Microbial Mechanisms of Pathogenicity Notes

Microbial Mechanisms of Pathogenicity

Key Concepts of Pathogenicity

Pathogenicity refers to the ability of a microorganism to cause disease. Microbes follow a series of steps to infect a host, which includes entering through portals, evading host defenses, damaging host cells, and exiting to infect new hosts. The common portals of entry include mucous membranes (respiratory, gastrointestinal, genitourinary tracts), skin, and parenteral routes. After entering, pathogens can penetrate or evade host defenses using various mechanisms such as capsules, enzymatic actions, and antigenic variation.

Virulence Factors (VFs)

Virulence factors are microbial components that help establish, disseminate, and maintain disease. They can be categorized into:

  1. Structural Components: Examples include fimbriae, pili, and the glycocalyx, which facilitate adherence to host cells.

  2. Extracellular Enzymes: Including coagulases that coagulate fibrinogen, hyaluronidases that hydrolyze hyaluronic acid, collagenases that degrade collagen, and IgA proteases that destroy IgA antibodies.

  3. Toxins: They can be categorized as exotoxins and endotoxins. Exotoxins, produced by gram-positive bacteria, can act as enzymes or other active biological agents, whereas endotoxins, found in the outer membrane of gram-negative bacteria, can provoke strong inflammatory responses in the host.

Capsules and Cell Wall Components
  • Capsules: These structures prevent phagocytosis by the host's immune cells. For instance, Bacillus anthracis has a capsule that protects it from phagocytes.
  • Cell Wall components such as M protein in Streptococcus pyogenes resist phagocytosis, while the Opa protein in Neisseria gonorrhoeae inhibits the immune response. The waxy lipid mycolic acid in Mycobacterium tuberculosis allows it to resist intracellular digestion.
Invasion Mechanisms
  • Pathogens have evolved various proteins to invade host cells. For example, Salmonella employs invasion proteins that modify the host cell’s actin, resulting in membrane ruffling, through which the pathogen is engulfed.
  • Type III Secretion System: This is a specialized mechanism through which bacteria inject proteins directly into host cells to facilitate invasion and manipulation of host cell functions.
Antigenic Variation

Antigenic variation allows pathogens to evade the immune response by altering surface proteins. This can lead to successful infection even in previously exposed hosts. Notable examples include:

  • Antigenic Drift: This occurs through small accumulative mutations in viral RNA, leading to new viral strains each flu season. Vaccinations must adapt accordingly.
  • Antigenic Shift: This major genetic change happens when a virus strain mixes RNA from different strains, often jumping between species (e.g., from birds to humans) and can result in pandemics.
Discussion Points to Consider
  1. Describe three examples of how pathogens can evade or penetrate host defenses.
  2. Consider why vaccines against the flu do not provide long-term protection, while many others can last for years.
  3. Discuss how Koch's postulates might be modified to demonstrate that a bacterial gene is a virulence factor.