Pathogenicity and Virulence
The ability of an organism to cause disease in a host. A binary, qualitative property: an organism is either pathogenic or not, in a given host context.
- The degree of pathogenicity, a quantitative measure of how severely or readily an organism causes disease.
- Virulence varies between strains of the same species (e.g. toxigenic versus nontoxigenic Corynebacterium diphtheriae) and is measurable experimentally, for example via LD50 (the dose required to kill 50% of an experimental host population).
| Mechanism | Function |
|---|---|
| Adhesion | Attachment to host cell surfaces or tissue, the first step in colonisation, mediated by adhesins (pili, fimbriae, surface proteins) |
| Invasion | Penetration into host cells or tissue beyond the initial site of attachment |
| Toxin production | Secretion or release of molecules that damage host cells or disrupt host physiology (full detail in the dedicated Microbial Toxins subsection) |
| Immune evasion | Strategies that avoid, resist, or subvert host immune clearance |
| Capsule formation | Antiphagocytic surface layer, masks antigens and impedes complement deposition (see Bacterial Cell Structure) |
| Intracellular survival | Ability to survive and replicate within phagocytic or nonphagocytic host cells, avoiding extracellular immune surveillance |
| Enzyme production | Secreted enzymes that degrade host tissue barriers or defence molecules, facilitating spread |
| Biofilm formation | Community-based growth conferring antimicrobial tolerance and immune evasion (full detail in the dedicated Biofilms subsection) |
| Antigenic variation | Alteration of surface antigens over time, allowing the organism to evade an immune response generated against a previous antigenic form |
- Minimum infectious dose: the smallest number of organisms required to establish infection in a susceptible host, varies enormously between organisms.
- Organisms with a very low infectious dose (e.g. Shigella species, as few as 10-100 organisms) are highly efficient at establishing infection and are associated with person-to-person and low-inoculum food/water transmission; organisms requiring a high infectious dose (e.g. Vibrio cholerae, often requiring 10^5-10^8 organisms in the absence of reduced gastric acidity) typically require heavily contaminated food or water for transmission.
- Relationship between inoculum and disease: a larger inoculum generally shortens the incubation period and increases the likelihood and severity of disease, though host factors (immune status, gastric acid, normal microbiota) substantially modify this relationship for any given exposure.
- Adhesins: surface structures or molecules mediating attachment (pili, fimbriae, specific outer membrane proteins).
- Invasins: molecules that trigger host cell uptake of the organism or otherwise facilitate tissue penetration.
- Capsules: antiphagocytic polysaccharide (occasionally polypeptide) surface layers.
- Toxins: exotoxins and endotoxin (LPS), covered in full in the dedicated Microbial Toxins subsection.
- Secretion systems: specialised molecular machinery (e.g. Gram negative Type III secretion systems) that inject effector proteins directly into host cells, manipulating host cell function to favour bacterial survival or invasion.
- Iron acquisition systems: siderophores and related molecules that scavenge iron from host iron-binding proteins (transferrin, lactoferrin), since free iron is normally kept at extremely low concentration in host tissue as a nonspecific defence mechanism (nutritional immunity); efficient iron acquisition is a virulence determinant for many pathogens.
