Virulence factor

Virulence factors are molecules produced by pathogens (bacteria, viruses, fungi and protozoa) that add to their effectiveness and enable them to achieve the following: colonization of a niche in the host (this includes attachment to cells); immunoevasion, evasion of the host's immune response; entry into and exit out of cells (if the pathogen is an intracellular one); and obtainment of nutrition from the host.

Types of Virulence Factors

Virulence factors can take many forms, including:

• Adhesins: These are used by pathogens to adhere to host cells. They are typically located on the surface of the pathogen and have specific receptors on the host cell that they bind to.

• Invasins: These are used by pathogens to invade host cells. They can either promote phagocytosis by the host cell (as seen in Salmonella) or they can trigger the host cell to engulf the pathogen by a process called receptor-mediated endocytosis (as seen in Listeria).

• Evasins: These are used by pathogens to evade the host's immune response. This can be done by hiding within host cells (as seen in Mycobacterium tuberculosis), by breaking down antibodies (as seen in Streptococcus) or by suppressing the immune response (as seen in HIV).

• Toxins: These are poisonous substances produced by pathogens that can damage host cells. They can either be endotoxins, which are part of the cell wall of Gram-negative bacteria, or exotoxins, which are secreted by both Gram-positive and Gram-negative bacteria.

• Nutrient acquisition: Pathogens also produce virulence factors that allow them to obtain nutrients from the host. This can be done by producing siderophores, which bind iron, or by producing enzymes that break down host tissues.

See Also

• Pathogenicity
• Infection
• Disease
• Bacteria
• Virus
• Fungi
• Protozoa