Bacterial Virulence Factors

Overview
  • Bacterial Virulence Factors are molecules synthesized by certain bacteria that increases their capacity to infect or damage human tissues. Virulence factors may be coded within the bacterial genome, thus being inherent aspects of the organism's structure, or may be coded within transmissible genetic elements and thus acquired from the environment as discussed in Bacterial Genetics. The titles of the following sections provide interlinks that list organisms with these virulence factors.
Capsule
  • Definition and Composition
    • Bacterial capsules are protective coatings that surround the entire bacterial cell wall. They are composed of sugars except in the case of Bacillus anthracis whose capsule is composed of D-glutamine.
  • Significance
    • Bacterial capsules inhibit the capacity of Macrophages and Neutrophils to phagocytose encapsulated bacteria, thus allowing their evasion of the host immune response. However, the immune system can develop antibodies to capsular antigens which coat and thus opsonize encapsulated bacteria, allowing for their phagocytosis. Vaccines can also be generated using capsular sugars and form the basis of conjugate vaccines.
  • Detection
    • Polysaccharide capsules are detected with "Quellung Reaction" which involves treatment of bacteria with capsule-binding antibodies. Binding of antibodies causes the capsule to swell with water which can be detected microscopically.
Flagella
  • Definition and Composition
    • Flagella are long, whip-like proteinacious filaments which are anchored within the bacterial cell wall and can extend several times the length of the bacteria itself. Rotation of bacterial flagella allow for bacterial movement, which is usually in the direction of nutrients.
  • Significance
    • The motility afforded by flagella enhances the capacity of bacteria to anatomically spread. This is best observed in the context of [[Urinary Tract Infection|UTIs] where flagellated bacteria can spread up the urethra.
Pili
  • Definition and Composition
    • Pili are short, hair-like proteinacious filaments, anchored to the bacterial cell wall, that extend only a short distance.
  • Significance
    • Pili enhance the capacity of bacteria to adhere to certain human tissues and thus improve the organism's infectivity.
Spore
  • Definition and Composition
    • Spores are small, metabolically inactive forms of bacteria that can survive for years. They contain little else beyond the bacterial genome, cell wall, and an extremely tough keratin-like outer coating.
  • Significance
    • Spores are highly resistance to environmental insults including heat, cold, radiation, dehydration, and detergents. Spores can germinate into metabolically active bacteria when exposed to a favorable environment years, decades, or even millennia later. Spores are the reason why simply boiling objects is insufficient for sterilization and instead requires autoclaving which involves heat up to 121C under intense pressures.
Exotoxins
  • Bacterial exotoxins are proteins released by certain bacteria which can severely dysregulate critical cellular processes or improve the capacity of proteins to invade tissues. The effects of particular bacterial exotoxins are discussed with the individual bacteria. Enterotoxins are a subset of exotoxins which specifically affect digestion and generally result in Infectious Diarrhea.