Virulence factors are molecules expressed and secreted by pathogens (bacteria, viruses, fungi and protozoa) that enable them to achieve the following:

• colonisation of a niche in the host (this includes adhesion to cells)
• evasion of the host's immune response
• entry into and exit out of cells (if the pathogen is an intracellular one)
• obtain nutrition from the host.

Virulence factors are very often responsible for causing disease in the host as they inhibit certain host functions.

Pathogens possess a wide array of virulence factors. Some are intrinsic to the bacteria (e.g. capsules and endotoxin) whereas others are obtained from plasmids (e.g. some toxins).

A major group of virulence factors are bacterial toxins. These are divided into two groups: endotoxins and exotoxins. The endotoxin is the lipopolysaccharide (LPS), which is part of the bacterial cell wall of Gram-negative bacteria. it is the Lipid A component of the LPS that has the toxic properties. The LPS is a very potent antigen and, as a result, stimulates an intense host immune response. As part of this immune response cytokines are released which cause the fever and other symptoms seen during disease. If a high amount of LPS is present then septic shock (or endotoxic shock) may result which, in severe cases, can lead to death. Exotoxins, on the other hand, are actively secreted by some bacteria and have a wide range of affects including inhibition of certain biochemical pathways in the host. The two most potent exotoxins known to man^{[citation needed]} are the tetanus toxin (tetanospasmin) secreted by Clostridium tetani and the botulinum toxin secreted by Clostridium botulinum. Exotoxins are also produced by a range of other bacteria including Escherichia coli; Vibrio cholerae (causative agent of cholera); Clostridium perfringens (causative agent of food poisoning as well as gas gangrene) and Clostridium difficile (causative agent of pseudomembranous colitis).

Toxins are also produced by some fungi as a competitive resource. The toxins, named mycotoxins, deter other organisms from consuming the food colonised by the fungi. As with bacterial toxins, there is a wide array of fungal toxins. Arguably one of the more dangerous mycotoxins is aflatoxin produced by certain species of the genus Aspergillus (notably Aspergillus flavus). If ingested repeatedly, this toxins can cause serious liver damage.

Another group of virulence factors possessed by bacteria are immunoglobulin (Ig) proteases. Immunoglobulins are antibodies expressed and secreted by hosts in response to an infection. These immunoglobulins play a major role in destruction of the pathogen through mechanisms such as opsonization. Some bacteria, such as Streptococcus pyogenes (causative agent of scarlet fever and many other conditions), are able to break down the host's immunoglobulins using proteases.

Some bacteria, such as Streptococcus pyogenes, Staphylococcus aureus and Pseudomonas aeruginosa, produce a variety of enzymes which cause damage to host tissues. Enzymes include hyaluronidase, which breaks down the connective tissue component hyaluronic acid ; a range of proteases and lipases ; DNAses, which break down DNA and haemolysins which break down a variety of host cells, including red blood cells.

Capsules, made of carbohydrate, form part of the outer structure of many bacterial cells including Neisseria meningitidis (causative agent of meningitis). Capsules play important roles in immune evasion, as they inhibit phagocytosis, as well as protecting the bacteria whilst outside a host.

More to follow.

[edit] Examples

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Examples of virulence factors for Staphylococcus aureus are hyaluronidase, protease, coagulase, lipases, deoxyribonucleases and enterotoxins

Some examples of virulence factors for Streptococcus pyogenes are M protein, lipoteichoic acid, hyaluronic acid capsul, invasins such as streptokinase, streptodornase, hyaluronidase, spenceronic, dorsettonic, and streptolysins, and excotoxins some other virulence factors are adhesion factors, biofilms(for example sortases or things that attach to integrins notably beta-1 and3 , extracellular enzymes, toxins and antiphagocytic factors.

[edit] Related links

• Virulence

[edit] References

v • d • e Toxins (enterotoxin/neurotoxin/hemotoxin/cardiotoxin) Bacterial toxins Exotoxin Gram positive Bacilli Clostridium: tetani (Tetanospasmin) · perfringens (Alpha toxin, Enterotoxin) · difficile (A, B) · botulinum (Botox) other: Anthrax toxin · Listeriolysin O Cocci Streptolysin · Leukocidin (Panton-Valentine leukocidin) · Staphylococcus (Staphylococcus aureus alpha/beta/delta, Exfoliatin, Toxic shock syndrome toxin, SEB) Actinobacteria Cord factor · Diphtheria toxin Gram negative Shiga toxin · Verotoxin/shiga-like toxin (E. coli) · E. coli heat-stable enterotoxin/enterotoxin · Cholera toxin · Pertussis toxin · Pseudomonas exotoxin · Extracellular adenylate cyclase By mechanism type I (Superantigen) · type II (Pore forming toxins) · type III (AB toxin/AB5) Endotoxin Lipopolysaccharide (Lipid A) · Bacillus thuringiensis delta endotoxin Virulence factor Clumping factor A · Fibronectin binding protein A Mycotoxins Aflatoxin · Amatoxin (alpha-amanitin, beta-amanitin, gamma-amanitin, epsilon-amanitin) · Citrinin · Cytochalasin · Ergotamine · Fumonisin (Fumonisin B1, Fumonisin B2) · Gliotoxin · Ibotenic acid · Muscimol · Ochratoxin · Patulin · Sterigmatocystin · Trichothecene · Vomitoxin · Zeranol · Zearalenone Invertebrates arthropod: scorpion: Charybdotoxin, Maurotoxin, Agitoxin, Margatoxin, Slotoxin, Scyllatoxin, Hefutoxin, Lq2 · spider: Latrotoxin (Alpha-latrotoxin) · Stromatoxin · PhTx3 mollusca: Conotoxin · Eledoisin · Onchidal · Saxitoxin Vertebrates fish: Ciguatera · Tetrodotoxin amphibian: (+)-Allopumiliotoxin 267A · Batrachotoxin · Bufotoxins (Arenobufagin, Bufotalin, Bufotenin · Cinobufagin, Marinobufagin) · Epibatidine · Histrionicotoxin · Pumiliotoxin 251D · Tarichatoxin reptile/snake venom: Bungarotoxin (Alpha-Bungarotoxin, Beta-Bungarotoxin) · Calciseptine · Taicatoxin · Calcicludine · Cardiotoxin III note: some toxins are produced by lower species and pass through intermediate species

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