pseudomonas. The fifth group includes those drugs that are effective against Mycobacteria. The following table details some examples of these antimicrobials.
Range of Activity Organisms Affected Example Antibiotics
Narrow Spectrum Gram-positives (Actinomyces, Corynebacteria, Bacillus, Clostridium, Pyogenic cocci, Spirochetes) Macrolides (Erythromycin)
Polypeptides (Polymyxin)
Moderate Spectrum Gram-positives plus systemic, enteric and urinary tract Gram-negatives Sulfonamides
Aminoglycosides
(Streptomycin, Gentamycin, Tobramycin)
Narrow/Moderate Spectrum Gram-positives plus Gram-negatives Beta-lactams
(Penicillin, Ampicillin, Cephalosporins)

Broad Spectrum All prokaryotes except Mycobacteria and Pseudomonas Chloramphenicol
Tetracycline

Anti-mycobacterial Mycobacteria Isoniazid
Ethambutol
Streptomycin
Rifampin

________________________________________
3. Sites of Activity
A third means of classifying antimicrobials is by their site of activity within the target cell. Further, antimicrobials may affect either the integrity or the synthesis of these sites. The various cellular targets include the cell wall, the plasma membrane, the nucleic acids and proteins. The following table lists these sites and gives examples of antimicrobials acting against them.
Site of Activity Example Antibiotics
Inhibition of cell wall integrity Lysozyme
Inhibition of cell wall synthesis
1. Biosynthetic enzymes (cytoplasmic) Fosfomycin, Cycloserine

2. Membrane-bound phospholipid carrier Bacitracin
3. Polymerization of subunits Beta-lactams

4. Combine with wall substrates Vancomycin
Inhibition of membrane integrity Surfacants, Polyenes, Polypeptides
Inhibition of membrane synthesis None
Inhibition of nucleic acid integrity Alkylating, Intercalating agents (mitomycin, chloroquin)
Inhibition of nucleic acid synthesis
1. Metabolism of DNA 5-Fluorocytosine, Acyclovir, NTP analogs
2. Replication of DNA Nalidixic acid, Novobiocin, Nitroimadazoles

3. Synthesis of RNA Rifampin

Protein integrity Phenolics, Heavy metals
Protein synthesis
1. 30S Subunit Streptomycin, Kanamycin, Tetracycline

2. 50S Subunit Chloramphenicol, Macrolides (Clindamycin, Erythromycin)

3. Folate metabolism Sulfonamides, Trimethoprim

________________________________________
RESISTANCE MECHANISMS
The problem of antibiotic resistance is becoming increasingly apparent as more and more strains of pathogenic microorganisms are untreatable with commonly used antimicrobials. This problem can be attributed to a variety of factors including overuse of antibiotics in agriculture and medicine and misuse of antibiotics by consumers. In addition, antibiotic resistance is often plasmid-borne, which means that resistance can be readily transferred from one organism to another. There are several mechanisms for antibiotic resistance and these relate to the sites of antimicrobial activity. These mechanisms include:
1. Altered receptors for the drug
2. Decreased entry into the cell
3. Destruction or inactivation of the drug
These mechanisms and some examples are outlined in the following table.
Altered Receptors
1. Beta-lactams Altered Penicillin Binding Proteins
2. Macrolides Methylation of 2 adenine residues in 23S RNA of the 50S subunit
3. Rifampin Single amino acid change in RNA polymerase ß-subunit
4. Sulfonamide/trimethoprim Altered synthetase binds pABA preferentially/altered reductase for TMP
5. Nalidixic acid Altered gyrase
6. Streptomycin Altered S12 protein in 30S subunit
Decreased Entry
1. Tetracycline Normally biphasic, active transport reduced
2. Fosfomysin (chromosomal) Glucose-6-phosphate transport reduced
Destruction/Inactivation
1. Chloramphenicol acetyltransferase Acetylates chloramphenicol
2. Beta-lactamase Cleaves ß-lactam ring
3. Aminoglycosides Acetylation or phosphorylation as drug passes membrane
________________________________________
TOXICOLOGY
While antimicrobials can be life-saving, they also pose certain dangers to the patient. Some antibiotics are relatively safe; others should only be used if there is no other means of controlling an infection. The following table lists some side effects/dangers of antimicrobial chemotherapy.
Side effects/Toxic effects Examples
Overgrowth of pathogens Intestinal (C. difficile), Vaginal (Candida)
Depression of intestinal symbiotes Several
Nephrotoxicity Polypeptides, Aminoglycosides
Ototoxicity - 8th cranial nerve Aminoglycosides
Ophthalmic toxicity Ethambutol
Aplastic anemia Chloramphenicol
Hypersensitivity Penicillin
Bone seeking Tetracycline
Staphylococcus
________________________________________
ORGANISM:
• Genus: Staphylococcus
• Species: aureus, epidermidis
________________________________________
GENERAL CONCEPTS:
• The staphylococci are divided in two groups based on the presence or absence of the enzyme coagulase. This enzyme converts fibrinogen into fibrin, causing blood plasma to clot. The species called S. aureus is coagulase-positive while S. epidermidis (and other "non-pathogens") is coagulase-negative.
• Typically, staphylococci are opportunistic pathogens or saprophytes.
________________________________________
DISTINCTIVE PROPERTIES:
• Staphylococci are Gram-positive cocci usually arranged in clusters like a bunch of grapes. This appearance is due to the fact that staphylococci divide along two separate planes.
• The morphologically similar streptococci can be differentiated from staphylococci by testing for the enzyme catalase; staphylococci possess this enzyme while streptococci do not.
• Staphylococci possess both group specific and type specific antigens: 90% of S. aureus isolates have protein A. This substance is capable of binding the Fc portion of immunoglobulin IgG. This property helps the bacterium escape the potentially lethal effects of immunoglobulin action and also serves as the basis for some serological tests (coagglutination).
• Toxins produced by S. aureus include: hemolysins, leukocidins, enterotoxin, exfoliative toxin and toxic shock syndrome (TSS) toxin.
• Extracellular enzymes produced by S. aureus include: coagulase, fibrinolysin, DNAse, lipases and hyaluronidase.
________________________________________
PATHOGENESIS:
• Coagulase negative strains of Staphylococcus are generally non-invasive. Under certain conditions, however, they may cause severe disease (e.g. S. epidermidis and subacute endocarditis).
• S. aureus is a common cause of boils, sties and skin infections. Serious (life-threatening) infections (pneumonia, deep abscesses, meningitis) may occur in debilitated persons.
• S. aureus is the most common cause of Gram-positive bacteremia, most commonly involving hospital strains of the organism.
• S. aureus is also responsible for scalded skin syndrome and toxic shock syndrome. It is the most common cause of food poisoning. Symptoms occur only a few hours following ingestion of preformed enterotoxin but large amounts of toxin are required.
________________________________________
HOST DEFENSES:
• Staphylococci have a long association with humans and make up a major portion of our skin flora. Because of this relationship, there are many factors acting both ways. Staphylococci generally resist host defenses. Antibody may help in certain circumstances but protein A prevents opsonization and the action of complement.
________________________________________
EPIDEMIOLOGY:
• These organisms are ubiquitous. Prior to 1950, most staphylococci were sensitive to penicillin; now, most are resistant (hospital strains). Synthetic penicillins have been very useful but now resistance to vancomycin is spreading.
________________________________________
DIAGNOSIS:
• Clinical: Generally, a Gram stain of exudate from a lesion can demonstrate the characteristic Gram-positive cocci arranged in clusters.
• Laboratory: Isolation techniques employ blood agar, mannitol salt agar or potassium-tellurite agar. Bacteriophage testing or serotyping may be utilized.
________________________________________
CONTROL:
• Sanitary: There is virtually no possibility to eliminate these organisms because they are (and have been) a significant part of human normal flora. To control the spread of disease, however, clean hospitals and proper food handling are paramount.
• Immunological: Nothing really available. But, a new vaccine is in clinical trials (see ICAAC Report)
• Chemotherapeutic: Antibiotics can be used if life-threatening. One should use penicillin if the particular strain is susceptible. Otherwise, methicillin, oxacillin, cephalosporins or vancomycin may be required. Often, surgical drainage is an important treatment.

Streptococcus
________________________________________
ORGANISM:
• Genus: Streptococcus, Enterococcus
• Species: S. pyogenes (Group A), S. agalactiae (Group B), S. mutans (viridans), S. pneumoniae, E. faecalis (Group D)
________________________________________
GENERAL CONCEPTS:
• The streptococci are a very heterogeneous group of bacteria. Some members are a part of our normal flora while others are potent pathogens.
• The primary pathogens are S. pyogenes and S. pneumoniae but other species can be opportunistic.
• For example, S. agalactiae can produce severe neonatal disease including meningitis, pneumonia and bacteremia in infants aged 7 days up to 3 months. E. faecalis may be implicated in endocarditis and urinary tract infections. S. mutans is an important contributor to dental caries.
• The streptococci are generally delineated into groups according to the Lancefield method.
• As important as the acute diseases produced by these microorganisms are the later sequelae of Group A streptococci. These sequelae include i) rheumatic fever following respiratory infections and ii) glomerulonephritis following skin infections.
________________________________________
DISTINCTIVE PROPERTIES:
• Streptococci are Gram-positive, non-motile cocci that divide in one plane, producing chains of cells. S. pneumoniae is a lancet-shaped diplococcus (formerly genus Diplococcus).
• The streptococci are catalase negative (unlike Staphylococcus) and may be either facultative or obligate anaerobes.
• Hemolysis (alpha, beta) on blood agar is an important differential characteristic.
• Lancefield groupings are based on the serology of cell wall polysaccharides (18 groups were originally established by Rebecca Lancefield).
• The M proteins of group A serve as important virulence factors that help the organism resist phagocytosis.
• Lipoteichoic acids (LTA) mediate attachment to epithelial cells.
• Many antigenic moieties on the streptococcal cell surface resemble human muscle and connective tissue and these similarities may be responsible for the late sequelae. For example, S. pyogenes membrane Ags resemble cardiac, skeletal, smooth muscle, heart valve fibroblasts and neuronal tissue.
• The capsule of S. pyogenes is composed of hyaluronic acid (like host connective tissue) so it is non-antigenic while the capsule of S. pneumoniae is very antigenic and is its sole virulence factor.
• Toxins produced by streptococci include: streptolysins (S & O), NADase, hyaluronidase, streptokinase, DNAses, erythrogenic toxin (causes scarlet fever rash by producing damage to blood vessels; requires cell to be lysogenized by phage that encodes toxin).
________________________________________
PATHOGENESIS:
• S. pyogenes is the leading cause of bacterial pharyngitis and tonsillitis. It may also produce sinusitis, otitis, arthritis and bone infections. Some strains prefer skin, producing either superficial (impetigo) or deep (cellulitis) infections.
• S. pneumoniae is the major cause of bacterial pneumonia in adults. Its virulence is dictated by its capsule.
• Post-infection sequelae of S. pyogenes occur 1-3 weeks after acute disease. These sequelae include i) acute rheumatic fever (following pharyngeal infections) and ii) glomerulonephritis (following either pharyngeal or skin infections). These sequelae may be due to an altered immune response (autoantibodies). Glomerulonephritis results from deposition of Ag:Ab complexes on basement membrane of kidney glomeruli.
• Other species/groups include:
o Group B strep (e.g. S. agalactiae) most often produce disease in animals but are also the leading cause of neonatal septicemia and meningitis.
o Group D strep (e.g. E. faecalis) produce urinary tract infections and endocarditis.
o Viridans species (e.g. S. mutans) are responsible for oral caries and subacute bacterial endocarditis following dental surgery.
o Anaerobic streptococci may cause genital, brain or abdominal infections.
________________________________________
HOST DEFENSES:
• The normal host resists streptococcal infection; they are often secondary invaders.
• The capsule of S. pyogenes is poorly immunogenic; anti-M protein Abs are important in host defense.
• The capsule of S. pneumoniae is very immunogenic; anti-capsule Abs are opsonizing, enhancing phagocytosis.
• Pneumococcal infections occur most often in debilitated persons (alcoholics, elderly, those with underlying disease).
________________________________________
EPIDEMIOLOGY:
• These organisms are widely distributed in nature.
• Five to 15% of normal healthy individuals carry S. pyogenes.
• Carriage of S. pneumoniae (a solely human organism) is age dependent:
Age % Carriage
Less than 5 40%
5 to 9 30%
10 to 15 17%
adults 6%
adults with children 25%
• Streptococci are labile organisms that require close personal contact for transmission; S. pyogenes respiratory disease peaks at 6 and 13 years old, showing a seasonal pattern (late winter, early spring).
________________________________________
DIAGNOSIS:
• Clinical: Diagnosis based solely upon symptomology is often not possible.
• Laboratory: To confirm the presense of S. pyogenes, throat swabs are used. For S. pneumoniae, sputum or blood samples are taken. The specimens may then be plated on blood agar for isolation of Gram-positive, catalase-negative cocci. Useful characteristics for differentiation include the pattern of hemolysis, bacitracin resistance or sensitivity and optochin resistance or sensitivity. Immunologically-based rapid test kits are often employed.
________________________________________
CONTROL:
• Sanitary: Since Streptococcus is a labile organism, close contact is required for spread; hence, avoiding contagious contacts can prevent disease.
• Immunological: Pneumococcal vaccines are available for persons at high risk, particularly the elderly.
• Chemotherapeutic: Penicillin is the drug of choice for S. pyogenes and S. pneumoniae, when the organisms are susceptible. Chemotherapy is given over a 10 and 7-145 days regimen, respectively. Group D streptococci are resistant to many antibiotics. Life long prophylaxis (low dose penicillin) is recommended for rheumatic fever patients.
Bacillus
________________________________________
ORGANISM:
• Genus: Bacillus
• Species: anthracis, cereus
________________________________________
GENERAL CONCEPTS:
• At least 48 species are known but only B. anthracis and B. cereus cause disease in humans.
• B. anthracis is responsible for the disease anthrax. This is a disease primarily of animals but humans can acquire via handling, inhaling or ingesting contaminated animal products.
• B. cereus is predominantly responsible for food poisoning in humans.
________________________________________
DISTINCTIVE PROPERTIES:
• Members of the genus Bacillus are Gram-positive, rod-shaped, spore-formers that require oxygen. However, this is a very diverse group of organisms and some species are actually Gram-negative or facultative.
• B. anthracis produces a single antigenic type of capsule and several exotoxins.
• B. cereus produces enterotoxins that causes food poisoning.
________________________________________
PATHOGENESIS:
• Anthrax infections result only if the bacteria produce a i) capsule and ii) exotoxins. The capsule allows the bacteria to survive phagocytosis. Three exotoxins (all of which are required for virulence) include:
1. edema factor (adenylate cyclase)
2. protective antigen factor
3. lethal factor.
• Anthrax infections are classified by route of entry:
1. Cutaneous anthrax; Bacillus spores enter the skin through a cut or animal bite and germinate. A small red lesion develops after 1-7 days, eventually producing local necrosis (the "black eschar"). Spread of the bacteria causes regional lymph tenderness which may be followed by a toxic septicemia and death. Only about 5% of cutaneous infections become septic.
2. Inhalation anthrax; Bacillus spores are inhaled and ingested by aveolar macrophages. These cells carry the bacteria to the regional lymph nodes, causing necrotic hemorrhaging which leads to death.
3. Gastrointestinal anthrax; ingestion of contaminated meat produces systemic symptoms which can lead to death. Mortality by gastrointestinal anthrax may be 50%.
• B. cereus food poisoning results from the ingestion of preformed enterotoxins, producing predominantly vomiting and diarrhea. The vomiting form is most often associated with ingestion of a heat stable toxin from contaminated rice, while the diarrheal form is most often associated with ingestion of a heat labile toxin from contaminated meat or vegetables.
________________________________________
HOST DEFENSES:
• Animals with high level phagocytic activity are more resistant. The capsule of B. anthracis is poorly immunogenic.
• Specific immunity can result due to the production of antitoxin Ab (if the affected animal/human survives).
________________________________________
EPIDEMIOLOGY:
• Bacillus species are worldwide soil saprophytes. The spores can survive 60 or more years under harsh environmental conditions.
• Most human anthrax infections occur in persons whose occupation brings them in contact with infected animals or their products. For example, woolsorters and hidehandlers are the most frequently affected.
• There are approximately 2000-5000 cases of human anthrax worldwide per year and 95% of these are of the cutaneous form.
• B. cereus outbreaks occur sporadically. There have been outbreaks associated with Chinese restaurants due to the way in which their fried rice was prepared.
________________________________________
DIAGNOSIS:
• Clinical: Cutaneous anthrax may be suspected upon observing the characteristic "black eschar" lesions. Inhalation and gastrointestinal anthrax are very difficult to diagnose based solely on clinical presentation. B. cereus food poisoning may present in two different forms: the vomiting form occurs within 1-6 hours (average 2 hours) following ingestion while the diarrheal form occurs from 8-12 hours (average 9 hours) following ingestion.
• Laboratory: A Gram stain of lesion material or feces can indicate the presence of these Gram-positive bacteria. Immunofluorescence techniques are also available.
________________________________________
CONTROL:
• Sanitary: Decontamination of infected animal products, deep burial of animal carcasses and the use of protective clothing can reduce the incidence of anthrax. Proper food handling, preparation and storage are essential to preventing food poisoning.
• Immunological: An avirulent spore vaccine for animals and those at high risk is available against anthrax.
Chemotherapeutic: Penicillin, erythromycin or tetracycline are drugs of choice for anthrax.
________________________________________
LINKS TO ANTHRAX AND BIOTERRORISM:
• Centers for Disease Control
• Illinois Department of Public Health
• Electronic Textbook of Dermatology
• Center for the Study of Bioterrorism and Emerging Infections at Saint Louis University School of Public Health
Clostridium
________________________________________
ORGANISM:
• Genus: Clostridium
• Species: perfringens, tetani, botulinum, difficile
________________________________________
GENERAL CONCEPTS:
• The clostridia are opportunistic pathogens. Nonetheless, they are responsible for some of the deadliest diseases including gas gangrene, tetanus and botulism. Less life-threatening diseases include pseudomembranous colitis (PC) and food poisoning.
• Clostridia cause disease primarily through the production of numerous exotoxins.
________________________________________
DISTINCTIVE PROPERTIES:
• Clostridium species are Gram-positive, rod-shaped, spore-formers. These generally obligate anaerobes are ubiquitous saprophytes or part of our normal flora.
• Clostridia employ butyric fermentation pathways to generate energy and, as a result, often produce a foul odor.
• C. perfringens produces large rectangular spores and is non-motile. This species is most often associated with wound infections but these are generally polymicrobic.
• C. tetani produces terminal spores, giving it the appearance of a squash racket. This species is motile and produces a single antigenic type of exotoxin.
• C. botulinum produces oval subterminal spores and is motile. Different strains within this species produce one of 8 exotoxin types (A,B,C1,C2,D,E,F,G). Types C and D are encoded by bacteriophage that infect the bacteria.
• C. difficile produces large oval subterminal spores and two different toxins; toxin A (an enterotoxin causing fluid accumulation in the intestine) and toxin B (a cytopathic agent). Ordinarily, this species can't compete with normal intestinal flora but, when antibiotics eliminate these normal flora, C. difficile can flourish, producing disease.
________________________________________
PATHOGENESIS:
• C. perfringens: Gas gangrene results from an anaerobic tissue environment caused by poor blood supply due to trauma, surgery, etc. This acute disease is often fatal. One to six days following trauma, a generalized fever and pain is observed in the affected area. This leads to rapid muscle necrosis because of the release of bacterial exotoxins (lecithinases, hemolysins, collagenases, proteases, lipases). A spreading infection ensues. Gas gangrene generally involves muscle extremities where anaerobiosis can occur.
• C. tetani: Tetanus results from trauma or a puncture wound leading to tissue contamination. Tetanus is a non-invasive disease occurring because of the release of exotoxins. C. tetani produces a spasmogenic toxin that fixes to gangliosides thereby blocking the release of the neurotransmitter glycine. Glycine normally prevents contraction of antagonistic muscles; therefore, muscle spasms and convulsions (lockjaw) may occur. Cardiac failure can lead to death in approximately 55-65% of affected persons.
• C. botulinum: Botulism results from the ingestion of bacterially produced neurotoxins. Types A, B, E and F are the most toxic for humans. These protein exotoxins are often released in an inactive form; proteolytic cleavage activates them. Type A is the most potent exotoxin known (10 ng can kill a normal adult). These toxins block the release of the neurotransmitter acetylcholine resulting in double vision, slurred speech, decreased saliva, difficult swallowing and general weakness. Paralysis with accompanying respiratory failure can be fatal in about 20% of those affected. Botulism food poisoning can be observed about 18-36 hours following ingestion of preformed toxin, which is heat labile. Infant botulism may occur via germination of spores in the intestinal tract with subsequent toxin production, possibly accounting for some cases of Sudden Infant Death Syndrome (SIDS).
• C. difficile: Pseudomembranous colitis (PC) results predominantly as a consequence of the elimination of normal intestinal flora through antibiotic therapy. Symptoms include abdominal pain with a watery diarrhea and leukocytosis. "Pseudomembranes" consisting of fibrin, mucus and leukocytes can be observed by colonoscopy. Untreated pseudomembranous colitis can be fatal in about 27-44%.
________________________________________
HOST DEFENSES:
• Gas gangrene: Host defenses are ineffective.
• Tetanus: No innate immunity. Disease episodes are ineffective (too little toxin released).
• Botulism: Toxin is poorly absorbed in intestine. Disease episodes are ineffective (too little toxin released).
• Pseudomembranous colitis: Normal flora play an important role. Other host defenses are unknown.
________________________________________
EPIDEMIOLOGY:
• The clostridia are ubiquitous in the soil and some are part of the normal human flora.
• Heroin addicts are particularly susceptible to tetanus as a consequence of their life style.
• Poorly canned foods create an anaerobic environment. Unkilled spores germinate and produce toxin.
• PC patients secrete large numbers of spores in feces. This provides a reservoir.
________________________________________
DIAGNOSIS:
• Clinical:
o Gas gangrene: Symptomology and the presence of bacilli in the wound.
o Tetanus: Cramping and twitching around a wound, auditory hyperacuity and pain in neck and jaw. Tetanus is similar to strychnine ingestion so must exclude the latter.
o Botulism: Difficult to diagnose. Must demonstrate a normal cerebrospinal fluid (CSF) to exclude other possibilities. The toxin is rarely found.
o Pseudomembranous colitis: Demonstration of pseudomembranes by colonoscopy is diagnostic.
o Laboratory: Members of the genus Clostridium can be differentiated from other bacteria by laboratory techniques including enzymatic digestion on egg-yolk agar plates and by using mice treated with or without antitoxin. For PC, the organisms can be isolated from feces.
________________________________________
CONTROL:
• Sanitary: Early cleansing of any wound, the surgical removal of affected tissues, suture sterilization and proper washing and canning of foods can prevent disease.
• Immunological: There are no vaccines available for gas gangrene and antitoxins are ineffective. The use of hyperbaric oxygen and chelating agents can help, however. A vaccine for tetanus made from the inactivated tetanus toxin ("toxoid") is available and required. Booster immunizations are recommended every 5 yrs or less to maintain high levels of circulating antitoxin. Botulism may be treated with antitoxin. PC patients should be certain to replace lost fluids and electrolytes to avoid dehydration.
• Chemotherapeutic: Penicillin or chloramphenicol may be employed but their use is debatable. For PC, vancomycin or metronidazole should be employed.

Note: Clostridium septicum has been strongly associated with an underlying malignancy. That is, 83% of patients having a C. septicum infection have also been diagnosed with cancer.
Anaerobic bacteria
________________________________________
Anaerobic bacteria are widely distributed in nature. Many anaerobes are common soil bacteria while many others make up part of the normal flora. The sensitivity of anaerobes to oxygen may be due to several factors, including the genetic inability to make enzymes such as superoxide dismutase (SOD), catalase or various peroxidases. In the absence of these enzymes, oxygen products that include superoxide, hydroxy radical and singlet oxygen can cause damage to cellular constituents. These highly reactive oxygen products are formed as outlined below:
O2
O2- (superoxide)
O2- + H2O2
OH- + O2 + OH• (free hydroxy radical)
O2- + OH•
OH- + O2* (singlet oxygen)
Notably, it is the superoxide moiety that is responsible for generation of the hydroxy radical and the singlet oxygen species. Therefore, aerobic bacterial species have evolved enzymes designed to eliminate superoxide (superoxide dismutase) thereby reducing the formation of the more potent species. Unfortunately, superoxide dismutase itself produces a harmful product (hydrogen peroxide) which must then be eliminated by other enzymes (catalase and peroxidase). The pathways for these enzymes are outlined below:
2 O2- + 2 H+
O2 + H2O2 (superoxide dismutase)
2 H2O2
2 H2O + O2 (catalase)
H2O2 + H2R
2 H2O + R (peroxidase)
________________________________________
Medical Relevance of Anaerobic Bacteria
The fact that most of the human normal flora is composed of anaerobic bacteria suggests that anaerobic infections might be of medical concern. Indeed, anaerobic infections can occur in a variety of body sites and involve many different genera. Most of the normal anaerobic flora are not overtly pathogenic; rather, they are considered to be opportunistic. That is, if given the opportunity, they can inflict serious and occasionally life-threatening disease. These types of infections most often occur due to trauma, injury or surgery. In general, a loss of natural barriers that introduce these bacteria into normally sterile body sites may result in infection. The sites commonly involved in anaerobic infection include the following:
1. intraabdominal infections
2. pulmonary infections
3. pelvic infections
4. brain abscesses
5. skin and soft tissue
6. oral and dental infections
7. bacteremia and endocarditis
Treatment of these infections can sometimes be difficult but, generally, moderate to broad spectrum antibiotics are usually effective.
Bacteroides
________________________________________
ORGANISM:
• Genus: Bacteroides
• Species: fragilis, thetaiotaomicron, melaninogenicus
________________________________________
GENERAL CONCEPTS:
• Bacteroides comprise a major portion of the human normal flora, predominating in the intestinal tract. These organisms are, like other anaerobes, generally opportunistic and can cause a variety of infections throughout the body. The most common infections include pleuropulmonary, intraabdominal and infections of the female urogenital tract. Bacteroides make up about one-third of the total anaerobic isolates obtained from various infections.
• B. fragilis is particularly important because of 1) the frequency of isolation and 2) its resistance to antibiotics.
• Bacteroides produce several exoenzymes including collagenase, neuraminidase, DNAse, heparinase and some proteases. These enzymes may play a role in the pathogenesis of the organism, assisting the bacteria in the invasion of host tissues following an initial trauma.
________________________________________
DISTINCTIVE PROPERTIES:
• Bacteroides are Gram-negative, anaerobic bacilli or cocco-bacilli. Each species is morphologically distinct and most are encapsulated.
• The activity of the Bacteroides LPS endotoxin is reduced compared to other Gram-negative endotoxins because the lipid A moiety has fewer fatty acids and phosphate groups.
• Most B. fragilis strains deconjugate bile acids.
• B. thetaiotaomicron converts lithocholic acid to ethyl ester. This is significant because lithocholic acid may promote the occurrence of tumors; conversion inactivates this carcinogenic potential.
________________________________________
PATHOGENESIS:
• Bacteroides are indigenous normal flora that may be opportunistic under predisposing conditions such as immunosuppression, aminoglycoside therapy, tissue damage, malignancy, etc.
• The production of collagenase seems important in the pathogenesis of B. melaninogenicus while the pathogenesis of B. fragilis is enhanced by production of an anti-phagocytic capsule.
________________________________________
HOST DEFENSES:
• Polymorphonuclear leukocytes provide an important first line of phagocytic defense.
• Anticapsular immunoglobulin plus complement enhance this phagocytosis.
________________________________________
EPIDEMIOLOGY:
• These are infections of endogenous origin.
• The percentage of Bacteroides in the gingival crevice equals about 16-20% of total flora (8-17% in plaque).
• Bacteroides predominate in the feces, reaching densities of 1011/g feces.
________________________________________
DIAGNOSIS:
• Clinical: Clinical diagnosis relies on factors that are general for anaerobic infections including a foul odor of discharge, the localization of the infection, tissue necrosis, production of gas, etc.
• Laboratory: A Gram stain and isolation of the responsible agent can be routine employing appropriate sampling, transport and growth procedures.
________________________________________
CONTROL:
• Sanitary: Generally, avoiding the introduction of bacteria is most important in preventing disease. Proper wound cleansing and surgical procedures can be highly effective.
• Immunological: None are currently in practice.
• Chemotherapeutic: Prophylaxis prior to dental, bowel, gynecological surgery can prevent infection. Active disease can be treated using carbenicillin, cefoxitin or chloramphenicol.
Salmonella
________________________________________
ORGANISM:
• Genus: Salmonella
• Species: typhi, enteritidis, cholerae-suis
________________________________________
GENERAL CONCEPTS:
• The diseases produced by different species of Salmonella are collectively known as "salmonelloses". These diseases occur worldwide and are most generally manifested as a self-limiting gastroenteritis.
• Salmonellae are pathogenic because of their capacity to i) invade intestinal mucosa and ii) produce toxins.
• The salmonellae infect a variety of animals, resulting in a large animal reservoir. S. typhi is more specific to humans, however.
• Approximately 2000 serotypes of Salmonella are known.
________________________________________
DISTINCTIVE PROPERTIES:
• The genus Salmonella is a member of the family Enterobacteriaceae. The genus is composed of Gram-negative bacilli that are facultative and flagellated (motile).
• Salmonellae possess 3 major antigens; the "H" or flagellar antigen (phase 1 & 2), the "O" or somatic antigen (part of the LPS moiety) and the "Vi" or capsular antigen (referred to as "K" in other Enterobacteriaceae).
• Salmonellae also possess the LPS endotoxin characteristic of Gram-negative bacteria. This LPS is composed of an "O" polysaccharide ("O" antigen) an "R" core and the endotoxic inner "Lipid A". Endotoxins evoke fever and can activate complement, kinin and clotting factors.
________________________________________
PATHOGENESIS:
• Salmonellosis may present as one of several syndromes including gastroenteritis, enteric (typhoid) fever or septicemia.
• Disease is initiated by oral ingestion of the bacteria followed by colonization of the lower intestine. The bacteria are capable of mucosal invasion, which results in an acute inflammation of the mucosal cells. This then leads to the activation of adenylate cyclase, increased fluid production and release of fluid into the intestinal lumen, resulting in diarrhea.
• Salmonella gastroenteritis is the most common form of salmonellosis and generally requires an 8-48 hour incubation period and may last from 2-5 days. Symptoms include nausea, vomiting and diarrhea. Salmonella enteritidis is the most common isolate.
• Enteric or typhoid fever occurs when the bacteria leave the intestine and multiply within cells of the reticuloendothelial system. The bacteria then re-enter the intestine, causing gastrointestinal symptoms. Typhoid fever has a 10-14 day incubation period and may last for several weeks. Salmonella typhi is the most common species isolated from this salmonellosis.
• Salmonella septicemia (bacteremia) may be caused by any species but S. cholerae-suis is common. This disease resembles other Gram-negative septicemias and is characterized by a high, remittent fever with little gastrointestinal involvement.
________________________________________
HOST DEFENSES:
• Several factors are important in preventing Salmonella infections including:
o Gastric factors such as the acidity or rate of emptying of the stomach contents. These factors affect the number of organisms that reach the intestine.
o Intestinal factors such as motility, the presence of normal flora, the amount of mucus, the presence of secretory IgA specific for Salmonella and genetic components. These factors affect the organism's ability to colonize and penetrate the intestinal mucosa.
o Nonspecific factors such as nutrition, the presence of iron-binding proteins and the enzyme lysozyme. These factors also affect the survivability of ingested Salmonella.
________________________________________
EPIDEMIOLOGY:
• The source of organisms for Salmonella gastroenteritis include contaminated food or water. Most commonly, persons acquire Salmonella from contaminated poultry (turkeys and chickens). S. enteritidis or S. choleraesuis are the most commonly isolated species.
• Enteric fever, in contrast, is generally transmitted from person to person and involves S. typhi(no animal reservoirs). Contamination of food or water with human feces and an asymptomatic human carrier state provide the reservoir.
________________________________________
DIAGNOSIS:
• Clinical: Clinical diagnosis of the salmonelloses is often difficult because the symptoms closely resemble other diarrheal diseases. Isolation of the organism is required for positive identification.
• Laboratory: Salmonella can be readily isolated and characterized using standard bacteriologic media or rapid identification systems. Salmonellae are motile, incapable of fermenting lactose and produce H2S. Serological techniques may be used for epidemiological characterization.
________________________________________
CONTROL:
• Sanitary: Since salmonellae are acquired through ingestion of contaminated foodstuffs, sanitary means of control are most important. Treatment of animal feeds reduces the overall level of organisms in the animal population, improved slaughtering practices prevent cross-contamination of animal products and proper hygienics by food-handlers prevents contamination at the consumer level.
• Immunological: A vaccine for typhoid is available, since only one serotype is responsible for the disease. However, the vaccine is not very effective because the bacteremic stage (i.e. where bacteria contact the vaccine-induced antibody) is brief.
• Chemotherapeutic: Typhoid fever and Salmonella septicemia may be treated using moderate to broad spectrum antibiotics. Gastroenteritis should only be treated by replacing lost fluids, since antibiotic therapy does not affect the course of the disease and may only increase the number of resistant species.
Shigella
________________________________________
ORGANISM:
• Genus: Shigella
• Species: dysenteriae
________________________________________
GENERAL CONCEPTS:
• Shigella dysenteriae is responsible for bacillary dysentery, a disease most often associated with crowded, unsanitary conditions.
• Other species of Shigella may produce milder forms of diarrheal disease.
________________________________________
DISTINCTIVE PROPERTIES:
• Shigellae are facultative, non-motile, Gram-negative bacilli. They possess the heat stable endotoxin (LPS) characteristic of Gram-negative bacteria.
• Shigellae are pathogenic primarily due to their ability to invade intestinal epithelial cells.
• S. dysenteriae also produces a heat labile exotoxin that is a neurotoxin acting upon the gray matter of the central nervous system.
________________________________________
PATHOGENESIS:
• Dysentery is an oral infection transmitted via fecal contamination of water or food. During the 1-4 day incubation period, penetration of bacteria into the mucosal epithelial cells of the intestine causes an intense irritation of the intestinal wall, producing cramps and a watery, bloody diarrhea.
________________________________________
HOST DEFENSES:
• Unlike the salmonellae, shigellae are acid tolerant. As a consequence, gastric acidity provides little protection against infection.
• Protective defenses include the normal flora, secretory IgA and phagocytosis.
________________________________________
EPIDEMIOLOGY:
• Dysentery and other shigelloses occur worldwide but the hosts are limited to humans and primates.
• Because of their acid tolerance, relatively few organisms are required to produce disease. Indeed, as few as 10 cells can cause disease in 10% of healthy persons; 200 cells may cause disease in 40% of persons.
• Contaminated food and water are the primary sources for contracting dysentery, but person to person transmission may occur because of the low dose required to produce disease.
• About 60% of cases occur in children aged 1-10 years.
________________________________________
DIAGNOSIS:
• Clinical: As with other diarrheal diseases, clinical diagnosis alone is equivocal. Diarrhea, fever and a watery bright red blood tinged stool are classical symptoms, but isolation of the organisms is required for confirmation.
• Laboratory: Shigella can be readily isolated and characterized using standard bacteriologic media or rapid identification systems. Shigellae are non-motile, incapable of fermenting lactose and do not produce H2S. Serological techniques may be used for epidemiological characterization.
________________________________________
CONTROL:
• Sanitary: As with other fecal-oral diseases, dysentery is best prevented by assuring a safe water supply and proper disposal of feces.
• Immunological: No vaccines are currently available.
• Chemotherapeutic: The use of antibiotics is debatable because the disease is self-limiting. If required, ampicillin or a trimethoprim-sulphamethoxizole combination may be employed. Replacing lost fluids to prevent dehydration is most important for treating the disease.
Campylobacter
________________________________________
ORGANISM:
• Genus: Campylobacter
• Species: jejuni
________________________________________
GENERAL CONCEPTS:
• The genus Campylobacter is a relatively recently discovered important human pathogen. The reason for this is that the organisms are microaerophilic, requiring low concentrations of oxygen only. Indeed, Campylobacter infections occur more often than Salmonella and Shigella diarrheas combined. In the United States, there are about 2 million cases annually, most among college students.
________________________________________
DISTINCTIVE PROPERTIES:
• Campylobacters are Gram-negative, curved rods (the name derives from the Greek "campylo", meaning curved). These organisms are microaerophilic and motile.
• Campylobacters possess a typical Gram-negative cell wall containing LPS endotoxin.
• There are approximately 50 heat-labile "K" (capsular) and "H" (flagellar) antigens and 60 different heat-stable "O" (somatic) antigens associated with different species of Campylobacter.
• These organisms are able to use amino acids and citric acid cycle intermediates for growth. C. jejuni grows best at 42°.
________________________________________
PATHOGENESIS:
• A relatively small inoculum is required to cause illness; as few as 800 bacteria can produce disease in healthy persons.
• Illness generally occurs following a 2-4 day incubation period when the bacteria multiply in the intestine, reaching numbers similar to Salmonella and Shigella infections (106-109 per gram of feces). Symptoms resemble an acute enteritis with fever, diarrhea, nausea and abdominal pain. The illness is generally self-limiting but may last a week.
• C. jejuni appears to produce an enterotoxin similar to both the cholera and Escherichia coli toxins.
________________________________________
HOST DEFENSES:
• Host defense mechanisms that help to combat Campylobacter infections are not well characterized but gastric acidity and secretory IgA may be important.
________________________________________
EPIDEMIOLOGY:
• The genus Campylobacter is widely distributed among cattle, sheep, dogs, cats and other animals, existing as normal flora commensals.
• Human infection results from the ingestion of contaminated water, milk or undercooked foods. Outbreaks resulting from ingestion of raw clams have been reported.
________________________________________
DIAGNOSIS:
• Clinical: Clinical diagnosis is difficult since the symptomology is non-specific.
• Laboratory: Special methods are required for isolation. Growth occurs in 5% O2, 10% CO2, 85% N2 at 42°. A Gram stain of fecal material may reveal curved ("seagull" or "comma") shaped organisms.
________________________________________
CONTROL:
• Sanitary: As with other fecal-oral diseases, sanitary means of control are most important. Proper disposal of feces, cooking foods, etc. can prevent disease.
• Immunological: None available.
• Chemotherapeutic: Erythromycin or tetracycline can be used for severe or prolonged illness.
Vibrio
________________________________________
ORGANISM:
• Genus: Vibrio
• Species: cholerae
________________________________________
GENERAL CONCEPTS:
• The name Vibrio derives from the Latin because these curved rods possess a single polar flagellum and appear "to vibrate".
• V. cholerae was first isolated in pure culture in 1883 by Robert Koch.
• V. cholerae produces the disease cholera, defined as "a metabolic disturbance of the epithelial cells of the small bowel". Cholera is caused, in part, by a potent enterotoxin (choleragen) and is usually a disease of poor sanitation.
• Humans are the only natural host for this organism and there have been 6 great pandemics of cholera.
• Two biotypes of V. cholerae are described: Classic and El Tor.
• Three serotypes (Ogawa, Inaba and Hikojima) are also recognized.
________________________________________
DISTINCTIVE PROPERTIES:
• The genus Vibrio is composed of Gram negative, curved rods that are motile by means of a single polar flagellum.
• These organisms are sensitive to acid pH but tolerate alkaline pH (9.0-9.6) very well.
• The El Tor biotype produces less toxin but colonizes better and is more resistant to environmental factors.
________________________________________
PATHOGENESIS:
• The acid sensitivity of V. cholerae means that a large dose is required to produced disease. Indeed, 1011 vibrios given orally fail to produce illness but if bicarbonate (e.g. Alka-Selzer®) precedes the inoculation, then only 104 are required.
• Cholera is a disease of the small intestine, unlike most other enteric illnesses. The bacteria penetrate the mucus layer and adhere to the mucosal cells where they subsequently produce toxin.
• The potent enterotoxin choleragen is well defined. This 84 kD protein enterotoxin is composed of 2 major domains; the A domain controls its biologic activity while the B domain binds the toxin to cellular receptors (GM1 receptor). Binding via B leads to penetration of the A1 peptide, which enzymatically transfers ADP-ribose from NAD to a GTP regulatory protein. This leads to activation of adenylate cyclase and the overproduction of cyclicAMP, which causes hypersecretion of chlorides and water (click the image to animate). A cholera patient may secrete 20 liters of fluid per day with 108 vibrios per ml!
________________________________________
HOST DEFENSES:
• Gastric acidity plays an important role in preventing cholera infection. Also, genetic factors may be important since the toxin must bind to specific cellular receptors in order to function. Secretory antitoxin IgA may also play a role.
________________________________________
EPIDEMIOLOGY:
• Three patterns of cholera have been observed: heavily endemic, neoepidemic and limited outbreaks.
• Transmission of disease is primarily via water that has been contaminated by human feces. Carriage of vibrios is rare.
________________________________________
DIAGNOSIS:
• Clinical: Gastrointestinal symptoms and the presence of a rice water stool are presumptive for cholera. Direct examination of the feces may indicate vibrios.
• Laboratory: Isolation of V. cholera employs thiosulfate-citrate-bile salts-sucrose (TCBS) agar, which is selective for the organism.
________________________________________
CONTROL:
• Sanitary: Sanitary measures are most important in controlling cholera outbreaks. The shear volume of liquid and concentration of viable organisms precludes measures for decontamination.
• Immunological: An experimental vaccine using a modified strain has been attempted. This strain, called "Texas Star", produces a toxin that has the B domain but not the A (biologically active) domain. Ingestion of this organism should allow production of sIgA against the cell-binding moiety (thereby preventing binding) without producing overt disease. In addition, researchers have recently suggested that a cholera vaccine might be incorporated into french fries! Click here to see how.
• Chemotherapeutic: Most important to the patient is the replacement of lost fluid and salts. Death from cholera results as a consequence of extreme and rapid dehydration. Moderate or broad spectrum antibiotics may be used to reduce the output of viable organisms.
Escherichia
ORGANISM:
• Genus: Escherichia
• Species: coli
________________________________________
GENERAL CONCEPTS:
• Members of the genus Escherichia are common bacteria that colonize the human large intestine. Most are opportunistic normal flora but some are potent pathogens.
• Transmission of diarrheal disease is generally person to person, usually related to hygiene, food processing and sanitation.
• Four general categories of pathogenic E. coli are recognized:
1. Enterotoxigenic (ETEC)
2. Enteroinvasive or "Shigella-like" (EIEC)
3. Enteropathogenic (EPEC)
4. Enterohemorrhagic (EHEC)
• Different groups are most often delineated by serology, in particular, by the immunogenic character of the O (somatic, LPS) and H (flagellar) antigens.
________________________________________
DISTINCTIVE PROPERTIES:
• Escherichia are Gram-negative bacilli that ferment lactose. Most are motile by peritrichious flagella.
• Escherichia possess a typical Gram-negative cell wall containing LPS.
• Approximately 170 different O antigens have been delineated and some of these are cross-reactive with Shigella, Salmonella and Klebsiella.
• Motile strains possess H (flagellar) antigens that can be used for epidemiologic purposes.
• Escherichia also possess K (capsular) antigens similar to the Vi antigen of Salmonella.
• Enterotoxigenic strains may also display colonization factor antigens (CFA/I, CFA/II).
________________________________________
PATHOGENESIS:
• E. coli diarrhea is generally acquired via ingestion of water or food that has been contaminated by an infected person. The following table outlines the four major classifications:
Classification
(Strain) Site of Infection Disease(s) Pathogenic Mechanism(s)
Enterotoxigenic
(ETEC) Small intestine Traveler's diarrhea
Watery stool, cramps, nausea, low fever Enterotoxins ST and LT (click here to view the mechanisms.)

Enteroinvasive
(EIEC) Large intestine Shigella-like diarrhea
Fever, cramps, watery diarrhea followed by scant, bloody stool Tissue invasion and destruction of epithelial cells (plasmid-mediated)
Enteropathogenic
(EPEC) Small intestine Infantile diarrhea
Salmonella-like with fever, nausea, vomiting Adherence and destruction of epithelial cells (plasmid-mediated)
Enterohemorrhagic
(EHEC, O157:H7) Large intestine Hemorrhagic colitis
Severe abdominal pain, watery diarrhea followed by grossly bloody stool SLT-I, SLT-II cytotoxins ("verotoxins")
________________________________________
HOST DEFENSES:
• A number of factors help prevent infection by E. coli. These include gastric acidity, intestinal motility and the normal intestinal flora.
• Some evidence suggests a possible genetic component because the bacterial fimbriae are used to attach to specific cellular receptors.
• In addition, breast milk contains neutralizing (non-immunoglobulin) factors that help to prevent disease in nursing infants.
________________________________________
EPIDEMIOLOGY:
• E. coli is a human organism and, as a consequence, is transmitted from person to person via contaminated food or water.
• E. coli is also a very commonly acquired nosocomial infection (this is described on the next page).
________________________________________
DIAGNOSIS:
• Clinical: Generally, clinical diagnosis of E. coli infection is equivocal. The enterotoxigenic and enteropathogenic forms cause a watery diarrhea and nausea while the enteroinvasive and enterohemorrhagic forms subsequently produce bloody stools. These presentations alone, however, are not sufficient for confirmation.
• Laboratory: E. coli infection might be generally suspected in absence of isolation of Salmonella or Shigella or intestinal parasites. The organisms themselves are easily isolated and identified by routine procedures. Specialized serotyping may be necessary for epidemiologic studies.
________________________________________
CONTROL:
• Sanitary: As with other fecal-oral diseases, proper food handling and personal hygiene are the best means for preventing infection.
• Immunological: New vaccines against fimbrial antigens are possible.
• Chemotherapeutic: Antibiotic therapy is not generally recommended unless disease becomes life-threatening. Oral rehydration is the best treatment.
Coliforms
________________________________________
ORGANISM:
• Genus: Escherichia, Enterobacter, Klebsiella, Serratia, Citrobacter, Proteus
• Species: many (Click here to view.)
________________________________________
GENERAL CONCEPTS:
• Collectively, this group of Gram-negative bacilli (with the exception of Proteus) are referred to as "coliforms" because they share similar morphological and biochemical characteristics.
• With the exception of Proteus, these organisms ferment lactose, which is a useful characteristic for differentiating them from Salmonella and Shigella.
• Most of these organisms are members of the normal flora of humans and/or animals and are considered opportunistic pathogens.
• Diseases produced by the coliforms and Proteus can be grouped into three general categories:
1. Nosocomial or hospital-acquired infections: Forty percent of all nosocomial infections involve coliforms or Proteus. The primary sites for infection include the urinary tract (E. coli), surgical wound (E. coli), lower respiratory tract (Klebsiella), and primary bacteremia (E. coli).
2. Infections in compromised patients: E. coli is responsible for 40% of neonatal bacterial meningitis infections.
3. Community acquired infections: E. coli accounts for 85% of urethrocystitis cases, 80% of chronic bacterial prostatitis cases and 90% of acute pyelonephritis cases. Proteus, Klebsiella and Enterobacter may produce urinary tract infections. Proteus may also be responsible for some renal infection stones, due to the production of the enzyme urease and subsequent alkalinization and supersaturation of urine. In addition, K. pneumonia is responsible for approximately 3% of bacterial pneumonia cases and is more severe than that produced by S. pneumoniae. E. coli can also produce several different types of diarrheal disease (click here).
________________________________________
DISTINCTIVE PROPERTIES:
• These bacteria are all facultative Gram negative bacilli. Except for Proteus, they all ferment lactose. Except for Klebsiella, they are all motile. Each genus has a typical Gram- negative cell wall containing LPS.
• Antigens possessed by these organisms include the H (flagellar), K (capsular) and O (somatic).
• Those organisms that possess fimbriae (common pili) use these appendages for adhesion purposes.
• All genera produce the characteristic LPS endotoxin and some genera/species produce exotoxins.
________________________________________
PATHOGENESIS:
• Generally, these organisms are opportunistic pathogens. E. coli is perhaps the best studied.
• The site of infection may be specific for a particular serotype. For example, Klebsiella capsular types 1 and 2 are most commonly associated with the production of bacterial pneumonia; types 8, 9, 10 and 24 are more commonly associated with urinary tract infections. This finding suggests that these antigenic specificities may provide a site-specific pathogenicity to the organisms. In support of this idea, E. coli displaying the K1 antigen have a propensity for producing neonatal meningitis. The K1 antigen provides the organism with an increased resistance to phagocytosis and the action of complement. The fact that this K1 antigen is cross reactive with the capsular antigen from group B meningococci (also capable of producing meningitis) suggests that the specific antigenic makeup of a particular organism may help to determine the sites where it can produce infections.
________________________________________
HOST DEFENSES:
• It is, in fact, the failure of host defense that leads to coliform disease.
• The normal flora are antagonistic and help prevent infection. Prolonged antibiotic therapy can compromise normal defense mechanisms.
• Loss of anatomic barriers or immunosuppression are also contributory to coliform disease.
________________________________________
EPIDEMIOLOGY:
• Multiple reservoirs and modes of transmission exist for these organisms. They may be found in water (tap or distilled), soil, food and the intestinal tract as well as contaminated hospital food or containers, respiratory equipment, hemodialysis units, intravenous (IV) fluids or caps or staff members. Thus, there are both exogenous and endogenous sources for contamination by these organisms.
________________________________________
DIAGNOSIS:
• Clinical: Clinical diagnosis is difficult if not impossible.
• Laboratory: The coliforms and Proteus are easily cultivated and identified by routine laboratory procedures. As a group, the Enterobacteriaceae are oxidase negative, capable of reducing nitrates to nitrites, ferment glucose, ferment lactose (except Proteus) and motile (except Klebsiella). Many conventional and rapid identification systems are available.
________________________________________
CONTROL:
• Sanitary: Frequent hand-washing by staff and a general awareness of microbial presence can reduce hospital-acquired infections. Disinfectants are not always effective.
• Immunological: There is the possibility of anti-serum or vaccine against these organisms but none are currently in use.
• Chemotherapeutic: Moderate or broad spectrum antibiotics are generally useful. Susceptibility tests should be performed when appropriate.
Pseudomonas
________________________________________
ORGANISM:
• Genus: Pseudomonas
• Species: aeruginosa, others
________________________________________
GENERAL CONCEPTS:
• Greater than 140 species of Pseudomonas have been described and most are saprophytic.
• In terms of human disease, Pseudomonas is generally an opportunistic pathogen. However, the genus is responsible for several specific diseases including glanders (P. mallei) and melioidosis (P. pseudomallei).
• Pseudomonas infections may be serious in hospitalized patients or those with cancer or cystic fibrosis.
________________________________________
DISTINCTIVE PROPERTIES:
• Pseudomonads are Gram negative rods. They are motile, nonfermentative aerobes that can utilize acetate for carbon and ammonium sulphate for nitrogen. Many species are resistant to high salt, dyes, weak antiseptics and most antibiotics. P. aeruginosa can grow at 42°.
• Pseudomonas possesses the LPS endotoxin characteristic of other Gram-negative bacteria and displays O and H antigens.
• Pseudomonas produces many exoenzymes including hemolysins, leukocidins and proteases. In addition, a toxin called Toxin A is the most toxic product produced by Pseudomonas. This product causes the ADP-ribosylation of translation factor EF-2, producing ADP-ribosyl-EF-2. The effect of this enzymatic activity is the loss of host cell protein synthesis capability. This mechanism is identical to that produced by diphtheria toxin.
• Also, a protein called exoenzyme S is another ADP-ribosyltransferase, transferring the ADP-ribose from NAD to other proteins (not EF-2).
• Pseudomonas has an antiphagocytic polysaccharide slime layer and many strains produce pigments, some which are fluorescent.
________________________________________
PATHOGENESIS:
• Pseudomonas produces localized infections following surgery or burns. Localized infections can lead to generalized, and occasionally fatal, bacteremia.
• Pseudomonas is also responsible for a number of nosocomial infections including urinary tract infections following catheterization, pneumonia resulting from contaminated respirators, and eye and ear infections.
________________________________________
HOST DEFENSES:
• Most Pseudomonas species are complement-resistent; the best host defense seems to be opsonization and PMN phagocytosis.
• Once an infection becomes established, however, antitoxin (a humoral response) is very important.
________________________________________
EPIDEMIOLOGY:
• Pseudomonas can be found in the soil, in water, or on vegetation.
• On average, 3% of persons entering the hospital have Pseudomonas in their stool. After a hospital stay of as little as 72 hrs, 20% have Pseudomonas. The organisms are spread from patient to patient via staff, contaminated reservoirs, respiratory equipment, food, sinks, taps, mops; most moist environments.
________________________________________
DIAGNOSIS:
• Clinical: Diagnosis is very difficult.
• Laboratory: Pseudomonas can be easily isolated on blood agar. These organisms are non-fermentative (oxidative), oxidase-positive, Gram-negative bacilli. They often give off a fruity odor and some may produce fluorescent pigments. P. aeruginosa grows well at 42°.
________________________________________
CONTROL:
• Sanitary: Several sanitary measures are important in the prevention of Pseudomonas infections. Proper isolation and topical therapy of burn patients, good aseptic technique and the careful cleaning and sterilization of instruments are important.
• Immunological: None currently available.
• Chemotherapeutic: Gentamycin, tobramycin or a gentamycin/carbenicillin combination are the drugs of choice for treating serious Pseudomonas infections.
Brucella
________________________________________
ORGANISM:
• Genus: Brucella
• Species: abortus (cow), melitensis (goat), suis (pig), canus (dog)
________________________________________
GENERAL CONCEPTS:
• The Brucellae are generally associated with animal infections but most are also pathogenic for humans.
• All human infections come from animals; there is no human to human transmission. Such diseases are called "zoonoses".
• B. melitensis is associated with a specific human disease called Malta fever.
• Brucellae are intracellular parasites.
________________________________________
DISTINCTIVE PROPERTIES:
• The genus Brucella is composed of Gram negative coccobacilli. Most are aerobic but grow best in a 5-10% CO2-enriched environment. Their metabolism is oxidative.
• Brucellae possess a typical Gram-negative LPS endotoxin, as well as two major serological determinants; A and M.
________________________________________
PATHOGENESIS:
• Of the four species that cause disease in humans, B. melitensis and B. suis are more transmissible to humans, particularly via the oral route.
• These bacteria are intracellular parasites of the reticuloendothelial (RE) system (e.g. spleen, liver, bone marrow, lymph nodes and kidneys).
• Following exposure, the organisms may produce a localized abscess, which is followed by bacteremia. Phagocytosis by macrophages and intracellular multiplication leads to localization in the RE tissues. Disease may remain subacute or become chronic with initial symptoms of malaise, chills, weakness and intermittent fever. Granulomas in various RE tissues may occur as a result of a hypersensitivity reaction.
• In animals, multiplication occurs in the uterus because of the presence of erythritol, which the bacteria prefer to glucose. This localization can lead to abortion or excretion in milk (human source for infection).
________________________________________
HOST DEFENSES:
• Because of the intracellular life-style of the Brucellae, humoral defenses play a minor role. Cell mediated defenses (T-lymphocytes, activated macrophages) are required.
________________________________________
EPIDEMIOLOGY:
• Brucella infections have a worldwide distribution but have been mostly eradicated in the United States.
• B. abortus affects primarily cows; B. melitensis affects goats and sheep; B. suis affects pigs; B. canus affects dogs. Humans generally acquire disease through occupational exposure, Thus, veterinarians, meat workers and animal handlers are those most likely to be afflicted.
________________________________________
DIAGNOSIS:
• Clinical: Symptoms of brucellosis are variable and diagnosis is, therefore, very difficult. Flu-like symptoms with limb and back pain, an intermittent fever with malaise may last up to 3 months for acute disease (a year or more for subacute or chronic disease).
• Laboratory: Isolation of Brucella from the blood is possible. Cultures must be incubated 3-4 weeks with added CO2.
________________________________________
CONTROL:
• Sanitary: Pasteurization of dairy products and use of protective clothing prevent human infection. More importantly, systematic identification and elimination of infected animals and vaccination of animals reduces the reservoir.
• Immunological: Vaccination for persons at high risk is possible, but they must first be tested to ensure that no hypersensitivity already exists.
• Chemotherapeutic: Tetracycline or a tetracycline/streptomycin combination is generally curative.
Yersinia
________________________________________
ORGANISM:
• Genus: Yersinia
• Species: pestis, enterocolitica
________________________________________
GENERAL CONCEPTS:
• The organism Yersinia pestis is responsible for the plague, a disease that has an extremely important place in human history. During the 6th century AD, the plague ravaged the known world over a 50 year period causing 100 million deaths. The "black death" again devastated Europe during the 14th century over a 5 year period causing 25 million deaths (25% of the European population).
• Other species of Yersinia generally produce a self-limiting gastroenteritis.
________________________________________
DISTINCTIVE PROPERTIES:
• The genus Yersinia is composed of Gram negative, bipolar staining coccobacilli. Like other Enterobacteriaceae, their metabolism is fermentative. Y. pestis produces a thick anti-phagocytic slime layer, while Y. enterocolitica is motile at 28°.
________________________________________
PATHOGENESIS:

• Plague is a disease that is primarily maintained among rodent populations and transmitted by infected fleas. Urban plague involves rats and has been the major source for human epidemics. Sylvatic plague exists in wild rodent populations.
• Two distinct forms of plague occur in human populations:
1. Bubonic plague occurs within a week of being bitten by an infected flea. Multiplication of the bacteria produces the characteristic "bubo" (swollen, painful lymph node). Bacteremia follows, causing death in about 75% of those affected.
2. Pneumonic plague occurs under crowded conditions when contaminated respiratory droplets expelled by infected persons are directly inhaled by another person. This form is characterized by a shorter incubation period and greater mortality (90%).
• Pathogenic Y. pestis produce two antiphagocytic components; F1 antigen and the VW antigens. Both are required for virulence and, interestingly, are only produced when the organism grows at 37°, not at lower temperatures. This might explain why the bacteria are not virulent in their alternate host, the flea, which has a body temperature near 25°. Moreover, the bacteria are capable of surviving and multiplying within monocytes, but not PMNs, and upon emerging from the monocytic host, the bacteria possess their F1 and VW antigens.
• Most disease produced by Y. enterocolitica is a typical gastroenteritis characterized by fever, abdominal pain, and diarrhea. Illness generally lasts from 1 to 2 weeks but chronic cases may persist for up to a year.
________________________________________
HOST DEFENSES:
• Antibodies specific for the F1 and VW antigens are opsonic and confer immunity by enhancing phagocytosis and intracellular killing by PMNs.
________________________________________
EPIDEMIOLOGY:
• Plague occurs in urban or wild rodent populations: humans acquire disease primarily via infected fleas. The Yersinia multiply in the flea intestinal tract. Upon biting a new host, the flea regurgitates, inoculating the new host with the organisms.
• Y. enterocolitica is an intestinal parasite of animals. Humans acquire disease via ingestion of contaminated food or water.
________________________________________
DIAGNOSIS:
• Clinical: Rapid diagnosis must be made because of the speed at which the disease progresses and the high mortality rate. Bubonic plague can result in 75% mortality with a few days; the pneumonic form can result in greater than 90% mortality within 24 hours. Diagnosis of Yersinia gastroenteritis requires laboratory confirmation.
• Laboratory: Examination of sputum or a lymph node biopsy will reveal Gram-negative, bipolar staining coccobacilli. However, for the safety of workers and the community, P-3 level containment is required for Y. pestis. Y. enterocolitica may be grown on a variety of standard bacteriologic medias.
________________________________________
CONTROL:
• Sanitary: Control of rat populations concurrent with elimination of their fleas prevents spread of the plague to humans. Decontamination of water and milk prevents gastroenteritis.
• Immunological: A short term vaccine against Y. pestis is available for persons at high risk.
Chemotherapeutic: Treatment of the plague must be rapid and aggressive. Y. pestis is generally susceptible to streptomycin and chloramphenicol but concomitant therapy is sometimes recommended. Treatment of Y. enterocolitica infections usually involves the use of ampicillin or tetracycline.
Bordetella
________________________________________
ORGANISM:
• Genus: Bordetella
• Species: pertussis, parapertussis, bronchiseptica
________________________________________
GENERAL CONCEPTS:
• The genus Bordetella is responsible for respiratory disease in humans and animals. B. pertussis, in particular, is the etiologic agent of pertussis, more commonly known as whooping cough. B. parapertussis causes a milder form of pertussis, while B. bronchiseptica mostly affects animals but occasionally humans.
• Bordetella cause disease by producing toxins that impair ciliary function in the respiratory tract.
________________________________________
DISTINCTIVE PROPERTIES:
• Bordetella are Gram negative coccobacilli. They produce a capsule and are strict aerobes. Only B. bronchiseptica is motile.
• Bordetella possess the heat stable endotoxin LPS and produce several exotoxins. These include:
1. Pertussigen: A 120 kD protein exhibiting the A-B model for toxin activity. Pertussigen is an ADP-ribosyl-transferase that interferes with the transfer of signals from cell surface receptors. Pertussigen is also involved in mediating attachment to respiratory epithelia.
2. Adenylate cyclase toxin: this toxin increases cAMP levels, inhibiting immune effector cell functions.
3. Tracheal cytotoxin: This toxin causes ciliostasis and extrusion of ciliated epithelia.
4. Dermonecrotic toxin: This heat labile substance causes tissue destruction.
5. Filamentous hemagglutinin: This is involved in attachment to host cells.
________________________________________
PATHOGENESIS:
• Whooping cough results from colonization and multiplication of Bordetella pertussis on the mucus membranes of the respiratory tract, in particular, the ciliated epithelial cells.
• Production of toxins irritates cells causing ciliostasis and leukocytosis.
• The hallmark of pertussis is the spasmatic cough that may last 6 weeks. Occasional secondary complications include encephalopathy, seizures and pneumonia.
________________________________________
HOST DEFENSES:
• Antibody against the pertussigen exotoxin affords immunity.
________________________________________
EPIDEMIOLOGY:
• The mucus membranes of the respiratory tract are the organism's natural habitat.
• Disease generally follows direct contact with an infected person.
• Pertussis is generally a disease of infants (50% of cases occur in children less than 1 year old).
• The disease is highly contagious.
________________________________________
DIAGNOSIS:
• Clinical: Whooping cough requires a 7-14 day asymptomatic incubation period. This is followed by the catarrhal stage, which lasts 1-2 weeks. Symptoms include fever, rhinorrhea and a highly infectious cough. The next 2-4 weeks define the paroxysmal phase, during which the spasmatic ("whooping") cough is observed. Vomiting and leukocytosis (> 100,000 lymphocytes/mm3) are also evident. Finally, the convalescent phase marks the end of disease and may last 3-4 weeks or longer. During this period, secondary complications may occur.
• Laboratory: The organisms can be grown on Bordet-Gengou agar media after 3-4 days incubation. Immunological techniques may also be employed.
________________________________________
CONTROL:
• Sanitary: This very contagious disease requires quarantine for a period of 4-6 weeks.
• Immunological: Pertussis vaccine is a part of the required "DPT" schedule.
• Chemotherapeutic: Antibiotic prophylaxis (erythromycin) may be used for contacts. Treatment of disease with antibiotics does not affect its course.
Neisseria
________________________________________
ORGANISM:
• Genus: Neisseria
• Species: gonorrhoeae, meningitidis
________________________________________
GENERAL CONCEPTS:
• Neisseria inhabit mucosal surfaces. There are 2 species that are pathogenic for humans:
1. N. gonorrhoeae. Also referred to as the gonococcus, N. gonorrhoeae is responsible for the disease gonorrhea, named by Galen in the year 130 AD from the literal "flow of seed".
2. N. meningitidis. Also referred to as the meningococcus, N. meningitidis is responsible for meningitis.
________________________________________
DISTINCTIVE PROPERTIES:
• Neisseria are Gram-negative diplococci with their adjacent sides flattened.
• These organisms are aerobic, strongly oxidase-positive, have an oxidative metabolism, are susceptible to drying and are fastidious (growth is inhibited by free fatty acids).
• There are four types of N. gonorrhoeae based on the presence of fimbriae; T1, T2, T3 and T4.
• N. meningitidis is serotyped by the antigenic character of its capsular polysaccharide; several groups including A, B, C, 29E, W-135 and Y are recognized.
________________________________________
PATHOGENESIS:
• Gonorrhea is a sexually transmitted disease. The sites of infection include the urethra (in men) and the cervix (in women).
• Fimbriae (pili) are very important for the gonococcus to attach to host cells. N. gonorrhoeae lacking fimbriae are avirulent.
• A substance called Protein I makes up 66% of the outer membrane protein of N. gonorrhoeae. This protein is antigenic and is used as the basis of some serological tests.
• N. gonorrhoeae produce cytotoxic substances that damage ciliated epithelial cells in fallopian tubes; the LPS endotoxin may be partly responsible.
• N. gonorrhoeae also produce an extracellular protease that cleaves a proline-threonine bond in immunoglobulin IgA. This causes loss of antibody activity.
• Approximately 9-15% of affected women contract Pelvic Inflammatory Disease (PID) as a consequence of gonorrhea. This is often a polymicrobic infection involving Bacteroides and other anaerobes.
• The virulence of N. meningitidis is associated with its antiphagocytic capsule.
• The meningococcal LPS is as toxic as Escherichia or Salmonella and causes suppression of leukotriene B4 (a chemokinetic/chemotactic factor) synthesis in PMNs.
• Untreated, meningococcal meningitis has a mortality approaching 85%.
• N. meningitidis produces proline-threonine and proline-serine proteases, but any particular isolate will only produce one or the other.
________________________________________
HOST DEFENSES:
• Gonorrhea: Infection stimulates local immunity (secretory Igs may enhance association with PMNs). Uncomplicated infection activates complement via the classical pathway while disseminated infections activate complement via the alternate pathway.
• Meningitis: Anti-capsular antibodies are bactericidal.
________________________________________
EPIDEMIOLOGY:
• N. gonorrhoeae normally colonizes mucosal surfaces. Humans are the only host and transmission is via sexual contact. The probability of acquiring disease from a single exposure varies: in men, the probability is about 25%; in women, the probability is about 40%; in women taking oral contraceptives, the probability is about 100%.
• Gonorrhea occurs worldwide and generally affects persons aged 15-29.
• N. meningitidis inhabits the human nasopharynx. There is a 3-30% normal carrier state lasting days to months that provides the reservoir for infection of susceptible persons. Attack rates are highest in children (usually less than one year old) and sporadic epidemics do occur.
________________________________________
DIAGNOSIS:
• Clinical: The symptoms of gonorrhea differ between the sexes. In men, a copious urethral exudate containing Gram- negative diplococci is common; in women, disease is often asymptomatic. The symptoms of meningitis usually begin abruptly with headache and fever. However, confirmation of meningococcal infection requires bacteriologic culture.
• Laboratory: Neisseria may be cultured on Thayer-Martin agar or other suitable media with incubation in 10% CO2. Neisseria are strongly oxidase-positive, Gram-negative diplococci. N. gonorrhoeae oxidizes glucose only; N. meningitidis oxidizes both glucose and maltose.
________________________________________
CONTROL:
• Sanitary: Condoms are useful for preventing gonorrhea. Avoiding contact with infected persons can prevent meningitis.
• Immunological: Only experimental vaccines are available. These vaccines target the meningococcal capsular polysaccharide but are not effective against group B (the most common isolate) because of its poor immunogenicity.
• Chemotherapeutic: Penicillin is the drug of choice for treating gonorrhea. However, the number of resistant isolates continues to increase and other drugs must be employed in these cases. Penicillin is also the drug of choice for treating meningococcal meningitis.
Haemophilus
________________________________________
ORGANISM:
• Genus: Haemophilus
• Species: influenzae
________________________________________
GENERAL CONCEPTS:
• Haemophilus influenzae is responsible for producing a variety of infections including meningitis and respiratory infections.
• Six serological types (a,b,c,d,e,f) based on the antigenic structure of the capsular polysaccharides are recognized. Nonencapsulated strains are (by definition) nontypable.
• Other species of Haemophilus include: H. parainfluenzae (pneumonia, endocarditis), H. ducreyi (venereal chancre) and H. aegypticus (conjunctivitis).
________________________________________
DISTINCTIVE PROPERTIES:
• The genus Haemophilus is composed of Gram-negative coccobacilli.
• These organisms are fastidious and require factors X (hemin) and/or V (NAD).
• Haemophilus possess LPS in the cell wall but produce no apparent extracellular toxins.
________________________________________
PATHOGENESIS:
• The organisms colonize the nasopharynx and are spread by direct contact. Haemophilus are capable of penetrating the epithelium to produce a bacteremia that may lead to localization of the organisms in many organs. The capsule is Haemophilus' major virulence determinant yet unencapsulated strains produce ear, sinus and respiratory infections.
• H. influenzae type b is the most common cause of bacterial meningitis in children aged 6 months-2 years. It is uncommon in adults because of protective antibody.
• Cellulitis, conjunctivitis, epiglottitis and arthritis may also result from Haemophilus infection.
• For pneumonia in adult men, unencapsulated H. influenzae is second only to the pneumococcus (S. pneumoniae). Those affected are usually chronic smokers, alcoholics or elderly.
________________________________________
HOST DEFENSES:
• Antibody directed against the polyribosyl-ribitol-phosphate (PRP) capsule is bactericidal.
________________________________________
EPIDEMIOLOGY:
• Spread of Haemophilus is human to human. Day care centers are common sites for transmission from healthy, unaffected adults to susceptible infants.
________________________________________
DIAGNOSIS:
• Clinical: A Gram stain of cerebrospinal fluid may reveal the organisms. One can also detect capsular material directly.
• Laboratory: The organisms are cultured on chocolate agar because it contains both factors X and V. Incubation in 10% CO2 is required.
________________________________________
CONTROL:
• Sanitary: Avoidance of carriers is not always possible.
• Immunological: A vaccine against type b is available. Unfortunately, the vaccine is nonimmunogenic in infants where it is needed the most.
• Chemotherapeutic: Third generation cephalosporins are probably the drugs of choice because of their ability to cross the blood-brain barrier and their bactericidal activity.
Legionella
________________________________________
ORGANISM:
• Genus: Legionella
• Species: pneumophila
________________________________________
GENERAL CONCEPTS:
• Legionellae are ubiquitous aquatic saprophytes
• Legionella is a relatively recent discovery in terms of human disease.
• L. pneumophila, in particular, is the etiologic agent of Legionnaire's disease, first described in Philadelphia in 1976.
• Legionnaire's disease is characterized by cough and fever with radiologic evidence of pneumonia.
• Legionella is a unique, previously unrecognized bacterium. One might say it is a new organism for an old disease.
________________________________________
DISTINCTIVE PROPERTIES:
• Legionella are motile, catalase-positive, Gram-negative bacilli. Some strains produce a yellow fluorescence under 366 nm ultraviolet light. Most are fastidious, requiring iron and L-cysteine for growth.
• There are 25 species of Legionella and 42 serogroups. Most (85%) clinical isolates are L. pneumophila serotype 1.
• The Legionella cell wall is mostly diaminopimelic acid (DAP) and contains little peptidoglycan. The LPS appears to lack the endotoxic lipid A moiety.
• Several species produce hemolysins and there are some reports of exotoxins.
• Legionella are capable of intracellular (macrophage) multiplication.
________________________________________
PATHOGENESIS:
• Legionella have, in general, a low potential for virulence and most infections occur in persons having compromised immunity or pulmonary function.
• There are two forms of disease produced by Legionella:
1. Pontiac fever: An epidemic flu-like condition described in Pontiac Michigan in 1968 was later found to be due to Legionella. This condition was marked by fever, chills, headache and malaise that lasted 2-5 days and resolved.
2. Legionnaire's disease: A severe pneumonia characterized by fever, chills and a non-productive cough. This multi-organ disease has significant mortality if not treated promptly.
• The virulence of Legionella is dependent upon their ability to survive and multiply within macrophages.
________________________________________
HOST DEFENSES:
• Most healthy individuals resist infection by Legionella but the mechanisms are not well understood.
________________________________________
EPIDEMIOLOGY:
• Legionella are typically associated with aerosolized water (central air conditioning, cooling towers, showers, whirlpools).
• Disease is generally waterborne; transmission occurs via airborne droplets.
• The organisms exist in nature; humans are an accidental host.
________________________________________
DIAGNOSIS:
• Clinical: Symptoms include headache, malaise, rapid fever, nonproductive cough, pneumonia. Generally, disease is difficult to diagnose but might be suspected in middle aged to older men who smoke and drink.
• Laboratory: Bacteria can be grown on Buffered Charcoal-Yeast Extract (BCYE) agar. Direct immunofluorescent may be used to visualize the organisms. An increase in Legionella-specific serum antibody is evidence of infection.
________________________________________
CONTROL:
• Sanitary: Regular maintenance of air conditioning or the inclusion of biocidal compounds into water cooling towers reduces the reservoir. Similarly, hyperchlorination of the water supply eliminates the source.
• Immunological: None available.
• Chemotherapeutic: Erythromycin is the drug of choice.
Treponema
________________________________________
ORGANISM:
• Genus: Treponema
• Species: pallidum, pertenue, carateum
________________________________________
GENERAL CONCEPTS:
• Three "treponematoses" are discussed: syphilis, yaws and pinta.
• Each of these diseases is characterized by distinct clinical stages. These stages are known as primary, secondary and tertiary.
o The primary stage involves multiplication of the bacteria at the site of entry to produce a localized infection.
o The secondary stage occurs following an asymptomatic period and involves dissemination of the bacteria to other tissues.
o The tertiary stage may occur after 20-30 years.
• The Treponema are highly invasive organisms; T. pallidum is the most invasive of the species, T. carateum the least invasive.
________________________________________
DISTINCTIVE PROPERTIES:
• The Treponema are motile, helically coiled organisms having a corkscrew-like shape. They stain very poorly because their thickness approaches the resolution of the light microscope.
• Treponema are delicate organisms requiring pH in the range 7.2 to 7.4, temperatures in the range 30°C to 37°C and a microaerophilic environment.
• The structure of these organisms is somewhat different: the cells have a coating of glycosamino-glycans, which may be host-derived, and the outer membrane covers the three flagella that provide motility.
• In addition, the cells have a high lipid content (cardiolipin, cholesterol), which is unusual for most bacteria. Cardiolipin elicits "Wassermann" antibodies that are diagnostic for syphilis.
• Treponema possess a complex antigenic makeup that is difficult to determine because the organisms cannot be grown in vitro.
________________________________________
PATHOGENESIS:
• Treponema pallidum is capable of infecting all body tissues.
• The disease caused by T. pallidum is syphilis. This is a relatively painless, slowly evolving disease. The host-parasite relationship leads to short symptomatic periods when the organism multiplies, followed by prolonged asymptomatic periods when host responses produce healing.
• Syphilis is strictly a person-person disease.
• An incubation period of from 10 to 90 days precedes the clinical presentation, despite the fact that the organisms disseminate immediately. The prominent feature of the disease is vascular involvement, particularly arterioles and capillaries.
• Treponemal antigen-host antibody complexes may cause some immunosuppression of the host and production of the distinct clinical stages:
o The primary stage occurs weeks to months following infection. The principal sign of infection is the hard chancre, generally found on the genitals. This lesion is essentially painless but filled with treponemes and is, therefore, highly contagious.
o The secondary stage occurs following an asymptomatic period of 2-24 weeks. In the secondary stage, a skin rash spreads from the palms and soles towards the trunk. This rash may last 2-6 weeks and is followed by recovery. On average, about 25% of patients experience relapses of the secondary stage.
o Following the secondary stage is a period of latency which may last many years and during which there are essentially no clinical symptoms.
o The tertiary stage may erupt following the period of latency and can affect all areas of the body and be fatal. Cardiovascular and neurological involvement are the most frequent causes of death. Typically, however, the appearance of lesions called "gummas" mark the tertiary stage. These lesions are, in fact, large granulomas resulting from hypersensitivity reactions and they can be extremely disfiguring.
• Syphilis that occurs in utero is termed congenital syphilis. About 50% of such fetuses abort or are stillborn. Of those surviving birth, two scenarios are observed: the "early" form shows symptoms that are apparent at birth; in the "late" form, infants appear normal until they are about 2 years old and only then display the traits known as "Hutchinson's triad", which include interstitial keratitis, notched incisors and eighth nerve deafness.
• Other treponematoses include:
o Yaws: Caused by T. pertenue, this disease occurs in tropical Africa, S. America, India, Indonesia and the Pacific Isles (equitorial regions). Symptoms involve the occurrence of a painless papule called the "Mother yaw" as the primary stage. Following healing and complete dissemination of the organisms, many papules return after 1-12 months and occur on the face and moist body areas.
o Pinta: Caused by T. carateum, this disease occurs only in tropical Central and South America. It is characterized by a painless papule (primary) followed 2-18 months later by secondary papules on the hands, feet and scalp. These lesions heal slowly after treatment (unlike syphilis, yaws).
________________________________________
HOST DEFENSES:
• It is the intricate interplay between the organism and the host immune system that defines the treponematoses.
• Immunity results in untreated persons but it is slow to evolve.
• Immunity is probably a combination of both humoral and cell-mediated defenses.
________________________________________
EPIDEMIOLOGY:
• Syphilis is found worldwide and is transmitted via sexual contact (ages 20-24 are most affected).
• Because the route of transmission is the same, 10% of gonorrhea patients also have syphilis.
• Yaws and pinta are not sexually transmitted and generally affect children or adolescents. These diseases are often geographically diagnosed.
________________________________________
DIAGNOSIS:
• Clinical: The clinical manifestations of the treponematoses are generally characteristic and readily identified.
• Laboratory: Darkfield examination of material from a chancre can show the presence of spirochetes. Immunological techniques including fluorescent treponemal antibody (FTA) or T. pallidum immobilization (TPI) can be of great assistance in observing the organisms. The Wassermann test looks for the presence of antibody against cardiolipin. Many other tests are also available.
________________________________________
CONTROL:
• Sanitary: As with other sexually transmitted diseases (STDs), use of a condom helps prevent infection.
• Immunological: None are available.
• Chemotherapeutic: Benzathine penicillin (long acting) or penicillin G are the drugs of choice. One must be aware of a possible Jarisch-Herxheimer reaction following treatment of secondary or tertiary syphilis, however. The rapid release of treponemal antigens after lysis by penicillin can cause hypersensitivity reactions in some persons.
Leptospira
________________________________________
ORGANISM:
• Genus: Leptospira
• Species: interrogans
________________________________________
GENERAL CONCEPTS:
• As the name implies, Leptospira are spiral shape organisms.
• The diseases produced by Leptospira are termed "leptospirosis" and can vary from subclinical to fatal.
• Leptospirosis is a zoonosis; man is an accidental host via contaminated animal urine.
• The leptospirosis known as "Weil's disease" was first described in 1886.
________________________________________
DISTINCTIVE PROPERTIES:
• Leptospira are thin, tightly coiled obligate aerobes that are highly motile.
• Their structure is similar to other spirochetes; a multilayered outer membrane, helical shaped peptidoglycan and flagella located in the periplasmic space.
• Their nutritional requirements include long-chain fatty acids and vitamins B1 and B12.
• There are more than 180 serotypes of Leptospira described.
________________________________________
PATHOGENESIS:
• Mucosa and broken skin provide the entry for leptospires.
• The organisms produce a generalized infection with bacteremia (leptospiremic phase). Antibody is produced and the organisms then become localized primarily in the kidneys. Multiplication in the kidneys leads to shedding in the urine (leptospiruric phase). This may persist for weeks, months or years.
• Leptospira produce no known exo- or endotoxins.
• Damage to the endothelial lining of capillaries and renal failure are the most common reasons for death. Occasionally the central nervous system may become involved.
• The host immune response is probably responsible for lesions associated with late phase of disease. This is suggested because antibiotics are ineffective after symptoms have persisted for more than 4 days.
________________________________________
HOST DEFENSES:
• Antibody plus complement is leptospiricidal. Immunity against Leptospira is primarily humoral.
• The cell-mediated response may be responsible for late manifestations.
________________________________________
EPIDEMIOLOGY:
• Leptospirosis is a worldwide zoonosis.
• The primary reservoir is rodents because, once infected, they shed the organisms for life. Dogs are a major source for human infections.
• Direct or indirect contact with infected urine is the mode of transmission.
________________________________________
DIAGNOSIS:
• Clinical: Leptosirosis is a general febrile disease that is often misdiagnosed as meningitis or hepatitis. Following a 7-14 day incubation period, patients experience fever, severe headache, pain and occasional jaundice. Symptoms last about 7 days then subside. The leptospiruric phase then lasts for several days before complete recovery.
• Laboratory: Darkfield microscopic examination of the blood or urine combined with serologic tests is confirmatory.
________________________________________
CONTROL:
• Sanitary: Reducing the prevalence in domestic animals via vaccine reduces human infection.
• Immunological: A vaccine is available for animals, but not humans.
• Chemotherapeutic: Penicillin or tetracycline is effective if given early.
Borrelia
________________________________________
ORGANISM:
• Genus: Borrelia
• Species: recurrentis, hermsii, burgdorferi
________________________________________
GENERAL CONCEPTS:
• Borreliae produce febrile diseases characterized by remittent fever.
• The organisms are transmitted to humans by lice or ticks.
• B. recurrentis produces epidemic relapsing fever (lice); B. hermsii causes endemic relapsing fever (ticks); B. burgdorferi is the agent responsible for Lyme disease (ticks).
________________________________________
DISTINCTIVE PROPERTIES:
• The Borreliae are similar to Leptospira but somewhat fatter and have more complex nutritional requirements.
• The cell wall contains various lipids including cholesterol.
________________________________________
PATHOGENESIS:
• Borreliae produce a generalized infection following an incubation period of about 1 week. Symptoms include fever, headache and muscle pain that lasts 4-10 days and subsides. An afebrile period lasting 5-6 days follows and then there is a recurrence of acute symptoms.
• Epidemic relapsing fever (transmitted by lice) is generally more severe than endemic relapsing fever (transmitted by ticks) and has an approximately 40% mortality if untreated. Also, the epidemic form is generally characterized by having a single relapse, while the endemic form may have several relapses due to cyclic antigenic variation of the Borrelia.
• Lyme disease (transmitted by ticks) involves the production of ulcerative lesions on the skin and may lead to arthritis or neurologic involvement.
________________________________________
HOST DEFENSES:
• Antibody is important in controlling disease because the organisms are capable of resisting non-specific defenses.
________________________________________
EPIDEMIOLOGY:
• B. recurrentis is transmitted via the human louse. Thus, this is a disease that occurs when people are crowded together under poor conditions; i.e. war, natural disaster, etc. Humans are the reservoir for B. recurrentis.
• B. hermsii is transmitted via the soft-shelled tick. This disease is maintained in (primarily) rat populations; humans acquire disease when bitten by an infected tick.
• B. burgdorferi is transmitted via the hard-shelled tick (deer tick). As with B. hermsii, the organisms are maintained within animal populations and humans are an accidental host.
________________________________________
DIAGNOSIS:
• Clinical: The symptomology of the recurrent fevers is not specific enough for accurate clinical diagnosis. With Lyme disease, however, the occurrence of a "bulls-eye" lesion on the skin (erythema chronicum migrans, ECM) is almost always (85%) associated with infection. This usually begins as a small red lesion that enlarges over several weeks to a reddened area that may cover several inches in diameter. Among cases that show ECM, about 20% progress to include arthralgia, about 50% involve intermittent episodes of arthritis and 10% progress to chronic arthritis.
• Laboratory: Darkfield smears can be used to observe the relapsing fever Borrelia but serologic tests (ELISA) are a better determinant for Lyme disease.
________________________________________
CONTROL:
• Sanitary: Measures to eliminate the tick and louse vectors are important for reducing the incidence of human disease.
• Immunological: A recombinant vaccine is available for Lyme disease, with a 75-90% efficacy.
• Chemotherapeutic: Tetracycline is the drug of choice.
NOTE: The Centers for Disease Control (CDC) have an excellent site describing Lyme disease and Borrelia burgdorferi located at http://www.cdc.gov/ncidod/dvbid/lyme/index.htm
Corynebacterium
________________________________________
ORGANISM:
• Genus: Corynebacterium
• Species: diphtheriae
________________________________________
GENERAL CONCEPTS:

• Corynebacteria belong in the family Mycobacteriaceae and are part of the CMN group (Corynebacteria, Mycobacteria and Nocardia).
• The family Mycobacteriaceae are Gram-positive, nonmotile, catalase-positive and have a rodlike to filamentous morphology (Corynebacteria are often pleomorphic).
• As a group, they produce characteristic long chain fatty acids termed mycolic acids. In the image to the right, the R-groups represent these chains. For Corynebacteria, chains of 28-40 carbons are common; for Nocardia, chains of 40-56 carbons are produced; for Mycobacteria, the chains are 60-90 carbons in length.
________________________________________
DISTINCTIVE PROPERTIES:
• Corynebacterial cell walls contain thin spots which leads to some Gram variability and "ballooning" that produces a "club-shaped" cell. Old cells store inorganic phosphate, which can appear as metachromatic granules when stained.
________________________________________
PATHOGENESIS:

• C. diphtheriae is the etiologic agent of diphtheria.
• These organisms colonize the mucus membranes of the respiratory tract and produce the enzyme neuraminidase which splits N-acetylneuraminic acid (NAN) from cell surfaces to produce pyruvate which acts as a growth stimulant.
• C. diphtheriae also produces diphthin, which is a protease that inactivates IgA.
• Toxigenic strains carry the gene tox, which resides on certain bacteriophages; lysogenization leads to toxigenicity.
• The toxin that is produced is a single polypeptide of 62,000 daltons and contains a single disulfide cross-link. Digestion with trypsin gives 2 fragments, A and B. The B (binding) fragment attaches to cell surfaces then proteases release the A (active) fragment to enter the cell. In the cell, the toxin acts as an ADP-ribosyltransferase, inactivating translation factor EF2.
________________________________________
HOST DEFENSES:
• Humoral immunity (antitoxin) is important in preventing disease.
________________________________________
EPIDEMIOLOGY:
• Diphtheria exists throughout the world and occasional outbreaks occur almost yearly.
• The Schick test can be used to ascertain population risk. This test involves the injection of a minute amount of the diphtheria toxin under the skin. The absence of a reaction indicates immunity.
________________________________________
DIAGNOSIS:
• Clinical: Muscle weakness, edema and a pseudomembranous material in the upper respiratory tract characterizes diphtheria.
• Laboratory: Tellurite media is the agar of choice for isolation of Corynebacteria, which produce jet black colonies.
________________________________________
CONTROL:
• Sanitary: Reduce carrier rate by use of vaccine.
• Immunological: A vaccine (DPT) prepared from an alkaline formaldehyde inactivated toxin (i.e. toxoid) is required. Passive immunization with antitoxin can be used for patients.
• Chemotherapeutic: Penicillin, erythromycin or gentamicin are drugs of choice.
Mycobacterium
________________________________________
ORGANISM:
• Genus: Mycobacterium
• Species: tuberculosis, leprae
________________________________________
GENERAL CONCEPTS:

• Mycobacteria belong in the family Mycobacteriaceae and are part of the CMN group (Corynebacteria, Mycobacteria and Nocardia).
• The family Mycobacteriaceae are Gram-positive, nonmotile, catalase-positive and have a rodlike to filamentous morphology (Corynebacteria are often pleomorphic).
• As a group, they produce characteristic long chain fatty acids termed mycolic acids. In the image to the right, the R-groups represent these chains. For Corynebacteria, chains of 28-40 carbons are common; for Nocardia, chains of 40-56 carbons are produced; for Mycobacteria, the chains are 60-90 carbons in length.
________________________________________
DISTINCTIVE PROPERTIES:
• Mycobacteria are considered "acid-fast", which means that they retain dyes following an acid-alcohol decolorization step.
• These organisms are very slow growers; a 5 hour division time is not uncommon.
• Different species can be differentiated based on growth rate, niacin secretion, reduction of nitrate, caratogenesis, etc.
• Mycobacteria produce "cord factors", which are dimycolates of trehalose. This gives rise to a pattern of growth in serpentine cords.
• The very lipid nature of the cell wall provides some resistance to drying, acid or alkaline conditions. It is also a good adjuvant that enhances humoral and cell-mediated immune responses. The responsible component may be N-acetyl-muramyl-L-alanyl-D-isoglutamine (MDP), which causes the inhibition of macrophage migration.
• M. leprae has never been cultured in vitro but can be grown on the footpads of armadillos.
________________________________________
PATHOGENESIS:
• M. tuberculosis is the agent responsible for the disease tuberculosis.
• Virulent strains of Mycobacteria have the capacity to disrupt phagosomal membranes of alveolar macrophages while the cord factors inactivate mitochondrial membranes of phagocytes. These properties enable the organisms to survive and multiply in phagocytes.
• Leprosy is caused by M. leprae and may appear in one of two forms; tuberculoid or lepromatous.
________________________________________
HOST DEFENSES:
• It is the cell-mediated immune response (delayed hypersensitivity) that leads to the production of the granulomatous lesion known as a tubercle. This lesion contains a core of rounded macrophages, surrounded by outer giant cells. Persistence of the lesion gives rise to a cheesy caseation necrosis and, eventually, calcification.
________________________________________
EPIDEMIOLOGY:
• Tuberculosis is spread by airborne droplets that must penetrate deep into the respiratory tree.
• With the AIDS epidemic, the number of multiple drug resistant (MDR) strains has increased, making it even more difficult to treat cases.
________________________________________
DIAGNOSIS:
• Clinical: Tuberculosis is characterized by a prolonged and productive cough.
• Laboratory: Performing an acid-fast stain on sputum can reveal the bacteria. A skin test can ascertain exposure, but only where the Bacille Calmette-Guerin (BCG) vaccine is not used regularly.
________________________________________
CONTROL:
• Sanitary: Treatment and quarantine of cases reduces the rate.
• Immunological: BCG vaccine is available. In the US, this is used to test for exposure; in other areas of the world, it is used to prevent disease.
• Chemotherapeutic: Isoniazid, rifampin and ethambutol are the drugs used to treat tuberculosis, often in combination and often for many months. Rifampin is used for leprosy.
Rickettsia
________________________________________
ORGANISM:
• Genus: Rickettsia, Rochalimaea, Coxiella
• Species: Rickettsia prowazekii (epidemic typhus), Rickettsia typhi (endemic typhus), Rickettsia rickettsii (spotted fever), Rochalimaea quintana (trench fever), Coxiella burnetii (Q fever)
________________________________________
GENERAL CONCEPTS:
• The Rickettsia are Gram-negative, obligate intracellular bacteria that infect mammals and arthropods.
• R. prowazekii is the agent of epidemic typhus. During World War I, approximately 3 million deaths resulted from infection by this bacterium. In World War II, the numbers were similar. This agent is carried by the human louse; therefore, disease is a consequence of overcrowding and poor hygiene.
• Rocky Mountain spotted fever and Q fever remain relatively common.
________________________________________
DISTINCTIVE PROPERTIES:
• These organisms are small, pleomorphic coccobacilli about 2 µm in length. Their structure is typical of Gram-negative bacteria.
• Rickettsia replicate in the cytoplasm and nucleus of their host cell; Coxiella replicate only in the phagolysosome.
________________________________________
PATHOGENESIS:
• Typhus, spotted fever and trench fever are transmitted via arthropod vectors; Q fever is acquired via inhalation or ingestion of contaminated milk or food.
• Within minutes, the bacteria enter host endothelial cells via an induced phagocytosis. The enzyme phospholipase A may help penetration.
• Replication of the bacteria causes lysis of the host cell and consequent spread to other cells.
• Initial replication occurs at the site of entry producing a local lesion. This is followed by dissemination via the vascular system producing vasculitis and a skin rash. These lesions may become necrotic.
• Virulence is probably due to many factors including release of endotoxin, the production of immune complexes and hypersensitivity reactions.
• A characteristic triad of symptoms include fever, headache and rash (no rash with Q fever).
________________________________________
HOST DEFENSES:
• Humoral immunity may be important because, following recovery from disease, persons become immune to further infection.
________________________________________
EPIDEMIOLOGY:
• Epidemic typhus and trench fever are transmitted from human to human via the louse.
• Endemic (murine) typhus is primarily maintained in rodent populations and is transmitted via the flea. Humans are an accidental host.
• Spotted fever is found predominantly in animals and is transmitted by the tick. Humans are accidental hosts. Most cases of Rocky Mountain spotted fever in the US occur during the summer months in North and South Carolina, Kansas and Oklahoma.
• Q fever is found mostly in animals. Humans acquire disease primarily by inhalation of contaminated aerosols.
________________________________________
DIAGNOSIS:
• Clinical: These diseases present as febrile illnesses after exposure to arthropods or animal hosts or aerosols in endemic areas and are easily misdiagnosed. A delay in diagnosis may be partly responsible for the high mortality from Spotted fever. The spread of the rash is often characteristic: spread from the trunk to the extremities (centrifugal) is typical for typhus; spread from the extremities to the trunk (centripetal) is typical for spotted fever.
• Laboratory: The use of immunofluorescent antibodies to examine a biopsy can be diagnostic. The organism can be inoculated into tissue culture and grown over 4-7 days but this is very hazardous to personnel. The Weil-Felix test looks for the production of serum antibody that is reactive against Proteus OX19, OX2 or OXK antigens but it is not always reliable.
________________________________________
CONTROL:
• Sanitary: Arthropod and rodent control are possible but difficult.
• Immunological: No vaccines are currently available.
• Chemotherapeutic: Tetracycline or chloramphenicol are drugs of choice.
Chlamydia
________________________________________
ORGANISM:
• Genus: Chlamydia
• Species: trachomatis, psittaci
________________________________________
GENERAL CONCEPTS:
• The Chlamydia are obligate intracellular parasites.
• C. trachomatis is responsible for the diseases trachoma, inclusion conjunctivitis, lymphogranuloma venereum (LGV) and nongonococcal urethritis (NGU). In other words, oculourogenital infections.
• C. psittaci produces systemic diseases including psittacosis, ornithosis and pneumonitis.
________________________________________
DISTINCTIVE PROPERTIES:
• The Chlamydia have an unusual developmental cycle that involves two distinct forms: infectious elementary bodies and intracellular reticulate bodies. Elementary bodies attach and are internalized by susceptible host cells. Once inside, they reorganize into a replicative form (the reticulate body). Over a 24 hour period, these reticulate bodies divide and begin to reorganize back into elementary bodies. About 48-72 hours after infection, the cell is lysed and numerous infectious elementary bodies are released.
• The genome of Chlamydia is only 25% the size of E. coli, making it one of the smallest prokaryotes.
• The pathogenic mechanisms employed by Chlamydia are not well understood.
________________________________________
PATHOGENESIS:
• C. trachomatis is spread via the fingers to the urogenital area and vis versa. In contrast, C. psittaci is acquired from infected birds, usually via the respiratory route.
• Trachoma is an infection of the epithelial cells of the conjunctiva, producing inclusion bodies. Vascularization and clouding of cornea along with trichiasis (inward growth of eyelashes) can produce scarring that may lead to blindness.
• Inclusion conjunctivitis is a milder form that occurs in both children and adults. This form generally heals without scarring or blindness.
• Sexually transmitted nongonococcal urethritis (NGU) is similar to gonorrhea and occurs with greater frequency. In 1997, approximately 320,000 cases were reported to the Centers for Disease Control.
• In men, a condition termed lymphogranuloma venereum (LGV) involving inguinal lymphadenopathy ("buboes") can occur.
• Psittacosis is a respiratory disease ranging from influenza-like to pneumonia-like and is generally acquired from infected birds.
________________________________________
HOST DEFENSES:
• Chlamydia induce interferon and are sensitive to it.
• During infection, antibodies are synthesized but recovery is not generally protective.
________________________________________
EPIDEMIOLOGY:
• Trachoma is prevalent in Africa and Asia, generally in hot and dry areas.
• The organisms are very persistent. Their habitat is similar to that of Neisseria and Haemophilus.
• Infection can occur via swimming in unchlorinated pools, sharing towels or by passage through the birth canal.
________________________________________
DIAGNOSIS:
• Clinical: Diagnosis of trachoma is usually good. Likewise, the genital vesicles associated with LGV are characteristic. NGU can only be suspected in the absence of laboratory findings.
• Laboratory: Iodine stained specimens usually show inclusion bodies that represent the replicating bacteria. The Chlamydia can be cultured in tissue culture and appropriate serological tests performed.
________________________________________
CONTROL:
• Sanitary: Good hygiene, treatment of sexual partners and the quarantine of birds all reduce the incidence.
• Immunological: No vaccine is available or likely since specific antibodies fail to neutralize elementary bodies in vivo.
• Chemotherapeutic: Tetracycline or erythromycin are drugs of choice.
Mycotic Infections
________________________________________
ORGANISM:
• Genus/Species: There are a large number of different genera and species of fungi that cause human diseases. Only a few of these specific agents will be presented and discussed on this page.
________________________________________
GENERAL CONCEPTS:
• The fungi represent a diverse, heterogeneous group of eukaryotes. Most of these organisms are plant pathogens and relatively few cause disease in humans.
• In nature, fungi generally grow by secreting enzymes that digest tissues but some are actually predacious.
• The growth of the fungi generally involves two phases; vegetative and reproductive.
o In the vegetative phase, the cells are haploid and divide mitotically. Most fungi exist as molds with hyphae but some fungi exist as unicellular yeast cells. Some fungi can change their morphology and are termed dimorphic. For example, Candida is found in the yeast form at 37°C but changes to the mold form at 25°C.
o In the reproductive phase, fungi may undergo either asexual or sexual reproduction. Asexual reproduction involves the generation of spores; sexual reproduction requires specific cellular structures that are used for taxonomic differentiation.
• The fungi are classified based on the characteristics of their sexual phase. For the kingdom Fungi, there are two phyla; Zygomycota and Dikaryomycota. The phylum Dikaryomycota is further divided into two subphyla; Ascomycotina and Basidiomycotina. A third group of the fungi for which a sexual phase has not been observed is termed Deuteromycotina. The table outlines this classification and gives representatives of each.
________________________________________
Group Representative Genera
Phylum Zygomycota Rhizopus, Absidia, Mucor
Phylum Dikaryomycota
•Subphylum Ascomycotina Trichophyton, Histoplasma, Blastomyces
•Subphylum Basidiomycotina Cryptococcus
Form-class Deuteromycotina Candida, Epidermophyton, Coccidioides
________________________________________
• Fungal membranes contain ergosterol rather than cholesterol and this provides a target for chemotherapy (azole derivatives interfere with ergosterol synthesis). Nonetheless, most anti-fungal antibiotics are still relatively toxic to the human host.
________________________________________
DISTINCTIVE PROPERTIES:
• Mycotic infections are classified by the tissue levels that are colonized.
o Superficial infections are generally limited to the outer layers of the skin and hair.
o Cutaneous infections are located deeper in the epidermis, hair and nails.
o Subcutaneous infections involve the dermis, subcutaneous tissues and muscle.
• In addition, mycotic infections may be systemic, generally originating in the lungs.
• Finally, some mycoses are termed opportunistic, and these may involve a variety of body sites.
• The following table outlines these different types of mycotic infection, giving examples of representative agents.
________________________________________
Type and Sites of Infection Representative Diseases Representative Genera
Superficial: Limited to outer layers of skin and hair. Pityriasis versicolor (skin)
Tinea nigra (skin)
Black/white piedra (hair) Malassezia
Exophiala
Piedra/Trichosporum

Cutaneous: Involves deep epidermis and keratinized body areas (skin, hair, nails). Diseases are generally cosmetic, not life-threatening. Diseases of the skin are termed Tinea; Diseases of hair and nails are termed Dermatophycoses. Trichophyton
Microsporum
Epidermophyton

Subcutaneous: Involves dermis, subcutaneous tissues and muscle. Fungi are generally implanted in skin; fungal growth produces a lesion. Lymphocutaneous sporotricosis
Chromoblastomycosis
Eumycotic mycetoma Sporothrix
Foncecaea
Pseudallescheria
Many others

Systemic: Originate in lungs, phagocytosis by macrophages, spread to many organs. Most primary infections are inapparent. Progression may produce pulmonary symptoms or ulcerative lesions. Host responses produce formation of syncytia, fibrous tissue, granulomas and calcified lesions. Representative organisms are dimorphic, except for Cryptococcus, which is a yeast. Histoplasmosis: Endemic in Ohio and Mississippi Rivers valleys, most infections are asymptomatic. Histoplasma capsulatus
Blastomycosis: Endemic in Ohio and Mississippi Rivers valleys, important veterinary problem Blastomyces dematitidis
Paracoccidioidomycosis: Endemic in Central and South America, primarily Brazil Paracoccidioides braziliensis
Coccidioidomycosis: Endemic in Southwestern United States Coccidioides immitis
Cryptococcosis: Worldwide distribution. Most common clinical presentation is meningitis. Cryptococcus neoformans

Opportunistic: These organisms generally have a low potential for virulence but can produce severe disease involving a variety of body tissues. Candidiasis
Aspergillosis
Zygomycosis Candida albicans
Aspergillis
Rhizopus
________________________________________
PATHOGENESIS:
• Mycotic disease is often a consequence of predisposing factors including age, stress or other pathologic conditions (e.g. cancer, diabetes, AIDS).
• Only the dermatophytes (Trichophyton, Microsporum) and Candida are communicable from human to human. The other agents are acquired from the environment (plants, soil, etc.).
• Fungi generally cause one of three distinct tissue responses;
1. chronic inflammation (scarring, accumulation of lymphocytes)
2. granulomatous inflammation (collections of modified epithelial cells, lymphocytes)
3. acute suppurative inflammation (vascular congestion, exudation of plasma, accumulation of PMNs).
• Some of the tissue responses may be due to mycotoxins, which are fungal metabolites that are toxic to the host. Some agents produce LPS-like endotoxins or hemolysins or steroid-like toxins that affect the nervous system
• Aspergillus produces a toxin called aflatoxin that has a strong association with liver cancer. For example, in Thailand, where people generally consume about 25-times more aflatoxin in their diets, the incidence of cancer is about 10-fold greater.
• Systemic mycoses are generally asymptomatic but may have generalized symptoms including low grade fever, shaking chills, night sweats, malaise or appetite loss.
________________________________________
HOST DEFENSES:
• Host defenses against the fungi include nonspecific and specific factors:
o Nonspecific defenses include the skin (lipids, fatty acids, normal flora), internal factors (mucous membranes, ciliated cells, macrophages), blood components, temperature, genetic and hormonal factors. In other words, both physical and chemical factors and phagocytic defenses play major roles in prevention and control of mycotic disease.
o Specific defenses include both humoral and cell-mediated. The role of humoral defenses is somewhat controversial, since certain antibodies are not protective. It is possible that high titers of certain antibodies actually suppress the cell mediated defenses. Nevertheless, some antibodies may be protective (e.g. antitoxins or opsonins). Generally, however, the cell-mediated defenses are probably more important. Acquired resistance is usually T-cell mediated and persons with compromised cell-mediated defenses generally show more disseminated disease.
________________________________________
EPIDEMIOLOGY:
• Dermatophytes may be communicated from person to person by combs, towels, etc. These infections (termed "tineas" when affecting the skin) include ringworm, athlete's foot, jock itch, etc. Candida is a member of the normal vaginal flora; candidiasis is often associated with diabetes.
• In some cases of mycosis, occupation seems an important contributor. For example, Sporothrix is normally found in woody plants; hence, agricultural workers acquire disease more often. Similarly, Histoplasma is often found in bird or bat excreta; hence spelunkers (caves) or persons involved in community clean up may acquire more often.
• Most of the systemic diseases are geographically distributed; histoplasmosis and blastomycosis predominate in the Ohio and Mississippi Rivers valleys, paracoccidioidomycosis is found primarily in Brazil, coccidioidomycosis predominates in the Southwestern United States while cryptococcosis has a worldwide distribution.
________________________________________
DIAGNOSIS:
• Clinical: For the dermatophytes, appearance of the lesions is usually diagnostic. For systemic mycoses, the epidemiology and symptomology are useful clues.
• Laboratory: Treatment of skin scrapings with 10% potassium hydroxide can reveal hyphae or spores. Most fungi can be grown on Sabouraud's dextrose agar but they are often very difficult to speciate. Some fungi show a yellow fluorescence under 365 nm ultraviolet light. Skin testing for a delayed hypersensitivity response is useful for epidemiologic purposes but often not for diagnosis.
________________________________________
CONTROL:
• Sanitary: Control by sanitary means is difficult, but the incidence of communicable disease can be reduced by good hygiene.
• Immunological: No vaccines are currently available.
• Chemotherapeutic: Many antifungals are available but some are very toxic to the host and must be used with caution. Topical powders and creams often contain tolnaftate or azole derivatives (miconazole, clotrimazole, econazole) and are useful against superficial dermatophytes. Hair or nail disease may be treated with oral griseofulvin, but it is rather toxic. Sporotrichosis may be treated using potassium iodide or amphotericin B (oral). Systemic infections are generally treated by amphotericin B, 5-flourocytosine, miconazole or ketoconazole.
General Virology
________________________________________
GENERAL CLASSIFICATION
The viruses are a large group of obligate intracellular parasites capable of infecting a variety of different cell types. Viruses may contain either DNA or RNA as their genomic material and this material may be either single- or double-stranded. Single-stranded genomes may be of positive (i.e. mRNA) or negative (i.e. anti-mRNA) polarity. Surrounding and protecting the genome is a coat of protein ("capsid") arranged in one of several possible morphologies. Certain viruses also contain an additional phospholipid bilayer ("envelope") derived from the host cell and surrounding the protein capsid. This page will discuss the similarities and differences of these biological entities. Some of the viruses that cause disease in humans are described in the following table (or click here to see a viral classification flowchart):
Virus Genome Polarity Segments Morphology Enveloped Diseases
Picorna RNA +ss 1 Icosahedral No Polio, Hepatitis A, Colds
Toga RNA +ss 1 Icosahedral Yes Encephalitis, Rubella
Retro RNA +ss 1+1 Icosahedral Yes AIDS
Orthomyxo RNA -ss 6-8 Helical Yes Influenza
Rhabdo RNA -ss 1 Helical Yes Rabies
Paramyxo RNA -ss 1 Helical Yes Parainfluenza, Mumps, Measles
Papova DNA ds 1 Icosahedral No Warts
Adeno DNA ds 1 Icosahedral No Respiratory Infections
Herpes DNA ds 1 Icosahedral Yes HS, VZ, Mononucleosis, Cancer
Pox DNA ds 1 Complex Yes Smallpox
Hepatitis B DNA ds 1 Icosahedral Yes Serum Hepatitis

________________________________________
MULTIPLICATION
The process of viral multiplication involves several discrete steps. First, the virus must recognize and attach to its host cell. Generally, viruses are limited as to the type of host cell in which they can multiply and so recognition is often very specific. Viruses adsorb to their host cell surface via specific antireceptor molecules, often glycoproteins. Adsorption is generally temperature and energy independent. Penetration into the host cell, however, is often energy dependent and may occur by three different mechanisms; 1) translocation of the plasma membrane, 2) pinocytosis into cytoplasmic vacuoles, or 3) fusion of the plasma membrane with the viral envelope. Non-enveloped viruses may enter via translocation or pinocytosis; enveloped viruses typical enter via fusion. Once inside the host cell, uncoating releases the viral genome to be replicated.
________________________________________
VIRAL REPLICATION
The replication scheme employed by the viruses depends upon the type of genome that each contains. The following panels describe each particular type:
________________________________________
Positive-stranded RNA Viruses Negative-stranded RNA Viruses
+RNA (mRNA)
Proteins (replicative) -RNA




RDRP
-RNA +RNA (mRNA)
Proteins (replicative)




+RNA
Proteins (structural) -RNA Proteins (structural)





Progeny
Virus Progeny
Virus
Positive-stranded RNA is essentially equivalent to mRNA and can often be immediately translated into proteins. These replicative enzymes then synthesize a negative-strand copy of the +RNA, which is then copied back into +RNA messages. Translation of these messages produces structural proteins that are used to package progeny +RNA into virions. Negative-stranded RNA must first be converted into +RNA (mRNA) by the RNA-dependent RNA polymerase (RDRP) incorporated in the virion. The mRNA can then be translated into proteins. Replicative enzymes (RDRP) synthesize a negative-strand copy of the +RNA. Structural proteins translated from the mRNA are then used to package progeny -RNA and RDRP into virions.
________________________________________
DNA Viruses Retroviruses
mRNA +RNA
RNA/DNA



Reverse
Transcriptase

Proteins (replicative) dsDNA Proteins (structural) Integration into
Host Genome
dsDNA




dsDNA +RNA
Proteins





Progeny
Virus Progeny
Virus
Most DNA viruses utilize a standard semi-conservative mode of replication. Often, the DNA is immediately transcribed by host proteins to produce mRNAs that encode virus-specific transcription factors. These factors lead to selective transcription of genes involved in DNA synthesis, and genes encoding additional transcription factors. This final set of factors produces mRNAs encoding structural proteins that enclose the newly-synthesized DNA, producing progeny virions. Retroviruses have a unique means of replication. The viral RNA is first reverse transcribed by a viral protein (reverse transcriptase, RT) to produce an RNA/DNA hybrid duplex. RT then removes the RNA strand while synthesizing a new DNA strand to produce dsDNA, which then integrates into the host genome. From this location, progeny RNA molecules and structural proteins are combined to produce new viral particles.
________________________________________
EFFECTS ON HOST CELLS
Viruses can have one of several different effects on their cellular hosts. Abortive infections may result when a virus mistakenly infects a cell that does not permit viral replication. At the other extreme, cytolytic infections lead to cell lysis and release of large numbers of virus. Persistent infections may be productive, latent or transforming. The table to the right outlines some of these effects.
Type of Effect Virus Production Fate of Cell
Abortive No No Effect
Cytolytic Yes Death
Persistent
•Productive Yes Senescence
•Latent No No Effect
•Transforming
•DNA No Immortalization
•RNA Yes Immortalization
________________________________________
HOST DEFENSES AGAINST VIRAL INFECTION
A number of host defenses contribute to the prevention and/or elimination of viral infections. Nonspecific defenses include (prior to infection) anatomical barriers, viral inhibitors in fluids and tissues. Phagocytosis is somewhat variable. After infection, factors such as fever (viral replication is strongly influenced by temperature) and inflammatory processes including edema, leukocyte accumulation, local hyperthermia, reduced oxygen tension and altered cell metabolism can all act to reduce viral replication. Another important anti-viral factor is interferon. This substance is produced by an infected cell. It then reacts with other cells to i) activate an RNA endonuclease causing mRNA degradation or ii) cause phosphorylation of eIF2, essentially turning off cellular protein synthesis. Specific host defenses include antiviral antibody, which may prevent adsorption to target cells and cytotoxic T-lymphocytes, which recognize virally-infected cells and destroy them, reducing viral production.
Papillomaviruses
________________________________________
VIRUS:
• Human Papilloma Virus (HPV)
________________________________________
GENERAL CONCEPTS:
• The Papillomaviruses are small, non-enveloped icosahedral particles containing a circular dsDNA genome.
• Papillomaviruses are a member of the Papovavirus family, which is divided into 2 genera:
o Polyomavirus, which contains a 5200 base pair DNA genome and has been employed as a good molecular model system. There are 2 polyoma viruses that cause disease in humans; BKV and JCV.
o Papillomavirus, which contains an 8000 base pair DNA genome, can induce benign tumors of the head and neck, several varieties of skin warts, and cervical cancers.
• Human papillomaviruses are trophic for epithelial cells of the skin and mucus membranes. They appear to replicate in the cell nucleus and have two modes of replication:
o Stable replication in basal cells and
o Vegetative replication in more differentiated cells that generates progeny virions.
• Human papillomavirus (HPV) is thought to be the most common sexually transmitted disease in the world.
• The CDC estimates that there are approximately 6.2 million new cases of sexually transmitted HPV infections annually and that over 20 million people are already infected.
________________________________________
DISTINCTIVE PROPERTIES:
• Human papillomaviruses (HPVs) produce epithelial tumors of the skin and mucous membranes. More than 100 HPV types have been detected.
• HPV infections may be latent (asymptomatic), subclinical, or clinical. Most HPV infections are latent; clinical infections are usually apparent as warts.
• Of the many types of HPV, types 6 and 11 are generally classified as "low risk" because infection with these types has a low potential for producing cancerous lesions. These two types of HPv are thought to be responsible for 90% of all genital warts cases.
• On the other hand, HPV types 16 and 18 are classified as "high-risk" because they are responsible for most of the lesions that may progress to cancers, particularly those in the anogenital and/or mucosal category. These two types are thought to be responsible for 70% of cervical cancer cases.
• In "low risk" infections, the HPV genome is thought to exist as a separate circular dsDNA molecule, while in "high risk" malignant infections, the genome is incorporated into the host genome. Some of the viral proteins inactivate host cell tumor suppressor proteins, and this may lead to carcinomas.
________________________________________
PATHOGENESIS:
• Clinical HPV infections may be described as:
o Nongenital cutaneous,
o Nongenital mucosal and
o Anogenital.
• Nongenital cutaneous diseases include common warts, plantar warts, flat warts and other skin lesions.
• Nongenital mucosal diseases include resiratory papillomatosis, laryngeal papillomas, conjunctival papillomas, carcinomas and others.
• Anogenital diseases include a variety of warts as well as cancers of the cervix, anus, vagina and penis.
________________________________________
HOST DEFENSES:
• Host defenses against the papillomaviruses are not entirely understood, but a variety of mechanisms probably contribute.
• The efficacy of the new vaccine, however, suggests that humoral responses are protective.
________________________________________
EPIDEMIOLOGY:
• Papillomaviruses are widespread and warts are common in children and young adults.
• Humans are the only host for HPV and infections are generally transmitted by direct contact. However, the virus can survive for extended periods (months) outside the host, and this may provide another means of transmission.
• While there is a strong correlation between HPV infection and certain forms of cancer (e.g. cervical cancer), infection alone does not result in maligancy; rather, additional factors such as radiation, immunosuppression, or tobacco use are involved.
________________________________________
DIAGNOSIS:
• Clinical: Warts of the skin, oral cavity and genital area are generally diagnosed by appearance.
• Laboratory: Microscopy of wart scrapings shows a characteristic histologic appearance. Molecular techniques (nucleic acid hybridization) can be used for other HPV infections.
________________________________________
CONTROL:
• Sanitary: Avoidance of contacts but this is not really practical.
• Immunological: In 2006, the first vaccine developed to prevent cervical cancer and genital warts in women due to HPV was approved. This vaccine, Gardasil®, can be administered to females aged 9-26 years of age through a series of three shots over a six-month period. The vaccine is a quadrivalent, recombinant viral protein suspension that protects against infection by types 6, 11, 16 and 18. The vaccine has been shown to be safe and nearly 100% effective.
• Chemotherapeutic: Warts can be treated by freezing, cauterization, surgery, laser vaporization, etc.
Herpesviruses
________________________________________
VIRUS:
• Herpesviruses: Herpes Simplex (HSV-1, HSV-2), Varicella-Zoster virus (VZV), Cytomegalovirus (CMV), Epstein-Barr virus (EBV)
________________________________________
GENERAL CONCEPTS:
• The Herpesviruses are a large group containing more than 70 members that infect organisms from fungi to humans.
• The Herpesviruses contain a linear DNA genome and are enveloped (bilayered with surface projections derived from the host cell nuclear membrane). Replication occurs in the host cell nucleus.
• Herpesviruses cause acute infections but they are also capable of latency. This can lead to recurrent infections, which are important to the mechanism of host to host transmission. HSV-1, HSV-2 and VZV are termed "neurotropic" while CMV and EBV are "lymphotropic", referring to the cell type in which the latent infection is established.
• Some Herpesviruses have been associated with the production of cancers, providing the best evidence for a viral etiology for these diseases.
• Humans are the natural host for HSV, VZV, CMV and EBV.
________________________________________
DISTINCTIVE PROPERTIES:
• The morphology of each the Herpesviruses is similar but unique.
• At the genome level, HSV-1 and HSV-2 share only about 50% DNA homology; the genome molecular weights, however are similar (96 x 106 daltons). The genome molecular weights of the other viruses are somewhat larger; EBV is about 114 x 106; CMV is about 150 x 106.
• Except for HSV-1 and HSV-2, the proteins surrounding the genome are immunologically unrelated.
• Herpes DNA replication involves three coordinately regulated sequential events. Upon entering the host cell, the virus travels to the nucleus. There, a set of genes termed immediate early are transcribed by cellular transcription factors to produce a set of proteins, some of which are new transcription factors. This groups of factors recognize viral promoters preceding the early genes. Some of the resultant proteins are replication enzymes while others are a third set of transcription factors that synthesize mRNAs encoding the late genes. The late gene products, then, are structural proteins that encapsidate the newly formed DNA genome to produce new progeny virions.
________________________________________
PATHOGENESIS:
• Herpesviruses infect a range of different cell types, causing different disease scenarios. The skin and mucus membranes are common sites of infection for HSV and VZV; CMV and EBV are more internal (EBV infects B lymphocytes).
• The viruses produce intranuclear inclusions and multinucleated giant cells
• With the exception of VZV, primary Herpes infections are often asymptomatic. The process of latency is poorly understood but, except for VZV, asymptomatic shedding is common. Recurrence of acute disease results because of emotional stress, surgery, trauma, cold, fever, immune suppression, etc. Recurrence of HSV is common, frequent and localized.
• More specifically:
o HSV-1 is responsible for a variety of infections. Most commonly, HSV-1 produces the condition known as gingivostomatitis in which oral cavity vesicles or ulcers form. These lesions may recur frequently as "cold sores" (herpes labialis). Another condition produced by HSV-1 is herpetic keratitis, which may be serious if accompanied by conjunctivitis because this can lead to corneal scarring and blindness. Another condition known as "whitlows" appears as lesions on the fingers.
o HSV-2 is commonly referred to as genital herpes. This virus produces lesions on the genitals, urethra and bladder. Recurrence may be frequent. In neonates, infection may be local or disseminated and has about 50% mortality if untreated. HSV-2 may also cause meningitis or encephalitis.
o VZV produces the disease varicella and zoster. Varicella is commonly known as chickenpox. This relatively mild infection in children can be more serious in adults, occasionally progressing to pneumonia. Varicella is characterized by a skin rash appearing first on the head and trunk, and later on the extremities. The skin lesions progress from macules to papules to vesicles to pustules to crusts. These lesions are not prone to scar. Zoster is commonly known as shingles and is a manifestation of varicella infection. Typically occurring in older individuals, the lesions are confined to skin areas innervated by sensory nerves of the dorsal ganglia, primarily in the thoracic and lumbar regions. These nervous tissues are thought to be the sight of latent infection by VZV.
o CMV infections are often asymptomatic but when infection occurs in utero, cytomegalic inclusion disease may result. This condition is characterized by jaundice, hepatosplenomegaly and central nervous system disorder. CMV may also produce a form of mononucleosis characterized by fever, fatigue and atypical lymphocytes. This form of mononucleosis is different than that produced by EBV (below).
o EBV is primarily responsible for infectious mononucleosis, characterized by fever, fatigue, malaise and pharyngitis. EBV latently infects B-cells, creating the potential for recurrence. In addition, a strong association between EBV and Burkitt's lymphoma in Africa and nasopharyngeal carcinoma in the Orient and Africa provides evidence for a viral etiology for some human cancers.
________________________________________
HOST DEFENSES:
• Primary infection by Herpesviruses induces antibody, which is protective, but recurrent infections still occur.
• The cell-mediated immune response is also important (viral antigens on the cell surface allow detection and killing of infected cells).
________________________________________
EPIDEMIOLOGY:
• In lower socioeconomic groups, most individuals are infected subclinically by HSV-1, CMV and EBV. In the higher socioeconomic groups, about half are infected. VZV infects both groups equally. HSV-2 is generally transmitted by sexual contact; the others more commonly by saliva. CMV can be spread transplacentally (0.5-2.5% newborns have CMV in the urine).
________________________________________
DIAGNOSIS:
• Clinical: Generally, the lesions are characteristic and clinical diagnosis is accurate.
• Laboratory: Smears of lesions show intranuclear inclusions.
________________________________________
CONTROL:
• Sanitary: Avoidance of contacts reduces the incidence of disease but virus may be transmitted by asymptomatic individuals.
• Immunological: Vaccines are in development but the problems associated with latency and possible cancers remains. A vaccine for VZV is available. Hyperimmune serum can be used for susceptible or high risk individuals.
• Chemotherapeutic: Acyclovir (a nucleoside analog) is effective against HSV infections. Acyclovir and other analogs (iododeoxyuridine, trifluorothymidine, adenine arabinoside) have been used to treat other Herpes infections but often show little benefit.
Poxviruses
________________________________________
VIRUS:
• Poxviruses (Orthopoxviruses): Smallpox
________________________________________
GENERAL CONCEPTS:
• A passage from T. B. Macaulay's The History of England from the Accession of James II, volume IV, serves to preface this page describing the disease smallpox:
"The smallpox was always present, filling the churchyards with corpses, tormenting with constant fears all whom it had stricken, leaving on those whose lives it spared the hideous traces of its power, turning the babe into a changeling at which the mother shuddered, and making the eyes and cheeks of the betrothed maiden objects of horror to the lover."
• The Poxviruses contain a dsDNA genome, have a complex morphology and replicate in the host cytoplasm.
• Poxviruses cause localized and generalized infections in humans and animals.
• Smallpox was the major human disease agent but it was officially declared eradicated by the World Health Organization (WHO) in 1980.
• Smallpox is transmitted via the respiratory route. In the body, it is spread by a transient viremia to internal sites, and then a second viremia to the skin where the characteristic lesions erupt.
• The lack of an animal reservoir made eradication of human disease possible.
________________________________________
DISTINCTIVE PROPERTIES:
• Poxviruses are large and brick shaped having a complex symmetry (not icosahedral or helical) and small surface tubules.
• The genomic DNA has a molecular weight of 100-200 x 106 daltons.
• The virion contains a DNA-dependent RNA polymerase (DDRP, transcription enzyme) that is required for infection. Other enzymes in the virion complete the uncoating process and initiate early replication, which occurs in acidophilic intracytoplasmic inclusions.
• Poxviruses are antigenically complex but may share common internal antigens.
________________________________________
PATHOGENESIS:
• Smallpox is a systemic infection with a very characteristic rash.
• Transmission of disease occurs via the respiratory tract. Following a 12 day incubation, lesions first appear in the mouth and throat and then on the skin. The rash develops in a synchronous and centrifugal manner with lesions first taking the macular form, then papular, vesicular, pustular and finally as crusts.
• Two forms of smallpox exist: the more serious form, variola major, has about a 50% mortality rate; the less severe form, variola minor results in less than 1% death.
________________________________________
HOST DEFENSES:
• Antiviral antibody is protective. Hyperimmune serum can prevent disease. The cell-mediated response is also important, however, since individuals with hypogammaglobulinemia still recover.
________________________________________
EPIDEMIOLOGY:
• Before eradication, Asia and India were endemic for variola major; South America for variola minor.
________________________________________
DIAGNOSIS:
• Clinical: The characteristic lesions with their centrifugal and synchronous development are diagnostic.
• Laboratory: Electron microscopy of crusts can reveal the intracytoplasmic inclusions.
________________________________________
CONTROL:
• Sanitary: Quarantine of diseased individuals.
• Immunological: Vaccination of all individuals.
• Chemotherapeutic: None.
Picornaviruses
________________________________________
VIRUS:
• Picornaviruses: Enteroviruses, Rhinoviruses
________________________________________
GENERAL CONCEPTS:
• Picornaviruses are small, nonenveloped viruses containing a single positive strand RNA genome. They possess an icosahedral symmetry.
• Picornaviruses are divided into two groups; the Enteroviruses (Poliovirus, Coxsackievirus and Echovirus) and the Rhinoviruses. There are about 63 serotypes of Enterovirus and more than 100 serotypes of Rhinovirus.
• Picornaviruses commonly produce subclinical infections; acute disease may range from minor illness to paralytic disease.
• The Enteroviruses enter via the intestinal tract and attach to receptors on intestinal epithelia. During this alimentary phase, the virus replicates in cytoplasm. They then spread into the lymphatic circulation (lymphatic phase) and then to the bloodstream (viremic phase). The viremic phase generally marks the end of the infection but an occasional neurologic phase can lead to more severe and permanent problems.
________________________________________
DISTINCTIVE PROPERTIES:
• Polioviruses: Poliovirus types 1, 2 and 3 are recognized. Their genome contains a 7000 base positive strand of RNA. These viruses adsorb only to intestinal epithelial cells and motor neuron cells of the central nervous system.
• Coxsackie: These viruses are divided into two groups; A and B. There are 23 serotypes of A, 6 serotypes of B. In humans, Coxsackieviruses produce respiratory disease, herpangitis, "hand, foot and mouth" disease, febrile rashes, pleurodynia, pericarditis, myocarditis, aseptic meningitis and paralytic disease.
• Echoviruses: An acronym for "Enteric Cytopathogenic Human Orphan viruses, the Echoviruses contain 31 serotypes and produce respiratory disease, febrile illness (with or without a rash), aseptic meningitis and paralytic disease.
• Rhinoviruses: This group of viruses are sensitive to acid pH and their optimal growth occurs at 33°. There are over 100 serotypes of Rhinoviruses and they produce the common cold.
________________________________________
PATHOGENESIS:
• Polioviruses: A number of syndromes can be observed, most (90-95%) are subclinical. About 4-8% present with a mild fever, sore throat and headache. Nonparalytic polio occurs in about 1% of cases while paralytic disease affects only 0.1%. Of the paralytic conditions, about 5-10% affect the cranial nerve or medulla while the others are spinal and limited to muscle weakness rather than complete paralysis.
• Coxsackievirus, Echovirus: Infection by Coxsackie- or Echoviruses resembles that produced by Poliovirus. A variety of syndromes are possible, ranging from trivial to severe.
• Rhinovirus: Infection results via the nasopharynx by direct contact. Viral replication leads to inflammation and edema, symptoms of the common cold. Inapparent infection is common.
________________________________________
HOST DEFENSES:
• Enteroviruses: Interferon is effective against these viruses. More specifically, IgA antibodies in the intestine and saliva are protective.
• Rhino: Susceptibility to the Rhinoviruses is dependent on prior exposure. Antibody of the IgA and IgG classes is important. Non-specific defenses including interferon, gastric acidity and temperature may play a major role in controlling infection.
________________________________________
EPIDEMIOLOGY:
• Enteroviruses: Found worldwide, the enteroviruses are spread via the fecal-oral route. Most illnesses occur in the summer and fall. The virus may be carried in the throat for a week and shed in the feces for several weeks.
• Rhinoviruses: These viruses are spread from person to person, usually by direct contact. Inapparent infections occur in about half. On average, most persons suffer with 2-4 colds per year during the fall and spring months and these represent different serotypes of the virus.
________________________________________
DIAGNOSIS:
• Clinical: Diagnosis of enteroviral infections is usually not possible based on clinical presentation. However, some symptoms (pleurodynia, myocarditis) or conditions (aseptic meningitis) are suggestive. Diagnosis of rhinoviral infections, in contrast, is usually based on clinical presentation.
• Laboratory: Recovery of Enterovirus from the throat or feces is diagnostic. Recovery of Rhinoviruses is simply not practical.
________________________________________
CONTROL:
• Sanitary: Avoidance of contacts is the best means for preventing disease.
• Immunological: Vaccines for Polio have been available for more than 40 years. The Salk vaccine is a trivalent (types 1, 2 and 3), formalin inactivated suspension that is given by injection (parenteral). The Sabin vaccine is an attenuated, live, trivalent, oral suspension that produces intestinal IgA but has rare vaccine-associated paralysis. Vaccines against the other picornaviruses are not practical (too many serotypes).
• Chemotherapeutic: Supportive care is best.
Hepatitis Viruses
________________________________________
VIRUS:
• Hepatitis A, B, C
________________________________________
GENERAL CONCEPTS:
• Type A hepatitis virus (HAV) is a member of the enterovirus group of Picornaviruses; Hepatitis B virus (HBV) is distinctly different and contains a DNA genome; Hepatitis C virus (HCV, also referred to as non-A, non-B or NANBH) is not well characterized but appears to contain an RNA genome and may belong in the Flavivirus group.
• The disease caused by these agents is hepatitis, a generalized infection with inflammation and necrosis of the liver. The course of hepatitis may range from inapparent disease to chronic liver disease.
________________________________________
DISTINCTIVE PROPERTIES:
• Hepatitis A: As a member of the Enterovirus group, HAV contains a linear, positive strand RNA genome. It is also known as Enterovirus type 72 and is resistant to heat and acid.
• Hepatitis B: HBV contains a circular, partly double-stranded DNA genome 3200 nucleotides in length. Interestingly, a 600-2100 single-stranded region is contained in the DNA molecule. The particle contains a DNA-dependent DNA polymerase and a reverse transcriptase and replicates via an RNA intermediate. HBV possesses several antigens; the "Australian antigen" is associated with the surface (HBsAg), the "core antigen" (HBcAg) is internal and the "e antigen" (HBeAg) is part of the same capsid polypeptide as the HBcAg. All of these antigens elicit specific antibodies.
• Hepatitis C: HCV (or NANBH) contains a positive stranded RNA genome and is related to the Flaviviruses.
________________________________________
PATHOGENESIS:
• Hepatitis A: Clinical presentation of HAV infection varies from subclinical and mild in children to jaundice in adults. The virus usually enters by intestinal infection (fecal-oral transmission), spreads via the blood to the liver, which is its target organ. HAV is detectible in the feces during the incubation period (average 4 weeks), preceding a rise in serum levels of aminotransferase enzymes and the occurrence of pathologic changes in the liver. Most disease resolves within two weeks. Chronicity or fulminant hepatitis is rare.
• Hepatitis B: HBV, in contrast, may produce a persistent carrier state in addition to liver damage. Infection early in life often produces a carrier state, but only about 5-10% of cases if infection occurs later. Most disease is acquired via the parenteral route (blood transfusions. There are generally two patterns of HBV-associated disease:
1. Chronic persistent: Infections are generally asymptomatic with a mild elevation of serum alanine transaminase (ALT) and little liver fibrosis.
2. Chronic active: Infections produce jaundice with elevated ALT levels, liver damage and cirrhosis. Liver failure may predispose those affected to cancer.
• Hepatitis C: HCV may also produce a persistent carrier state, a higher level of chronic disease and cirrhosis. At least 50% of HCV infections result from blood transfusion.
• The following table outlines some of the differences between the Hepatitis viruses:
Hepatitis A Hepatitis B Hepatitis C
Genome +RNA DNA +RNA
Onset Abrupt Insidious Insidious
Transmission Fecal-Oral Parenteral Parenteral
Incubation (days) 15-40 60-180 28-112
Asymptomatic infection usual common common
Carrier State no yes yes
Chronicity 0% 10% 30-60%
Sequelae no cirrhosis cirrhosis
________________________________________
HOST DEFENSES:
• Hepatitis A: Antibody usually develops late in the infection; IgM after a week, IgG later. These help to clear virus from the body via complement and antibody-dependent cell-mediated cytotoxicity, which may account for some of the liver damage.
• Hepatitis B: Antibody and the cell-mediated responses are induced but do not seem to protect against infection and probably cause autoimmune responses (liver damage may relate to the host immune response). The possibility of immune complexes with surface antigen can also lead to immune complex disease.
• Hepatitis C: Little is known about host defenses against this agent but immune serum does not seem to be an effective prophylactic remedy. Probably a combination of humoral and cell-mediated defenses are important.
________________________________________
EPIDEMIOLOGY:
• Infectious hepatitis (HAV) is endemic throughout the world, but the incidence is difficult to estimate because many cases are subclinical. HAV has a low mortality but patients may be incapacitated for several weeks. It is spread via the fecal-oral route, person to person and reflects conditions of poor sanitation and overcrowding. The virus may be shed in the feces but there is no carrier state or progression to chronic liver disease.
• Serum hepatitis (HBV, HCV) is generally passed via blood, unsterile syringes, transfusions, etc. A carrier state can occur; in North America and Europe, the rate is about 0.1%. The rate increases to about 25-30% in Africa, Asia and the Pacific.
________________________________________
DIAGNOSIS:
• Clinical: Clinical diagnosis depends on the symptomology, which may include fever, malaise, headache, dark-colored urine and jaundice.
• Laboratory: Serology is the best method for determining infection.
________________________________________
CONTROL:
• Sanitary: HAV infection may be prevented by ensuring a safe water supply; HBV and HCV may be prevented by testing the blood supply.
• Immunological: Human immunoglobulin prevents or attenuates HAV and vaccines are under study. A vaccine for HBV is available and effective. A vaccine against HCV is under study.
• Chemotherapeutic: None.
Togaviruses
________________________________________
VIRUS:
• Togaviruses: Alphavirus, Flavivirus
________________________________________
GENERAL CONCEPTS:
• The Togaviruses are divided into two groups; Alphaviruses and Flaviviruses.
• Togaviruses are enveloped and contain a positive stranded RNA genome. This RNA is 5'-capped and 3'-polyadenylated.
• This group of viruses are called "arboviruses" because their life cycle involves alternating between vertebrates and arthropod vectors, mostly mosquitoes.
• The Togavirus' natural cycle usually involves birds and mammals and rarely humans with the exception of those responsible for yellow fever and dengue fever.
• Overall, there are approximately 25 types that are pathogenic for humans.
________________________________________
DISTINCTIVE PROPERTIES:
• Alphaviruses: Members of this group contain a nucleoprotein capsid surrounded by a lipid bilayer envelope that is derived from the host cell membrane plus two 50 kD glycoproteins. Their RNA is 4 x 106 daltons and the capsid is composed of a single 30 kD capsid protein. Antibodies specific for the glycoproteins are neutralizing. The viruses enter host cells by pinocytosis and they replicate in the cytoplasm. Translation of the genomic RNA gives a large polyprotein that is cleaved to yield an RNA polymerase and the structural proteins. Once assembled, the progeny virions bud from the host cell, picking up their envelope.
• Flaviviruses: This group is very similar to the Alphavirus group with a few exceptions. First, the genomic RNA is not polyadenylated until it enters the host cell. Second, the single capsid protein is smaller (14 kD). Third, the completed virion forms in the cytoplasm in association with the endoplasmic reticulum instead of budding from the cell.
________________________________________
PATHOGENESIS:
• Alphaviruses: There are three agents that cause disease in humans;
1. Chikungunya: This disease is transmitted by a mosquito, resulting in viremia that presents as an acute, febrile illness with malaise, a rash and arthritis.
2. Eastern and Western Equine Encephalitis (EEE, WEE): Following the bite of an infected mosquito, the resultant viremia is often asymptomatic. However, if the virus invades neural tissue, encephalitis may result. This condition is marked by high fever, delirium, coma and possibly death due to convulsions and paralysis.
3. Venezuelan Equine Encephalitis (VEE): Similar to EEE and WEE, VEE has more systemic manifestations with less neural involvement.
• Flaviviruses: There are four agents that cause disease in humans;
1. St. Louis Encephalitis (SLE): Transmitted by mosquitos, a viremia occurs but in most cases no disease results. In some, however, central nervous system involvement gives inflammation and neuronal degeneration, clinically presenting with fever, headache, convulsions, coma and death.
2. West Nile Encephalitis (WNE): WNE is very similar to SLE and it is also transmitted by mosquitos. In most cases no disease results. Mild, flu-like cases may be referred to as "West Nile fever". More severe cases of "West Nile encephalitis" or "West Nile meningitis" indicate central nervous system involvement that can lead to death (see http://www.cdc.gov/od/oc/media/wncount.htm for a current US update).
3. Yellow fever: This is a severe systemic disease (unlike SLE). The virus replicates in reticuloendothelial (RE) cells in many organs, producing liver damage and intestinal hemorrhages. Typically, phase 1 presents with a fever, headache, nausea and vomiting, while phase 2 shows toxicity, jaundice, shock and death.
4. Dengue fever and Dengue Hemorrhagic fever: The virus producing these diseases initially replicates in the skin at the site of the mosquito bite. Next, lymph nodes and the RE system become involved and viremia results. Fever and rash lasting 3-9 days defines the symptomology. Dengue fever is usually self-limiting but Dengue Hemorrhagic fever often involves additional processes (probably immunopathologic) that produce extreme vascular permeability, shock and death.
________________________________________
HOST DEFENSES:
• Togaviruses induce interferon and are susceptible to its effects. Infections usually resolve once antiviral antibody is produced. The role of the cell-mediated response is less well known but is probably important.
________________________________________
EPIDEMIOLOGY:
• Alphaviruses are generally associated with birds and horses and their mosquitoes. Flaviviruses are generally associated with birds and bird-feeding mosquitoes for SLE and WNE. Yellow fever and Dengue fever are human in origin.
________________________________________
DIAGNOSIS:
• Clinical: The epidemiology of the infection along with clinical suspicion and laboratory results are diagnostic.
• Laboratory: One can isolate the virus from the blood during the viremic phase.
________________________________________
CONTROL:
• Sanitary: Surveillance and vector control are excellent means for disease prevention.
• Immunological: A live vaccine against Yellow fever and killed vaccines against EEE and WEE are available for those at high risk.
• Chemotherapeutic: None available.
Rubellavirus
________________________________________
VIRUS:
• Rubellavirus
________________________________________
GENERAL CONCEPTS:
• Rubella virus is a member of the Togaviridae family and produces the disease commonly known as German measles.
• Prior to the introduction of the vaccine, rubella was a common, mild disease with a rash. It was found worldwide and mostly affected children. It was uncommon in adults but had severe manifestations when affecting a pregnant woman during the first trimester because of the potential for birth defects.
________________________________________
DISTINCTIVE PROPERTIES:
• The structure of Rubellavirus is similar to other Togaviruses.
• Humans are the only known reservoir for disease.
________________________________________
PATHOGENESIS:
• Transmission of the virus is via the respiratory route.
• Initial multiplication occurs in the respiratory tract. One week after infection, viremia occurs and this is later followed by the skin rash. The highest concentration of virus in the respiratory tract begins 3 days prior to the rash and lasts until 3 days after.
• If the virus infects a woman during the first 3-4 months of pregnancy, it may infect the placenta or fetus, multiply in any fetal organ and cause damage or death of certain cell types. This congenital rubella syndrome affects the eyes, heart and brain. The virus may persist in the child's tissues for 3-4 years and be shed for up to a year after birth.
________________________________________
HOST DEFENSES:
• Humoral defenses soon after the viremia help to clear the virus from the blood and prevent continued spread. Cell-mediated defenses clear the virus from tissues. The immune responses generated by natural infections provide life-long immunity.
________________________________________
EPIDEMIOLOGY:
• Before the vaccine, sporadic outbreaks and epidemics occurred every 6-9 years following a seasonal pattern (late winter, early spring, typical for respiratory disease).
• An epidemic in 1964-65 in the US produced 12.5 million cases of disease and resulted in 20,000 cases of congenital rubella syndrome.
________________________________________
DIAGNOSIS:
• Clinical: Following a 14-20 day incubation period (the "prodrome"), a rash develops on the face and neck and then the trunk and extremities. Lymph nodes become enlarged about a week before the rash and persist for 1-2 weeks after. Congenital rubella symptoms vary depending on the organs affected but typically the eyes, ears, heart and brain are involved. Confirmation of disease is critical in pregnant women.
• Laboratory: Illustrating a 4-fold increase in serum titer against Rubellavirus is diagnostic.
________________________________________
CONTROL:
• Sanitary: Avoidance of contacts.
• Immunological: A live vaccine is given in combination with measles and mumps (MMR) to all children over 15 months and selective young and adult women NOT pregnant or 3 months prior to becoming pregnant. Since its introduction in the late 1960's, the vaccine has greatly reduced the incidence of disease.
• Chemotherapeutic: None available.
Rhabdoviruses
________________________________________
VIRUS:
• Rhabdovirus: Rabies
________________________________________
GENERAL CONCEPTS:
• The Rhabdoviruses are uniquely bullet-shaped. They contain a negative stranded RNA genome and are very stable to drying.
• This group of viruses has a broad host range but there is only one that affects humans.
• The viruses are generally introduced through a bite wound.
________________________________________
DISTINCTIVE PROPERTIES:
• Rhabdoviruses possess a lipid envelope displaying a surface glycoprotein. The RNA-binding nucleocapsid protein surrounds the RNA genome. The virion itself contains an RNA-dependent RNA polymerase (RDRP).
• During replication, the virus first makes short positive strand RNAs (mRNAs) which are translated to produce proteins. Later, a full length positive RNA is transcribed and this is used to produce the full length negative strand RNA that is packaged into progeny.
________________________________________
PATHOGENESIS:
• Rhabdoviruses generally enter via a bite or a wound infected with saliva.
• Initially, the virus replicates at the site and then infects central nervous system (CNS) tissue. Following a variable (6 days up to 1 year, average 30-70 days) incubation period, the virus spreads rapidly via the nerves. CNS damage produces the symptoms of disease.
• Neurons accumulate ribonucleoprotein as intracytoplasmic inclusions (Negri bodies). Infection of the thalamus, hypothalamus or pons may occur.
• Symptoms include fever, excitation, dilation of the pupils, excessive lacrimation, salivation, anxiety, hydrophobia due to spasms of the throat muscles and, eventually, death.
________________________________________
HOST DEFENSES:
• Many factors come into play. Interferon, humoral factors and the cell-mediated response are all important.
• Only about half of those bitten by an infected animal actually acquire disease; however, once symptomatic, death is certain.
________________________________________
EPIDEMIOLOGY:
• Normally, rabies can be found in domestic and wild animals (dogs, cats, cattle, bats, foxes, skunks, raccoons). In the US, 8% of animals screened tested positive. Approximately 10,000 humans become infected per year worldwide.
________________________________________
DIAGNOSIS:
• Clinical: The patient's history along with symptoms of encephalitis are suggestive.
• Laboratory: Direct immunofluorescence is a highly specific and sensitive method. Also, confirmation in the animal (if available) is important.
________________________________________
CONTROL:
• Sanitary: Rigorous cleansing of a bite wound to reduce the number of viral particles can help to prevent disease. Use of the vaccine for all dogs (a modified live vaccine) and cats (a dead virus suspension) is important in preventing spread to humans.
• Immunological: Both active and passive vaccination may be used to prevent human disease. The active vaccines are inactivated virus grown in human diploid cell cultures (HDCV) while the passive vaccine uses immunoglobulin.
• Chemotherapeutic: None available.
Paramyxoviruses
________________________________________
VIRUS:
• Paramyxoviruses: Parainfluenza, Mumps, Measles, Respiratory Syncytial Virus
________________________________________
GENERAL CONCEPTS:
• The Paramyxoviruses are an important cause of respiratory disease in children. Illnesses include croup, bronchiolitis and pneumonitis.
• This group of viruses share similar features; they possess a double layered envelope with spikes, have a helical symmetry and contain a negative stranded RNA genome. An RNA-dependent RNA polymerase (RDRP) is contained within the virus particle.
• Paramyxoviruses replicate in the cytoplasm and are released by budding.
• Virus-specific antigens include those associated with the envelope and those contained within the nucleocapsid.
• The host range for the Paramyxoviruses includes humans and monkeys.
• These viruses produce multinucleated giant cells (syncytia) by production of a cell fusing factor, and then cause host cell lysis.
________________________________________
DISTINCTIVE PROPERTIES:
• Parainfluenza: About 30-40% of acute respiratory infections in infants and children can be attributed to the Parainfluenza virus. Disease ranges from mild and cold-like to life-threatening (croup, bronchiolitis and pneumonia). Parainfluenza viruses are the most common cause of croup and there are 4 serotypes, numbered 1-4.
• Mumps: A common acute disease of children, the mumps virus produces inflammation of salivary glands leading to obvious enlargement. Some severe manifestations can result from infection but there is only one serotype.
• Measles: Another acute childhood disease, measles virus commonly causes a fever with a rash, occasionally producing conjunctivitis and, sometimes, pneumonia. More serious complications include encephalitis and subacute sclerosing panencephalitis (SSPE). Only one serotype exists and disease is limited to humans and monkeys.
• RSV: Respiratory Syncytial virus is a major cause of bronchiolitis and pneumonia in infants under 1 year. Reinfection in adults usually involves the upper respiratory tract. The viruses produce a characteristic syncytia formation, hence the name. There is only one serotype that affects humans.
________________________________________
PATHOGENESIS:
• Parainfluenza: These viruses generally produce local infections in the upper and lower respiratory tract. The viruses implant in ciliated epithelia of respiratory tract (nose and throat). The virus can be shed over 3-16 days and the main pathologic response is inflammation. The most important (i.e. serious) diseases are croup, bronchiolitis and pneumonia. The severe diseases occur most often with types 1 and 2.
• Mumps: Mumps is a systemic infection spread by viremia. The major targets include glandular and nervous tissue. The virus enters via the pharynx or conjunctiva, there is local multiplication followed by viremia. A secondary viremia disseminates the virus to salivary glands, testes, ovaries, pancreas and the brain. The incubation period is 18-21 days and disease is asymptomatic in about 35% of those infected. The most characteristic response is painful enlargement of the parotid glands. Severe cases may progress to include epididymoorchitis in prepubescent males, which can cause atrophy of the testes, but rarely sterility. Mumps can also produce a transient high frequency deafness.
• Measles: Measles is also a systemic infection spread by viremia. Acute disease affects the lymphatic and respiratory systems and occasionally the brain. Persistence of the virus can produce SSPE. Measles virus generally enters via the oropharynx. Local multiplication precedes the viremia and spread to the reticuloendothelial system. Extensive multiplication produces a secondary viremia 5-7 days later with spread to the mucosa of the respiratory, urogenital or gastrointestinal tracts or the central nervous system. Clinically, there are respiratory symptoms during the prodromal stage (malaise, fever, cough) and a rash during the eruptive phase (Koplik's spots, rash on head then body). Complications are mainly bacterial superinfection but other rare complications (e.g. SSPE) can occur.
• RSV: The RS virus initiates a local infection in the upper or lower respiratory tract but illness varies with age and previous experience. The virus infects ciliated epithelia of the nose, eye and mouth and remains generally confined. Virus spreads extracellularly and by fusion. Severe disease may present as bronchiolitis, pneumonia or croup, particularly in infants. Some evidence suggests that there are possible immunopathologic mechanisms involved.
________________________________________
HOST DEFENSES:
• Parainfluenza: Interferon and neutralizing IgA are important but IgA is often short lived.
• Mumps: Interferon limits viral spread while IgM and IgG are protective. Long lasting IgG affords life-long immunity.
• Measles: Interferon affects viral spread but the cell-mediated response is associated with recovery (disease is more severe in persons with a T-cell immunodeficiency). Life long persistence of IgG affords protection.
• RSV: Interferon, age, immune competence and neutralizing IgA (short lived) all contribute to host defense. Breast milk may contain neutralizing factors as well.
________________________________________
EPIDEMIOLOGY:
• Parainfluenza: Found worldwide, Parainfluenza affects mostly children. The virus is endemic in some areas and epidemic at times. The source for infections is the respiratory tract of humans. Disease is contagious for 3-16 days, transmission is person to person or by droplets. At 5 years of age, about 90-100% have antibodies against type 3, about 74% for type 1 and about 58% for type 2.
• Mumps: Also found worldwide, mumps is endemic with peaks of acute disease appearing from January through May. Epidemics occur every 2-7 years and humans are the only host. Transmission is via salivary secretions and disease is contagious just before and after the symptoms. It is less contagious than Parainfluenza so intimate contact is usually required. In unvaccinated populations, about 45% of people are infected by age 5 and about 95% by 15 years of age. About 35% of those infected are subclinical, however.
• Measles: Endemic worldwide, epidemics occur every 2-4 years in developed countries when the population susceptibility reaches about 40%. The source of the virus is human (respiratory) secretions. Disease generally affects those in the 4-7 year age group. The incidence of SSPE is about 6-20 per million.
• RSV: Also endemic worldwide, epidemics occur yearly, usually between January and March. Reinfection is common. The source for infection is the human respiratory tract and disease is usually contagious about 4-5 days after the symptoms. Most infants experience upper respiratory infections but between 15% and 50% experience lower (more serious) respiratory infections.
________________________________________
DIAGNOSIS:
• Parainfluenza: Clinically not possible. Isolation of virus or serotests are required.
• Mumps: Typical symptoms on clinical presentation make diagnosis relatively easy.
• Measles: Typical symptoms on clinical presentation make diagnosis relatively easy.
• RSV: Strongly suspect in infants with lower respiratory tract infection.
________________________________________
CONTROL:
• Parainfluenza: Supportive care and isolation.
• Mumps: Immunize at 15 months (measles, mumps, rubella live vaccine).
• Measles: Immunize at 15 months (measles, mumps, rubella live vaccine).
• RSV: Supportive care and isolation.
Orthomyxoviruses
________________________________________
VIRUS:
• Orthomyxoviruses: Influenza
________________________________________
GENERAL CONCEPTS:
• The Orthomyxoviruses are composed of one genus and 3 types; A, B and C.
• The disease caused by these viruses, influenza, is an acute respiratory disease with prominent systemic symptoms despite the fact that the infection rarely extends beyond the respiratory tract mucosa.
• Type A is responsible for periodic worldwide epidemics; types A and B cause regional epidemics during the winter.
• The recurring pattern of the influenza viruses is due to their ability to exhibit variation in surface antigens. Two phenomenon account for this variability:
1. Antigenic drift is due to mutations in the RNA that leads to changes in the antigenic character of the H and N molecules. Antigenic drift involves subtle changes that may cause epidemics but not pandemics.
2. Antigenic shift is due to rearrangement of different segments of the viral genome that produces major changes in the antigenic character of the H and N molecules. Antigenic shift usually occurs in animal hosts and is responsible for producing both epidemics and pandemics.
• Influenza epidemics have been documented since 1173 AD; a pandemic in 1918 was responsible for 20 million deaths worldwide.
• The table outlines some of the differences between the three types of influenza virus:
A B C
Severity ++++ ++ +
Animal Reservoir Yes No No
Population Spread Pandemic, epidemic Epidemic Sporadic
Antigenic Changes shift, drift drift drift
________________________________________
DISTINCTIVE PROPERTIES:
• Orthomyxoviruses contain a single stranded, negative RNA genome divided into 8 segments.
• The viruses have a lipid bilayer envelope with surface glycoproteins (hemagglutinin and neuraminidase)
• There are 3 viral antigens of importance: the nucleoprotein antigen that determines the virus type (A, B or C), the hemagglutinin (H) antigen, and the neuraminidase (N) antigen. The H and N antigens are variable. There are about 13 different H antigens and 9 different N antigens found in birds. This provides a total of 117 (13 x 9) possible combinations, 71 of which have been observed. There are only about 3 combinations that affect humans, however.
• Viral attachment is mediated by the hemagglutinin. The virus enters host cells by pinocytosis and uncoating occurs by fusion of the viral envelope with the membrane of the vacuole. The RNA is capped and replication proceeds in the nucleus. The progeny are released by budding and cell death ensues.
• The segmented genome of the influenza virus allows rearrangements to occur in simultaneously infected cells. This accounts for the periodic appearance of new variants. The new variants are responsible for the process of antigenic shift.
________________________________________
PATHOGENESIS:
• Transmission of disease is airborne. The viruses deposit in lower respiratory tract, their primary site is the tracheobronchial mucosa.
• Neuraminidase produces liquefaction, which leads to viral spread.
• Respiratory symptoms include a cough, sore throat and nasal discharge. There is no viremia but systemic symptoms such as fever and muscle aches do occur.
• The extent of respiratory tract cell destruction is a probable factor in the disease.
• Severe complications include pneumonia (viral or bacterial).
________________________________________
HOST DEFENSES:
• Interferon is one non-specific defense but antibody is the prime defense. IgA is produced in the upper respiratory tract and IgG is produced in the lower respiratory tract. These antibodies are directed primarily against the hemagglutinin and neuraminidase. Cell-mediated (i.e. CTL) defenses are important in recovery.
________________________________________
EPIDEMIOLOGY:
• Influenza displays a typical pattern: school children bring the disease home and infect siblings and parents.
• Epidemics usually last from 3-6 weeks and the highest attack rates are for 5-19 year olds (generally Type A).
• Every winter, the recurring population susceptibility is due to changes in the surface antigens; major changes are referred to as antigenic shift. These changes are responsible for pandemics and they result from rearrangement of the viral genome segments. Minor changes are called antigenic drift and these are responsible for many epidemics. They result from mutation in the viral RNA. Antigenic drift occurs every 2-3 years while antigenic shift only occurs every 10 years.
• The table illustrates this pattern for pandemics due to antigenic shift. Notice the 10 year span between pandemics and the 30 year (generational) span between recurrence of the same antigenic character:
Year Antigenic character
1947 H1N1
1957 H2N2
1968 H3N2
1978 H1N1
________________________________________
DIAGNOSIS:
• Clinical: Influenza usually displays a sudden onset with fever, malaise, headache, muscle aches, sore throat, cough and rhinorrhea, generally in winter. The presence of disease in the community (i.e. epidemiology) is helpful in diagnosis.
• Laboratory: Serology on the patient's serum can be performed or the viruses may be isolated in chick embryos if necessary.
________________________________________
CONTROL:
• Sanitary: Avoid contacts.
• Immunological: Every year, inactivated vaccines are prepared using the most likely types and antigenic characters expected for any particular season. These vaccines are given parenterally in the fall, primarily for those at risk (older persons or those with chronic disease). Protection against disease is variable (50-90%).
• Chemotherapeutic: The drug amantadine HCl can be used for influenza type A but not type B in patients with other disease conditions. Generally, however, pain relievers (e.g. acetaminophen) are more generally employed.
Retroviruses
________________________________________
VIRUS:
• Retroviruses: HTLV, HIV
________________________________________
GENERAL CONCEPTS:
• The Retroviruses are composed of three subfamilies, two that infect humans. They are:
o Oncornaviruses: HTLV 1, HTLV 2, HTLV 5
o Lentiviruses: HIV 1, HIV 2
• The HTLV or Human T Cell Lymphotrophic Viruses are divided into three types based on the type of diseases they produce:
o HTLV-I produces cutaneous T-cell lymphomas,
o HTLV-II produces hairy T-cell leukemias,
o HTLV-V produces T-cell lymphomas and leukemias.
• The HIV or Human Immunodeficiency Viruses are divided into two types based on the type of diseases they produce:
o HIV-1 produces Acquired Immunodeficiency Syndrome (AIDS),
o HIV-2 produces a related disease syndrome, restricted to W. Africa.
• The Retroviruses are RNA viruses and their name is derived from the viral enzyme Reverse Transcriptase, which makes circular DNA from linear RNA. The viral DNA has the capacity to integrate into the host cell genome.
• In general, the viruses are not cytopathic. The Oncornaviruses generally transform cells, causing leukemias, sarcomas and carcinomas. The Lentiviruses attack T-cells, all but abolishing the host immune response.
________________________________________
DISTINCTIVE PROPERTIES:
• Most of the information about Retroviruses comes from studies of HIV.
• HIV is enveloped and displays a viral glycoprotein (gp120) that recognizes and binds to the CD4 receptor on T-helper cells. This glycoprotein is antigenically variable.
• The viral genome is composed of 2 positive strands of RNA that are 5'-capped and 3'-polyadenylated. A cellularly derived tRNA binds the 3'-end and serves as a primer for DNA synthesis.
• The order of the genes encoded by the viral RNA is as follows:
5'- LTR gag pol env tax/rex LTR -3'
• These genes correspond to the following viral products or functions:
o LTR: The Long Terminal Repeats are used for integration of the virus into the host genome and also contain promoter and enhancer sequences.
o gag: The Group-specific Antigen corresponds to the core and capsid proteins.
o pol: This gene encodes the reverse transcriptase, which actually has several functions.
o env: This Envelope gene encodes the gp120 glycoprotein.
o tax/rex: This region encodes factors involved in transactivation and other regulatory functions.
• Upon entering a host cell, translation of the RNA produces a polyprotein that is cleaved to give the individual components. Cleavage of the polyprotein requires a specific protease that is the target for new anti-HIV drugs.
• During replication, the reverse transcriptase (RT) uses the tRNA as a primer for DNA synthesis, creating a DNA/RNA hybrid duplex. Next, RT degrades the RNA strand using its RNAseH function and synthesizes a new DNA strand to produce a DNA duplex that circularizes. The circular DNA then integrates into the host genome, where it remains to be transcribed to produce new progeny RNA molecules. Following replication, the viruses escape by budding.
________________________________________
PATHOGENESIS:
• HIV is transmitted via body fluids. Blood is the best and persons can acquire through sex, parenteral drug usage or transfusions.
• The virus has a specific trophism for CD4+ (T-helper) cells. Following infection, it may remain latent for many years. Eventually, the infected T-cells lose their ability to function, resulting in a loss of both humoral and cell-mediated immunity.
• Patients generally die of secondary manifestations including Kaposi's sarcoma or opportunistic infections.
• Several specific syndromes are associated with infection by HIV. These include:
1. Lymphadenopathy and fever has an insidious onset and is characterized by weight loss and malaise.
2. Opportunistic infections: Many diseases that rarely affect normal individuals may occur in persons infected with HIV. These include: Pneumocystis carinii pneumonia, Candidiasis, severe Herpesvirus infections and frequent diarrhea caused by Salmonella, Shigella and Campylobacter.
3. Malignancies: Kaposi's sarcoma is a rare type of cancer that occurs in HIV-infected persons. These normally benign lesions become malignant and disseminate to involve visceral organs.
4. Wasting: Also known as "slim disease", this syndrome is common in Africa.
5. AIDS dementia: This condition mimics Alzheimer's disease and may involve HIV infection of the brain.
________________________________________
HOST DEFENSES:
• Host defenses are essentially eliminated, but there is some evidence suggesting a possible genetic component in some individuals that causes suppression of the virus.
• In addition, the ability of the virus to remain latent and frequent antigenic changes in the gp120 protein (antigenic drift) diminish host defense capabilities.
________________________________________
EPIDEMIOLOGY:
• The first case of AIDS was described on June 5, 1981. Now, there are estimated to be more than 30 million total HIV infections worldwide. About half of these are in sub-Saharan Africa and about one-tenth of them occurred in 1996 alone, giving daily infection rates of about 8500 per day, approximately 1000 in children.
• Current global trends include the following:
o The majority of new adult HIV infections involves persons 15-24 years old,
o Between 75% and 85% of HIV-positive adults have been infected through unprotected sexual intercourse, with heterosexual (male-female) intercourse accounting for more than 70% and male-to-male intercourse accounting for approximately 5-10%,
o Transfusion of HIV-infected blood and the sharing of HIV-infected injection equipment by drug users account for 3-5% and 5-10% of all global adult infections, respectively.
• In the US, approximately 750,000 persons are infected with HIV and about 580,000 have AIDS. The following graphic illustrates the distribution of AIDS throughout the US.


• While the focus on HIV/AIDS has always been homosexual men, a much larger percentage of infections now occur in women and children. Men who have sex with men continue to provide about 40-50% of new HIV infections, while about 20% of new infections involve women and about 10% involve children. Many (92%) of these children are infected by vertical transmission (in utero or during birth).
• In terms of race/ethnicity, overall AIDS infections in the US predominate in non-Hispanic whites, but the number of new cases in blacks has now exceeded those in whites. However, dividing the percent of cases by the percent of the population represented by each group reveals that a disproportionate number of cases occur in African-Americans and Hispanics. The following table illustrates these values:
Race or Ethnicity # of AIDS Cases % of AIDS Cases % of Population Cases/Population
Caucasian, not Hispanic 268,856 46.2 72.8 0.6
African American, not Hispanic 203,189 34.9 12.1 2.9
Hispanic 103,023 17.7 10.9 1.6
Asian/Pacific Islander 4,131 0.7 3.5 0.2
Native American/Alaskan Native 1,569 0.3 0.9 0.3
Race/Ethnicity not known 661 0.1 - -

________________________________________
DIAGNOSIS:
• Clinical: Diagnosis is often helped by the occurrence of rare diseases or infections such as Kaposi's sarcoma or Pneumocystis pneumonia or recurrent or serious opportunistic infections. The patient's history (life-style, drug use, etc.) is also informative.
• Laboratory: Laboratory diagnosis is based on measuring HIV antibodies using the ELISA (Enzyme-Linked Immuno-Sorbent Assay) test. Positive results are confirmed with another test known as a Western Blot. Together, the two tests are more than 99.9% accurate.
________________________________________
CONTROL:
• Sanitary: The use of condoms during sexual intercourse can greatly reduce the chance of infections. Some spermicidal creams may also have anti-HIV properties. Non-use of intravenous drugs or sharing of syringes/needles prevents direct inoculation or the virus into the blood. Testing of the blood supply reduces transmission by transfusion. Education is perhaps the best means of preventing disease.
• Immunological: No vaccines are available but some possibilities do exist.
• Chemotherapeutic: Anti-HIV drugs fall into three categories: the nucleosides, the non-nucleosides, and the protease inhibitors. Nucleosides and non-nucleosides are both known as reverse transcriptase inhibitors.
o Nucleoside reverse transcriptase inhibitors include: Retrovir (zidovudine, AZT), Videx (didanosine, ddI), Hivid (zalcitabine, ddC), Zerit (stavudine, d4T) and Epivir (lamivudine, 3TC).
o Non-nucleoside reverse transcriptase inhibitors include: Viramune (nevirapine) and Rescriptor (delavirdine).
o Protease Inhibitors include: Invirase (saquinavir), Norvir (ritonavir), Crixivan (indinavir) and Viracept (nelfinavir).
These drugs are often given in combinations of two or three in order to attack the HIV virus in different ways.
NOTE: Some of the information contained on this page was excerpted from the publication "World AIDS Day 1997 - Give Children Hope in a World with AIDS". This entire article can be downloaded (in PDF format, 772,591 bytes) from The Centers for Disease Control or locally. The HIV/AIDS Surveillance Report and additional HIV/AIDS information are also available from The Centers for Disease Control.
General Immunology
________________________________________
Host defense is present in many forms. Overall, the Immune Response (IR) can be divided into two major classifications; humoral and cell-mediated. While these responses are not mutually exclusive, they provide distinctly different avenues for dealing with pathogenic organisms or altered host cells. These different responses will be discussed in more detail later.
________________________________________
Immune Response



Humoral Immunity
Cell-mediated Immunity




(Antibody) (Cytotoxicity)
________________________________________
Some of these responses are specific, others are non-specific. This page will introduce host defense mechanisms by defining some commonly used terms and describing the specific cells and tissues involved in these immune responses.
________________________________________
DEFINITIONS:
Antigen (Ag): A molecule which elicits a specific immune response when introduced into an animal. More specifically, antigenic (immunogenic) substances are:
1. Generally large molecules (>10,000 daltons in molecular weight),
2. Structurally complex (proteins are usually very antigenic),
3. Accessible (the immune system must be able to contact the molecule), and
4. Foreign (not recognizable as "self").
Antibody (Ab): A glycoprotein produced in response to an antigen that is specific for the antigen and binds to it via non-covalent interactions. The term "immunoglobulin" is often used interchangeably with "antibody". We will use the term "immunoglobulin" to describe any antibody, regardless of specificity, and the term "antibody" to describe an antigen-specific "immunoglobulin". Immunoglobulins (Igs) come in different forms (IgA, IgD, IgE, IgG, IgM) that reflect their structure. More information can be found here.

Antibody kinetics: The figure illustrates the production of antibody in response to antigenic substances. In this figure, an animal was injected with Antigen A at day 0. Antigen A invokes a primary response beginning about day 4, as indicated by a rise in the specific antibody titer (titer = measure of the amount of antibody in the animal's serum per unit volume). Initially, this antibody is mostly IgM (and some IgG). After a peak titer between days 7 and 10, the response decreases rapidly. If the animal is then reinjected with Antigen A at day 28, the production of antibody begins almost immediately and reaches a level 1000-fold greater that that seen in the primary response. This is known as the secondary response and the principal antibody produced is IgG. If a second antigen (Antigen B) is also injected at the same time as the reinjection of Antigen A, however, only a primary response to Antigen B is observed. These results demonstrate that:
1. The immune response is specific.
2. The immune response has memory.
Clonal selection hypothesis (Jerne and Burnet): The clonal selection hypothesis attempts to explain the findings described above by suggesting the following:
1. Animals contain numerous cells called lymphocytes,
2. Each lymphocyte is responsive to a particular antigen by virtue of specific surface receptor molecules,
3. Upon contacting its appropriate antigen, the lymphocyte is stimulated to proliferate (clonal expansion) and differentiate,
4. The expanded clone is responsible for the secondary response (more cells to respond) while the differentiated ("effector") cells secrete antibody,
Click here to see a graphic illustration of clonal selection.
________________________________________
CELLS OF THE IMMUNE RESPONSE
Immune responsive cells can be divided into five groups based on i) the presence of specific surface components and ii) function: B-cells (B lymphocytes), T-cells (T lymphocytes), Accessory cells (Macrophages and other antigen-presenting cells), Killer cells (NK and K cells), and Mast cells. Some of the properties of each group are listed below.
Cell group Surface components Function
B-lymphocytes • Surface immunoglobulin (Ag recognition)
• Immunoglobulin Fc receptor
• Class II Major Histocompatability Complex (MHC) molecule (Ag presentation) • Direct antigen recognition
• Differentiation into antibody-producing plasma cells
• Antigen presentation within Class II MHC
________________________________________ ________________________________________ ________________________________________
T-lymphocytes • CD3 molecule
• T-cell receptor (TCR, Ag recognition) • Involved in both humoral and cell-mediated responses
• Helper T-cells (TH) • CD4 molecule • Recognizes antigen presented within Class II MHC
• Promotes differentiation of B-cells and cytotoxic T-cells
• Activates macrophages
• Suppressor T-cells (TS) • CD8 molecule • Downregulates the activities of other cells
• Cytotoxic T-cells (CTL) • CD8 molecule • Recognizes antigen presented within Class I MHC
• Kills cells expressing appropriate antigen
________________________________________ ________________________________________ ________________________________________
Accessory cells • Variable • Phagocytosis and cell killing
• Macrophages • Immunoglobulin Fc receptor
• Complement component C3b receptor
• Class II MHC molecule • Bind Fc portion of immunoglobulin (enhances phagocytosis)
• Bind complement component C3b (enhances phagocytosis)
• Antigen presentation within Class II MHC
• Secrete IL-1 (macrokine) promoting T-cell differentiation and proliferation
• Can be "activated" by T-cell lymphokines
• Dendritic cells • Class II MHC molecule • Antigen presentation within Class II MHC
• Polymorphonuclear cells (PMNs) • Immunoglobulin Fc receptor
• Complement component C3b receptor • Bind Fc portion of immunoglobulin (enhances phagocytosis)
• Bind complement component C3b (enhances phagocytosis)
________________________________________ ________________________________________ ________________________________________
Killer cells • Variable • Direct cell killing
• NK cells • Unknown • Kills variety of target cells (e.g. tumor cells, virus-infected cells, transplanted cells)
• K cells • Immunoglobulin Fc receptor • Bind Fc portion of immunoglobulin
• Kills antibody-coated target cells (antibody-dependent cell-mediated cytotoxicity, ADCC)
________________________________________ ________________________________________ ________________________________________
Mast cells • High affinity IgE Fc receptors • Bind IgE and initiate allergic responses by release of histamine
________________________________________
LYMPHOID TISSUES

Primary Secondary
(Responsible for maturation of Ag-reactive cells) (Sites for Ag contact and response)





Thymus
(T-cell maturation) Bone marrow Lymph nodes Spleen
(T-cell maturation) (B-cell maturation) (Expansion of lymphatic system, separate from blood circulation. Deep cortex harbors mostly T-cells, superficial cortex harbors mostly B-cells) (Similar to lymph nodes but part of blood circulation. Collects blood-borne Ags)


Immunoglobulins
________________________________________
Immunoglobulins generally assume one of two roles: immunoglobulins may act as i) plasma membrane bound antigen receptors on the surface of a B-cell or ii) as antibodies free in cellular fluids functioning to intercept and eliminate antigenic determinants. In either role, antibody function is intimately related to its structure and this page will introduce immunoglobulins (antibodies) and relate their structure to their function in host defense.
________________________________________
BASIC IMMUNOGLOBULIN STRUCTURE

Immunoglobulins are composed of four polypeptide chains: two "light" chains (lambda or kappa), and two "heavy" chains (alpha, delta, gamma, epsilon or mu). The type of heavy chain determines the immunoglobulin isotype (IgA, IgD, IgG, IgE, IgM, respectively). Light chains are composed of 220 amino acid residues while heavy chains are composed of 440-550 amino acids. Each chain has "constant" and "variable" regions as shown in the figure. Variable regions are contained within the amino (NH2) terminal end of the polypeptide chain (amino acids 1-110). When comparing one antibody to another, these amino acid sequences are quite distinct. Constant regions, comprising amino acids 111-220 (or 440-550), are rather uniform, in comparison, from one antibody to another, within the same isotype. "Hypervariable" regions, or "Complementarity Determining Regions" (CDRs) are found within the variable regions of both the heavy and light chains. These regions serve to recognize and bind specifically to antigen. The four polypeptide chains are held together by covalent disulfide (-S-S-) bonds.

Click here to visualize these 3D structures in real time!
Structural differences between immunoglobulins are used for their classification. As stated above, the type of heavy chain an immunoglobulin possesses determines the immunoglobulin "isotype". More specifically, an isotype is determined by the primary sequence of amino acids in the constant region of the heavy chain, which in turn determines the three-dimensional structure of the molecule. Since immunoglobulins are proteins, they can act as an antigen, eliciting an immune response that generates anti-immunoglobulin antibodies. However, the structural (three-dimensional) features that define isotypes are not immunogenic in an animal of the same species, since they are not seen as "foreign". For example, the five human isotypes, IgA, IgD, IgG, IgE and IgM are found in all humans and a result, injection of human IgG into another human would not generate antibodies directed against the structural features (determinants) that define the IgG isotype. However, injection of human IgG into a rabbit would generate antibodies directed against those same structural features.
Another means of classifying immunoglobulins is defined by the term "allotype". Like isotypes, allotypes are determined by the amino acid sequence and corresponding three-dimensional structure of the constant region of the immunoglobulin molecule. Unlike isotypes, allotypes reflect genetic differences between members of the same species. This means that not all members of the species will possess any particular allotype. Therefore, injection of any specific human allotype into another human could possibly generate antibodies directed against the structural features that define that particular allotypic variation.
A third means of classifying immunoglobulins is defined by the term "idiotype". Unlike isotypes and allotypes, idiotypes are determined by the amino acid sequence and corresponding three-dimensional structure of the variable region of the immunoglobulin molecule. In this regard, idiotypes reflect the antigen binding specificity of any particular antibody molecule. Idiotypes are so unique that an individual person is probably capable of generating antibodies directed against their own idiotypic determinants. This probability forms the basis of the Idiotypic Network Hypothesis to be described later.
________________________________________
BASIC IMMUNOGLOBULIN FUNCTION
Antibodies function in a variety of ways designed to eliminate the antigen that elicited their production. Some of these functions are independent of the particular class (isotype) of immunoglobulin. These functions reflect the antigen binding capacity of the molecule as defined by the variable and hypervariable (idiotypic) regions. For example, an antibody might bind to a toxin and prevent that toxin from entering host cells where its biological effects would be activated. Similarly, a different antibody might bind to the surface of a virus and prevent that virus from entering its host cell. In contrast, other antibody functions are dependent upon the immunoglobulin class (isotype). These functions are contained within the constant regions of the molecule. For example, only IgG and IgM antibodies have the ability to interact with and initiate the complement cascade. Likewise, only IgG molecules can bind to the surface of macrophages via Fc receptors to promote and enhance phagocytosis. The following table summarizes some immunoglobulin properties.
Isotype Structure Placental transfer Binds mast cell surfaces Binds phagocytic cell surfaces Activates complement Additional features
IgM
- - - + First Ab in development and response.
IgD
- - - - B-cell receptor.
IgG
+ - + + Involved in opsonization and ADCC. Four subclasses; IgG1, IgG2, IgG3, IgG4.
IgE
- + - - Involved in allergic responses.
IgA

- - - - Two subclasses; IgA1, IgA2. Also found as dimer (sIgA) in secretions.
________________________________________
GENERATION OF ANTIBODY DIVERSITY
The immune system has the capacity to recognize and respond to about 107 different antigens. This extreme diversity can be generated in at least three possible ways:
1. Multiple genes in the germ line DNA.
2. Variable recombination during the differentiation of germ line cells into B-cells.
3. Mutation during the differentiation of germ line cells into B-cells.
It is known that all three of these possibilities take place to produce antibody diversity. The following figures illustrate these possibilities:

1. The figure shows the genetic makeup of a germ line cell and a mature B-cell at the loci controlling heavy chain production. Germ line DNA has many (up to 200) different variable (V) region genes, in addition to 12 diversity (D) region genes and four joining (J) region genes. During differentiation of this cell into the B-cell, rearrangement of the DNA occurs. This rearrangement aligns one of the many V genes with one of the D genes and one of the J genes, producing a functional VDJ recombinant gene. Since any of the genes may recombine with any others, this rearrangement has the potential to generate 200 x 12 x 4 = 9600 different possible combinations. The same type of event occurs in the genes encoding the immmunoglobulin light chains where about 200 different V regions may recombine with about 5 different J regions giving rise to 200 x 5 = 1000 possible light chains. Since in any particular B-cell, any light chain combination can occur along with any heavy chain combination, the total possible immunoglobulin combinations approaches 107 (9600 x 1000).

2. A second way that diversity can result is through a process of variable or "inaccurate" recombination. The figure illustrates three possible recombination events between the variable (V) and joining (J) regions of an immunoglobulin light chain. In the first event, a proline-tryptophan dipeptide sequence is produced in the resulting protein. However, in the second and third events, differential recombination places proline-arginine or proline-proline sequences into the resulting immunoglobulin. These types of events may also occur between the V and D regions and the D and J regions of the heavy chain DNA sequence.

3. A third way that diversity can result is through a process of mutation. This process simply involves changes in DNA sequence that occur during differentiation of the B-cell. The figure illustrates how an A:T to G:C transition mutation could change a serine residue into a glycine residue in the resulting immunoglobulin. This process may, in part, explain the diversity observed in hypervariable (CDR) regions.
________________________________________
IMMUNOGLOBULIN PRODUCTION
The production of immunoglobulins by B-cells or plasma cells occurs in different stages. During differentiation of the B-cells from precursor stem cells, rearrangement, recombination and mutation of the immunoglobulin V, D, and J regions occurs to produce functional VJ (light chain) and VDJ (heavy chain) genes. At this point, the antigen specificity of the mature B-cell has been determined. Each cell can make only one heavy chain and one light chain, although the isotype of the heavy chain may change. Initially, a mature B-cell will produce primarily IgD (and some membrane IgM) that will migrate to the cell surface to act as the antigen receptor. Upon stimulation by antigen, the B-cell will differentiate into a plasma cell expressing large amounts of secreted IgM. Some cells will undergo a "class switch" during which a rearrangement of the DNA will occur, placing the VDJ gene next to the genes encoding the IgG, IgE or IgA constant regions. Upon secondary induction (i.e. the secondary response), these B-cells will differentiate into plasma cells expressing the new isotype. Most commonly, this results in a switch from IgM (primary response) to IgG (secondary response). The factors that lead to production of IgE or IgA instead of IgG are not well understood.
Click here to see a graphic illustration of immunoglobulin differentiation and production.
Histocompatibility
________________________________________
MAJOR HISTOCOMPATIBILITY COMPLEX
The Major Histocompatibility Complex (MHC) is a set of molecules displayed on cell surfaces that are responsible for lymphocyte recognition and "antigen presentation". The MHC molecules control the immune response through recognition of "self" and "non-self" and, consequently, serve as targets in transplantation rejection. The Class I and Class II MHC molecules belong to a group of molecules known as the Immunoglobulin Supergene Family, which includes immunoglobulins, T-cell receptors, CD4, CD8, and others. This page will describe the MHC molecules and the process of antigen presentation.
The major histocompatibility complex is encoded by several genes located on human chromosome 6. Class I molecules are encoded by the BCA region while class II molecules are encoded by the D region. A region between these two on chromosome 6 encodes class III molecules, including some complement components.
________________________________________
CLASS I MOLECULES
Class I molecules are composed of two polypeptide chains; one encoded by the BCA region and another (ß2-microglobulin) that is encoded elsewhere. The MHC-encoded polypeptide is about 350 amino acids long and glycosylated, giving a total molecular weight of about 45 kDa. This polypeptide folds into three separate domains called alpha-1, alpha-2 and alpha-3. ß2-microglobulin is a 12 kDa polypeptide that is non-covalently associated with the alpha-3 domain. Between the alpha-1 and alpha-2 domains lies a region bounded by a beta-pleated sheet on the bottom and two alpha helices on the sides. This region is capable of binding (via non-covalent interactions) a small peptide of about 10 amino acids. This small peptide is "presented" to a T-cell and defines the antigen "epitope" that the T-cell recognizes (see below). The following images illustrate the structure of the class I MHC as seen schematically, and three dimensionally from the side and from the top (T-cell perspective). The MHC-encoded polypeptide is shown in blue, the ß2-microglobulin is green and the peptide antigen is red.
Class I MHC Side view Top view




Click here to visualize this 3D structure in real time!
________________________________________
CLASS II MOLECULES
Class II molecules are composed of two polypeptide chains, both encoded by the D region. These polypeptides (alpha and beta) are about 230 and 240 amino acids long, respectively, and are glycosylated, giving molecular weights of about 33 kDa and 28 kDa. These polypeptides fold into two separate domains; alpha-1 and alpha-2 for the alpha polypeptide, and beta-1 and beta-2 for the beta polypeptide. Between the alpha-1 and beta-1 domains lies a region very similar to that seen on the class I molecule. This region, bounded by a beta-pleated sheet on the bottom and two alpha helices on the sides, is capable of binding (via non-covalent interactions) a small peptide of about 10 amino acids. This small peptide is "presented" to a T-cell and defines the antigen "epitope" that the T-cell recognizes (see below). The following images illustrate the structure of the class II MHC as seen schematically, and three dimensionally from the side and from the top (T-cell perspective). The MHC-encoded polypeptides are shown in yellow and green, while the peptide antigen is shown in red.
Class II MHC Side view Top view




Click here to visualize this 3D structure in real time!
________________________________________
CLASS I vs CLASS II MOLECULES
While class I and class II molecules appear somewhat structurally similar and both present antigen to T-cells, their functions are really quite distinct. First, class I molecules are found on virtually every cell in the human body. Class II molecules, in contrast, are only found on B-cells, macrophages and other "antigen-presenting cells" (APCs). Second, class I molecules present antigen to cytotoxic T-cells (CTLs) while class II molecules present antigen to helper T-cells (TH-cells). This specificity reflects the third difference, the type of antigen presented. Class I molecules present "endogenous" antigen while class II molecules present "exogenous" antigens. An endogenous antigen might be fragments of viral proteins or tumor proteins. Presentation of such antigens would indicate internal cellular alterations that if not contained could spread throughout the body. Hence, destruction of these cells by CTLs is advantageous to the body as a whole. Exogenous antigens, in contrast, might be fragments of bacterial cells or viruses that are engulfed and processed by e.g. a macrophage and then presented to helper T-cells. The TH-cells, in turn, could activate B-cells to produce antibody that would lead to the destruction of the pathogen.
________________________________________
T-CELL RECEPTOR (TCR) MOLECULES
The T-cell receptor molecule (TCR) is structurally and functionally similar to the B-cell immunoglobulin receptor. TCR is composed of two, disulfide-linked polypeptide chains, alpha and beta, each having separate constant and variable domains much like immunoglobulins. The variable domain contains three hypervariable regions that are responsible for antigen recognition. Genetic diversity is ensured in a manner analogous to that for immunoglobulins (click here for more information). Thus, just like the B-cell surface immunoglobulin provides antigen specificity to its B-cell, the TCR allows T-cells to recognize their particular antigenic moiety. However, T-cells cannot recognize antigen without help; the antigenic determinant must be presented by an appropriate (i.e. self) MHC molecule. Upon recognition of a specific antigen, the signal is passed to the CD3 molecule and then into the T-cell, prompting T-cell activation and the release of lymphokines. The following images illustrate the structure of the TCR as seen schematically, and three dimensionally from the side.
TCR Side view



Click here to visualize this 3D structure in real time!
________________________________________
ANTIGEN RECOGNITION BY T-CELLS
The TCR provides the specificity for an individual T-cell to recognize its particular antigen. However, this recognition is "MHC-restricted" because the TCR also requires interactions with MHC. Also, interactions between the CD4 molecule (found on helper T-cells) and class II MHC or the CD8 molecule (found on cytotoxic T-cells) and class I MHC stabilize and consummate the antigen recognition process, allowing helper T-cells to respond to "exogenous" antigens (leading to B-cell activation and the production of antibody) or cytotoxic T-cells to respond to "endogenous" antigens (leading to target cell destruction). The following images illustrate these processes schematically, and three dimensionally.
TCR - APC (class II) TCR - Target cell (class I)



Antigen Presentation by MHC-II to TCR


Humoral Immunity
________________________________________
The production of antibody involves three distinct phases:
Induction phase: Ag reacts with specific T and B cells
Expansion and Differentiation phase: Induced lymphocyte clones proliferate and mature to a functional stage (i.e. Ag receptor cells mature to Ag effector cells)
Effector phase: Abs or T cells exert biological effects either:
1. Independently or
2. Through the action of macrophages, complement, other non-specific agents
This page will discuss induction, differentiation and regulation of the humoral immune response, focusing on the production of Abs.
________________________________________
ANTIGEN PRESENTING CELLS (APCs)
Induction of the humoral immune response begins with the recognition of antigen. Through a process of clonal selection, specific B-cells are stimulated to proliferate and differentiate. However, this process requires the intervention of specific T-cells that are themselves stimulated to produce lymphokines that are responsible for activation of the antigen-induced B-cells. In other words, B cells recognize antigen via immunoglobulin receptors on their surface but are unable to proliferate and differentiate unless prompted by the action of T-cell lymphokines. In order for the T-cells to become stimulated to release lymphokines, they must also recognize specific antigen. However, while T-cells recognize antigen via their T-cell receptors, they can only do so in the context of the MHC molecules. This "antigen-presentation" is the responsibility of the antigen-presenting cells (APCs).
Several types of cells may serve the APC function. Perhaps the best APC is, in fact, the B-cell itself. When B-cells bind antigen, the antigen becomes internalized, processed and expressed on the surface of the B-cell. Expression occurs within the class II MHC molecule, which can then be recognized by T-helper cells (CD4+). Click the image to animate.
Other types of antigen-presenting cells include the macrophage and dendritic cells. These cells either actively phagocytose or pinocytose foreign antigens. The antigens are then processed in a manner similar to that observed for the B-cells. Next, specific antigen epitopes are expressed on the macrophage or dendritic cell surface. Again, this expression occurs within the class II MHC molecule, where T-cell recognition occurs. The stimulated T-cells then release lymphokines that act upon "primed" B-cells (B-cells that have already encountered antigen), inducing B-cell proliferation and differentiation. Click the image to animate.
________________________________________
DIFFERENTIATION OF B-LYMPHOCYTES
B-cells begin their lives in the bone marrow as multipotential stem cells. These completely undifferentiated cells serve as the source for all of the cellular components of the blood and lymphoid system. The initial differentiation step that ultimately leads to the mature B-cell involves DNA rearrangements joining the D and J segments of the immunoglobulin heavy chain genes (click here for more information). Next, DNA rearrangements joining the variable (V) region to the DJ segments of the immunoglobulin heavy chain, as well as similar rearrangements within the light chain genes gives rise to the pre-B-cell. Establishment of the B-cell specificity and consequent expression of surface immunoglobulin gives rise to the "virgin", fully functional B-cell. Each of these steps is entirely independent of antigen.
The antigen-dependent stages of B-lymphocyte differentiation occur in the spleen, lymph nodes and other peripheral tissue. These stages are, of course, initiated upon encounter with antigen and activation by T-cell lymphokines. The activated B-cell first develops into a B-lymphoblast, becoming much larger and shedding all surface immunoglobulin. The B-lymphoblast then develops into a plasma cell, which is, in essence, an antibody factory. This terminal differentiation stage is responsible for production of primarily IgM antibody during the "primary response". Some B-cells, however, do not differentiate into plasma cells. Instead, these cells undergo secondary DNA rearrangements that place the constant region of the IgG, IgA or IgE genes in conjunction with the VDJ genes. This "class switch" establishes the phenotype of these newly differentiated B-cells; these cells remain as long-lived "memory cells". Upon subsequent encounter with antigen, these cells respond very quickly to produce large amounts of IgG, IgA or IgE antibody, generating the "secondary response".
________________________________________
REGULATION OF THE HUMORAL RESPONSE
Regulation of the immune response is possibly mediated in several ways. First, a specific group of T-cells, suppressor T-cells, are thought to be involved in turning down the immune response. Like helper T-cells, suppressor T-cells are stimulated by antigen but instead of releasing lymphokines that activate B-cells (and other cells), suppressor T-cells release factors that suppress the B-cell response. While immunosuppression is not completely understood, it appears to be more complicated than the activation pathway, possibly involving additional cells in the overall pathway.

Other means of regulation involve interactions between antibody and B-cells. One mechanism, "antigen blocking", occurs when high doses of antibody interact with all of the antigen's epitopes, thereby inhibiting interactions with B-cell receptors. A second mechanism, "receptor cross linking", results when antibody, bound to a B-cell via its Fc receptor, and the B-cell receptor both combine with antigen. This "cross-linking" inhibits the B-cell from producing further antibody.

Another means of regulation that has been proposed is the idiotypic network hypothesis. This theory suggests that the idiotypic determinants of antibody molecules are so unique that they appear foreign to the immune system and are, therefore, antigenic. Thus, production of antibody in response to antigen leads to the production of anti-antibody in response, and anti-anti-antibody and so on. Eventually, however, the level of [anti]n-antibody is not sufficient to induce another round and the cascade ends.

Antigen-Antibody Interactions
________________________________________
AFFINITY
Interactions between antigen and antibody involve non-covalent binding of an antigenic determinant (epitope) to the variable region (complementarity determining region, CDR) of both the heavy and light immunoglobulin chains. These interactions are analogous to those observed in enzyme-substrate interactions and they can be defined similarly. To describe the strength of the antigen-antibody interaction, one can define the affinity constant (K) as shown:
Affinity K = [Ab - Ag]

[Ab] × [Ag] = 104 to 1012 L/mol

If the interaction between antigen and antibody were totally random, one would expect the concentrations of free antigen, free antibody and bound Ag-Ab complex to all be equivalent. In other words,
Affinity K = 1

1 × 1 = 100 L/mol

Therefore, the greater the K, the stronger the affinity between antigen and antibody. These interactions are the result of complementarity in shapes, hydrophobic interactions, hydrogen bonds and Van der Waals forces.
________________________________________
ANTIGEN-ANTIBODY RATIOS
Experimentally, if one adds a known concentration of antibody to a tube and then adds increasing amounts of the specific antigen, the Ag-Ab complexes will begin to precipitate. If one continues to add increasing amounts of antigen, the complexes will begin to dissolve and return to solution. The following graph illustrates this process.

Tube # 1 2 3 4 5 6 7
Amount of precipitate







Amount of Ag
(arbitrary units) 1 2 3 4 5 6 7

If one then measures the amount of antigen and antibody remaining in the supernatant, one sees the following:
Excess Ab + + + - - - -
Excess Ag - - - - + + +

Click the button to illustrate the process

The left portion of the graph (tubes 1-3) illustrates "Antibody Excess", since not all of the antibody that is available to bind to antigen has actually bound antigen. The right portion of the graph (tubes 5-7) illustrates "Antigen Excess", where there is not enough antibody to bind to all of the available antigen. In the middle (tube 4) is a region known as "Equivalence". Here, the ration of antigen to antibody is perfect, so that all the antigen molecules and all of the antibody molecules are part of a complex. These are interesting experimental observations that do have relevance to situations occurring in the human body. For example:
• Antibody excess might occur when a person is exposed to a virus from which they have recently recovered. Hence, their body would contain a relatively large concentration of antiviral antibodies. These antibodies could quickly act to block cell receptors on the viral surface and prevent adsorption to host cells, thereby preventing disease.
• Antigen excess might occur early in the first infection by a microorganism. A person would have relatively few antibodies and these would form complexes but they would be very small. Such small complexes probably would not be phagocytosed or removed by the kidneys and could become lodged near tissue surfaces. Later, when antibody becomes available, the size of the complexes can increase leading to effective elimination by phagocytes or tissue damage where the smaller complexes had become lodged. Click here for more information.
• Equivalence would occur when a person is exposed to an agent to which they have circulating antibodies. The correct ratio of antigen to antibody would produce extensive lattice formation, leading to enhanced phagocytosis, opsonization or agglutination, effectively eliminating the foreign agent.
________________________________________
CROSSREACTIVITY
Crossreactivity can occur when two (or more) antigens share similar structural features. Consider three different antigens, as shown on the right. Antibody produced in response to Ag1 is very specific and would, therefore, have a large affinity constant (K) when combining with Ag1. However, Ag2 is similar in shape to Ag1 and is capable of interacting with anti-Ag1 antibody via two of three sites. The interaction between Ab and Ag2 is not as strong as the interaction between Ab and Ag 1 (i.e. K is much smaller) but is still strong enough to allow binding. Hence, Ag1 and Ag2 are said to cross-react. Ag3, in contrast, cannot interact very well with anti-Ag1 antibody and would have a K value so low that significant binding would not occur. Ag3, therefore, would not cross-react with Ag1. Would antibody produced in response to Ag2 bind Ag3? Would antibody produced in response to Ag2 bind Ag1?
Crossreactivity also forms the basis for several diagnostic tests. For example, infection with Treponema pallidum (syphilis) causes the production of antibodies that cross-react with a substance found in cardiac muscle, cardiolipin. Since it is much easier to obtain pure cardiolipin than pure Treponemal antigens, this cross-reaction is used to test for syphilis (Wassermann test). Likewise, antibodies produced against certain Rickettsia cross-react with antigens from Proteus. Since the latter are much easier to obtain, they can be used to test for the former.
Cell Mediated Immunity
________________________________________
The second arm of the immune response is refered to as Cell Mediated Immunity (CMIR). As the name implies, the functional "effectors" of this response are various immune cells. These functions include:
• Phagocytosis and killing of intracellular pathogens
• Direct cell killing by cytotoxic T cells
• Direct cell killing by NK and K cells
These responses are especially important for destroying intracellular bacteria, eliminating viral infections and destroying tumor cells. This page will discuss the cell-mediated immune response, focusing on the mechanisms involved.
________________________________________
MACROPHAGE ACTIVATION
While the production of antibody through the humoral immune response can effectively lead to the elimination of a variety of pathogens, bacteria that have evolved to invade and multiply within phagocytic cells of the immune response pose a different threat. The following graphics illustrate this dilemma:
________________________________________
Extracellular microorganisms










Non-encapsulated microorganisms are easily phagocytosed and killed within macrophages.














Encapsulated microorganisms require the production of antibody in order to be effectively phagocytosed. Once engulfed, however, they are easily killed.
Intracellular microorganisms














Intracellular microorganisms elicit the production of antibody, which allows effective phagocytosis. Once engulfed, however, they survive within the phagocyte and eventually kill it.







IFN
TNF




Intracellular microorganisms also activate specific T-cells, which then release lymphokines (e.g. IFN, TNF) that cause macrophage activation. Activated ("killer") macrophages are then very effective at destroying the intracellular pathogens.
________________________________________
This process can be further illustrated by considering the following experiment known as "Koch's phenomenon":
• Inoculation of an unimmunized guinea pig with a lethal dose of the intracellular pathogen Mycobacterium tuberculosis (MT) results in death of the animal. Inoculation with a sub-lethal dose induces immunity.
• Inoculation of an MT-immunized guinea pig with a lethal dose of MT causes a local reaction ("delayed hypersensitivity") one to two days later.
• Inoculation of an MT-immunized guinea pig with a lethal dose of a different intracellular pathogen, Listeria monocytogenes (LM) again results in death of the animal.
• Inoculation of an MT-immunized guinea pig with a lethal dose of LM and MT causes a delayed hypersensitivity reaction.
These results demonstrate the specific (T-cell mediated) and non-specific (macrophage mediated) aspects of this type of cell mediated immunity.
________________________________________
CELL MEDIATED CYTOTOXICITY
The second half of the cell-mediated immune response is involved in rejection of foreign grafts and the elimination of tumors and virus-infected cells. The effector cells involved in these processes are cytotoxic T-lymphocytes (CTLs), NK-cells and K-cells. Each of these effector cells recognizes their target by different means, described below.
Cytotoxic T-lymphocytes
CTLs, like other T-cells are both antigen and MHC-restricted. That is, CTLs require i) recognition of a specific antigenic determinant and ii) recognition of "self" MHC (Click here to review these requirements). Briefly, CTLs recognize antigen via their T-cell receptor. This receptor makes specific contacts with the antigenic determinant and the target cell's class I MHC molecule. CTLs also express CD8, which may assist the antigen recognition process. Once recognition is successful, the CTL "programs" the target cell for self-destruction. This process is thought to occur in one of several possible ways. First, CTLs may release a substance known as perforin in the space between the CTL and its target. In the presence of calcium ions, the perforin polymerizes, forming channels in the target cell's membrane. These channels may cause the target cell to lyse. Second, the CTL may also release various enzymes that pass through the polyperforin channels, causing target cell damage. Third, the CTL may release lymphokines and/or cytokines that interact with specific receptors on the target cell surface, causing internal responses that lead to destruction of the target cell. CTLs principally act to eliminate endogenous antigens. Click here for more information.
NK cells
NK cells are part of a group know as the "large granular lymphocytes". These cells are generally non-specific, MHC-unrestricted cells involved primarily in the elimination of neoplastic or tumor cells. The precise mechanism by which they recognize their target cells is not clear. Probably, there is some type of NK-determinant expressed by the target cells that is recognized by an NK-receptor on the NK cell surface. Once the target cell is recognized, killing occurs in a manner similar to that produced by the CTL.
K cells
K-cells are probably not a separate cell type but rather a separate function of the NK group. K-cells contain immunoglobulin Fc receptors on their surface and are involved in a process known as Antibody-dependent Cell-mediated Cytotoxicity (ADCC). ADCC occurs as a consequence of antibody being bound to a target cell surface via specific antigenic determinants expressed by the target cell. Once bound, the Fc portion of the immunoglobulin can be recognized by the K-cell. Killing then ensues by a mechanism similar to that employed by CTLs. This type of CMIR can also result in Type II hypersensitivities.

Back
Home
Next

Complement
________________________________________
COMPONENTS AND FUNCTIONS OF THE COMPLEMENT SYSTEM
The complement system found in the blood of mammals is composed of heat labile substances (proteins) that combine with antibodies or cell surfaces. This complex, multicomponent system is composed of about 26 proteins. The "complement cascade" is constitutive and non-specific but it must be activated in order to function. The functions of complement include:
• making bacteria more susceptible to phagocytosis
• directly lysing some bacteria and foreign cells
• producing chemotactic substances
• increasing vascular permeability
• causing smooth muscle contraction promoting mast cell degranulation
The complement system can be activated via two distinct pathways; the classical pathway and the alternate pathway. Once initiated, a cascade of events (the "complement cascade") ensues, providing the functions listed above.
Most of the complement components are numbered (e.g. C1, C2, C3, etc.) but some are simply refered to as "Factors". Some of the components must be enzymatically cleaved to activate their function; others simply combine to form complexes that are active. The following table lists these components and their functions.
Components of the Classical Pathway
Native component Active component(s) Function(s)
C1(q,r,s) C1q Binds to antibody that has bound antigen, activates C1r.
C1r Cleaves C1s to activate protease function.
C1s Cleaves C2 and C4.
C2 C2a Unknown.
C2b Active enzyme of classical pathway; cleaves C3 and C5.
C3 C3a Mediates inflammation; anaphylatoxin.
C3b Binds C5 for cleavage by C2b.
Binds cell surfaces for opsonization and activation of alternate pathway.
C4 C4a Mediates inflammation.
C4b Binds C2 for cleavage by C1s. Binds cell surfaces for opsonization.
Components of the Alternate Pathway
Native component Active component(s) Function(s)
C3 C3a Mediates inflammation; anaphylatoxin.
C3b Binds cell surfaces for opsonization and activation of alternate pathway.
Factor B B Binds membrane bound C3b. Cleaved by Factor D.
Ba Unknown.
Bb Cleaved form stabilized by P produces C3 convertase.
Factor D D Cleaves Factor B when bound to C3b.
Properdin P Binds and stabilizes membrane bound C3bBb.
Components of the Membrane-Attack Complex
Native component Active component(s) Function(s)
C5 C5a Mediates inflammation; anaphylatoxin, chemotaxin.
C5b Initiates assembly of the membrane-attack complex (MAC).
C6 C6 Binds C5b, forms acceptor for C7.
C7 C7 Binds C5b6, inserts into membrane, forms acceptor for C8.
C8 C8 Binds C5b67, initiates C9 polymerization.
C9 C9n Polymerizes around C5b678 to form channel that causes cell lysis.
________________________________________
ACTIVATION OF THE COMPLEMENT CASCADE
Classical Pathway
The classical pathway starts with C1; C1 binds to immunoglobulin Fc (primarily IgM and IgG); C1 is recognition complex composed of 22 polypeptide chains in 3 subunits; C1q, C1r, C1s. C1q is the actual recognition portion, a glycoprotein containing hydroxyproline and hydroxylysine that looks like a tulip flower. Upon binding via C1q, C1r is activated to become a protease that cleaves C1s to a form that activates (cleaves) both C2 and C4 to C2a/b and C4a/b. C2b and C4b combine to produce C3 convertase (C3 activating enzyme). C4a has anaphylactic activity (inflammatory response).
C3 is central to both the classical and alternative pathways. In classical, C4b2b convertase cleaves C3 into C3a/b. C3a is a potent anaphylatoxin. C3b combines with C4b2b to form C4b2b3b complex that is a C5 convertase. C3b can also bind directly to cells making them susceptible to phagocytosis.
C5 is converted by C5 convertase (i.e. C4b2b3b) to C5a/b. C5a has potent anaphylatoxic and chemotaxic activities. C5b functions as an anchor on the target cell surface to which the lytic membrane-attack complex (MAC) forms. MAC includes C5b, C6, C7, C8 and C9. Once C9 polymerizes to form a hole in the cell wall, lysis ensues.
Classical Pathway

Component cleavage
Enzymatic activity
Component assembly
Alternate Pathway
The alternate pathway may be initiated by immunologic (e.g. IgA or IgE) or non-immunologic (e.g. LPS) means. The cascade begins with C3. A small amount of C3b is always found in circulation as a result of spontaneous cleavage of C3 but the concentrations are generally kept very low (see below). However, when C3b binds covalently to sugars on a cell surface, it can become protected. Then Factor B binds to C3b. In the presence of Factor D, bound Factor B is cleaved to Ba and Bb; Bb contains the active site for a C3 convertase. Next. properdin binds to C3bBb to stabilize the C3bBb convertase on cell surface leading to cleavage of C3. Finally, a C3bBb3b complex forms and this is a C5 convertase, cleaving C5 to C5a/b. Once formed, C5b initiates formation of the membrane attack complex as described above.
Generally, only Gram-negative cells can be directly lysed by antibody plus complement; Gram-positive cells are mostly resistant. However, phagocytosis is greatly enhanced by C3b binding (phagocytes have C3b receptors on their surface) and antibody is not always required. In addition, complement can neutralize virus particles either by direct lysis or by preventing viral penetration of host cells.
Alternate Pathway

Component cleavage
Enzymatic activity
Component assembly
Click here to see how complement can be used in diagnostic testing.
________________________________________
REGULATION OF THE COMPLEMENT CASCADE
Because both the classical and alternate pathways depend upon C3b, regulation of the complement cascade is mediated via 3 proteins that affect the levels and activities of this component.
1. C1 Inhibitor inhibits the production of C3b by combining with and inactivating C1r and C1s. This prevents formation of the C3 convertase, C4b2b.
2. Protein H inhibits the production of C3b by inhibiting the binding of Factor B to membrane-bound C3b, thereby preventing cleavage of B to Bb and production of the C3 convertase, C3bBb.
3. Factor I inhibits the production of C3b by cleaving C3b into C3c and C3d, which are inactive. Factor I only works on cell membrane bound C3b, mostly on red blood cells (i.e. non-activator surfaces).
Hypersensitivity
________________________________________
Occasionally, the immune system responds inappropriately to the presence of antigen. These responses are refered to as hypersensitivities. There are four different types of hypersensitivities that result from different alterations of the immune system. These types are classified as:
• Type I: Immediate Hypersensitivity
• Type II: Cytotoxic Hypersensitivity
• Type III: Immune Complex Hypersensitivity
• Type IV: Delayed Hypersensitivity
This page will describe the four types of hypersensitivity, giving examples of diseases that may result.
________________________________________
TYPE I HYPERSENSITIVITY
Type I or Immediate Hypersensitivity can be illustrated by considering the following experiment:
1. First, a guinea pig is injected intravenously with an antigen. For this example, bovine serum albumin (BSA, a protein) will be used. After two weeks, the same antigen will be reinjected into the same animal. Within a few minutes, the animal begins to suffocate and dies by a process called anaphylactic shock.
2. Instead of reinjecting the immunized guinea pig, serum is transferred from this pig to a "naive" (unimmunized) pig. When this second guinea pig is now injected with BSA, it also dies of anaphylactic shock. However, if the second pig is injected with a different antigen (e.g. egg white albumin), the pig shows no reaction.
3. If immune cells (T-cells and macrophages instead of serum) are transfered from the immunized pig to a second pig, the result is very different; injection of the second pig with BSA has no effect.

These results tell us that:
• The reaction elicited by antigen occurs very rapidly (hence the name "immediate hypersensitivity").
• The hypersensitivity is mediated via serum-derived components (i.e. antibody).
• The hypersensitivity is antigen-specific (as one might expect for an antibody-mediated reaction).
The details of this reaction can be summarized as follows (click the image to animate):
1. Initial introduction of antigen produces an antibody response. More specifically, the type of antigen and the way in which it is administered induce the synthesis of IgE antibody in particular.
2. Immunoglobulin IgE binds very specifically to receptors on the surface of mast cells, which remain circulating.
3. Reintroduced antigen interacts with IgE on mast cells causing the cells to degranulate and release large amounts of histamine, lipid mediators and chemotactic factors that cause smooth muscle contraction, vasodilation, increased vascular permeability, broncoconstriction and edema. These reactions occur very suddenly, causing death.
Examples of Type I hypersensitivities include allergies to penicillin, insect bites, molds, etc. A person's sensitivity to these allergens can be tested by a cutaneous reaction. If the specific antigen in question is injected intradermally and the patient is sensitive, a specific reaction known as wheal and flare can be observed within 15 minutes. Individuals who are hypersensitive to such allergens must avoid contact with large inocula to prevent anaphylactic shock.
________________________________________
TYPE II HYPERSENSITIVITY
Type II or Cytotoxic Hypersensitivity also involves antibody-mediated reactions. However, the immunoglobulin class (isotype) is generally IgG. In addition, this process involves K-cells rather than mast cells. K-cells are, of course, involved in antibody-dependent cell-mediated cytotoxicity (ADCC). Type II hypersensitivity may also involve complement that binds to cell-bound antibody. The difference here is that the antibodies are specific for (or able to cross-react with) "self" antigens. When these circulating antibodies react with a host cell surface, tissue damage may result. Click the image to animate the process.
There are many examples of Type II hypersensitivity. These include:
• Pemphigus: IgG antibodies that react with the intracellular substance found between epidermal cells.
• Autoimmune hemolytic anemia (AHA): This disease is generally inspired by a drug such as penicillin that becomes attached to the surface of red blood cells (RBC) and acts as hapten for the production of antibody which then binds the RBC surface leading to lysis of RBCs.
• Goodpasture's syndrome: Generally manifested as a glomerulonephritis, IgG antibodies that react against glomerular basement membrane surfaces can lead to kidney destruction.
________________________________________
TYPE III HYPERSENSITIVITY
Type III or Immune Complex hypersensitivity involves circulating antibody that reacts with free antigen. These circulating complexes can then become deposited on tissues. Tissue deposition may lead to reaction with complement, causing tissue damage. this type of hypersensitivity develops as a result of systematic exposure to an antigen and is dependent on i) the type of antigen and antibody and ii) the size of the resulting complex (click here for more information). More specifically, complexes that are too small remain in circulation; complexes too large are removed by the glomerulus; intermediate complexes may become lodged in the glomerulus leading to kidney damage. Click the image to animate the process.
One example of a Type III hypersensitivity is serum sickness, a condition that may develop when a patient is injected with a large amount of e.g. antitoxin that was produced in an animal. After about 10 days, anti-antitoxin antibodies react with the antitoxin forming immune complexes that deposit in tissues. Type III hypersensitivities can be ascertained by intradermal injection of the antigen, followed by the observance of an "Arthus" reaction (swelling and redness at site of injection) after a few hours.
________________________________________
TYPE IV HYPERSENSITIVITY
Type IV or Delayed Hypersensitivity can be illustrated by considering the following experiment:
1. First, a guinea pig is injected with a sub-lethal dose of Mycobacterium tuberculosis (MT). Following recovery of the animal, injection of a lethal dose of MT under the skin produces only erythema (redness) and induration (hard spot) at the site of injection 1-2 days later.
2. Instead of reinjecting the immunized guinea pig, serum is transfered from this pig to a "naive" (unimmunized) pig. When this second guinea pig is now injected with MT, it dies of the infection.
3. If immune cells (T-cells and macrophages instead of serum) are transfered from the immunized pig to a second pig, the result is very different; injection of the second pig with MT causes only erythema and induration at the site of injection 1-2 days later.
4. In a separate experiment, if the immunized guinea pig is injected with a lethal dose of Listeria monocytogenes (LM) instead of MT, it dies of the infection. However, if the pig is simultaneously injected with both LM and MT, it survives.

These results tell us that:
• The reaction elicited by antigen occurs relatively slowly (hence the name "delayed hypersensitivity").
• The hypersensitivity is mediated via T-cells and macrophages.
• The hypersensitivity illustrates both antigen-specific (T-cell) and antigen non-specific (macrophage) characteristics.
The details of this reaction can be summarized as follows (click the image to animate):
1. Initial introduction of antigen produces a cell-mediated response. Mycobacterium tuberculosis is an intracellular pathogen and recovery requires induction of specific T-cell clones with subsequent activation of macrophages.
2. Memory T-cells respond upon secondary injection of the specific (i.e. MT) antigen, but not the non-specific (i.e. LM) antigen.
3. Induction of the memory T-cells causes activation of macrophages and destruction of both specific (MT) and non-specific (LM) microorganisms.
Immunological Defects
________________________________________
Primary Defects Acquired Defects
Bone Marrow
Stem Cells
SCID

Immunosuppressives

Lymphoid Stem Cells



HypoGG
Pre B-cells Thymus DiGeorge Syndrome


Corticosteroids

B-cells T-cells PNP Deficiency

Selective Deficiency



Myeloma
Plasma cells TH-cells AIDS

Hypercatabolism

TS-cells
Immunoglobulins CTLs
________________________________________