Bacterial pathogenesis is the process by which bacteria cause disease in a host. Pathogenicity is the ability to cause disease, while virulence refers to the degree of damage inflicted. Virulence factors are molecules produced by pathogens that enable colonization, immune evasion, and tissue destruction.
Koch’s Postulates
Koch’s postulates establish the causal relationship between a microorganism and a disease: the microorganism must be found in diseased but not healthy individuals; it must be isolated and grown in pure culture; the pure culture must cause the same disease when inoculated into a healthy susceptible host; and the same microorganism must be re-isolated from the experimentally infected host.
Adhesion and Colonization
Adhesins are surface structures that mediate attachment to host cells. Pili (fimbriae) are the most common adhesive structures, including type I pili (E. coli), P pili (pyelonephritis-associated), and curli fibers. Non-pilus adhesins include surface proteins such as invasin (Yersinia), internalin (Listeria), and fibronectin-binding proteins (Staphylococcus, Streptococcus). Many bacteria form biofilms — surface-attached communities encased in an extracellular polymeric substance (EPS) matrix of polysaccharides, proteins, and DNA — which resist antibiotics and host immune defenses.
Invasion
Some pathogens invade host cells to avoid immune detection. The mechanism can be zipper-like (Listeria, Yersinia, binding host receptors) or trigger-like (Salmonella, Shigella, injecting effector proteins via type III secretion systems). Intracellular pathogens survive and replicate within host cells — Mycobacterium tuberculosis survives in macrophages by inhibiting phagosome-lysosome fusion. Invasion can cause direct tissue damage and facilitate spread to deeper tissues.
Toxins
Exotoxins are secreted proteins that cause specific damage, often at sites distant from the infection; they are immunogenic (stimulate antitoxin antibodies) and can be inactivated to form toxoids for vaccination. Types of exotoxins include A-B toxins (cholera toxin, diphtheria toxin, tetanus toxin, botulinum toxin), membrane-damaging toxins (hemolysins, leukocidins), and superantigens (toxic shock syndrome toxin, staphylococcal enterotoxins). Endotoxin (LPS) is the lipid A component of the Gram-negative outer membrane; released during bacterial lysis, it triggers fever, inflammation, and potentially septic shock through Toll-like receptor 4 (TLR4) activation.
Immune Evasion Strategies
The polysaccharide capsule inhibits phagocytosis; encapsulated pathogens include Streptococcus pneumoniae, Haemophilus influenzae type b, Neisseria meningitidis, and Klebsiella pneumoniae. Antigenic variation allows pathogens to alter surface antigens to evade antibodies — Neisseria gonorrhoeae varies pilin proteins, and influenza A varies hemagglutinin and neuraminidase. IgA protease cleaves secretory IgA at the hinge region and is produced by Neisseria, Haemophilus, and Streptococcus pneumoniae. For intracellular survival, some pathogens (Listeria, Shigella, Rickettsia) escape the phagosome and replicate in the cytosol, while others (Coxiella, Legionella) survive within modified phagosomes.
Iron Acquisition
Iron is essential for bacterial growth but is sequestered by host proteins (transferrin, lactoferrin, ferritin, hemoglobin). Siderophores are small, high-affinity iron chelators secreted by bacteria — examples include enterobactin (E. coli), pyoverdine (Pseudomonas), and mycobactin (Mycobacterium). Pathogens also utilize heme acquisition systems (HemR, HasR) and heme-binding proteins to extract iron directly from host hemoproteins.
Regulation of Virulence
Quorum sensing allows bacteria to regulate virulence gene expression in response to population density using autoinducer molecules (acyl homoserine lactones in Gram-negative, autoinducing peptides in Gram-positive). Two-component regulatory systems (e.g., PhoP/PhoQ in Salmonella, BvgS/BvgA in Bordetella) sense environmental signals and coordinate virulence gene expression. Many virulence factors are regulated by global regulators such as ToxR (Vibrio cholerae), PrfA (Listeria monocytogenes), and Agr (Staphylococcus aureus).
