Microbial Virulence

The development of infectious disease requires a convergence of three factors: microbial virulence, host susceptibility, and environmental conditions. Virulence is the measure of a pathogen's capacity to cause infection in a susceptible host, distinct from pathogenicity, which is the general ability to cause disease. Pathology arises from both microbe-mediated damage (virulence factor expression) and host-mediated damage from excessive or inappropriately sustained immune responses.

Obligate pathogens possess specialised virulence factors that enable infection in healthy hosts. They cause disease in most individuals regardless of immune status. In contrast, opportunistic pathogens have low intrinsic virulence and cause infection primarily in immunocompromised hosts or when introduced into normally sterile body sites. Pseudomonas aeruginosa exemplifies an opportunistic pathogen: a gram-negative, rod-shaped organism found in soil and water, transiently colonising the skin and gut of 3 to 5% of the population. It carries very high mortality when causing hospital-acquired infection in susceptible patients.

Virulence factors are gene products that enable colonisation, invasion, and evasion of host defences. Key categories include adhesins (e.g., pili, fimbriae, capsules) that promote attachment to epithelial surfaces; invasins (tissue-destroying enzymes) that enable deeper invasion; toxins that damage host cells directly; capsules that resist phagocytosis and complement; motility structures (flagella); and biofilms that protect against antibiotics and immune clearance.

Influenza A demonstrates virulence factor adaptability: haemagglutinin (HA) attaches to sialic acid residues on respiratory epithelial cells (adhesin function), while neuraminidase (NA) cleaves sialic acid to release virions from cells. Antigenic drift occurs through RNA replication errors producing mutations in HA or NA, reducing antibody recognition. Antigenic shift, which can generate pandemic strains, results from genetic reassortment during simultaneous infection of a host with multiple influenza A strains.

Antimicrobial resistance further amplifies virulence. Bacteria acquire resistance genes through horizontal gene transfer via three mechanisms: transduction (bacteriophage-mediated), conjugation (plasmid transfer through a pilus), and transformation (uptake of free environmental DNA). Any antibiotic use creates selection pressure favouring resistant strains, underscoring the importance of antibiotic stewardship.