Pathogenicity and virulence
Pathogenicity is whether a microbe can cause disease; virulence is how severe that disease is, often compared by LD50 or ID50. Outcomes also depend on the host's immune status, as in opportunistic infections.
Virulence factors and infection
- Adherence: pili and adhesins attach to host cells.
- Invasion and spread: enzymes such as hyaluronidase break through tissue.
- Immune evasion: capsules resist phagocytosis, and some microbes vary their antigens.
- Toxins: secreted protein exotoxins (tetanus, botulinum, diphtheria) and the lipopolysaccharide endotoxin of Gram-negative bacteria damage tissue.
Clinically, disease passes through incubation, prodromal, illness and convalescent stages. The chain of infection runs from agent and reservoir through exit, transmission (contact, droplet, airborne, vector) and entry to a susceptible host; breaking a link is the basis of infection control.
Koch's postulates
Robert Koch's criteria hold that a microbe causes a disease if it is found in every case, can be grown in pure culture, causes the disease when introduced into a healthy host, and can be re-isolated. Unculturable organisms and carriers led to molecular versions of these criteria.
References
- Tortora, G. J., Funke, B. R., & Case, C. L. (2021). Microbiology: An Introduction (13th ed.). Pearson.
- Casadevall, A., & Pirofski, L. (1999). Host-pathogen interactions: Redefining the basic concepts of virulence and pathogenicity. Infection and Immunity, 67(8), 3703-3713.
- Fredricks, D. N., & Relman, D. A. (1996). Sequence-based identification of microbial pathogens: A reconsideration of Koch's postulates. Clinical Microbiology Reviews, 9(1), 18-33.