Question
Describe various virulence factors in bacteria with examples.
Answer
Virulence factors are the bacterial attributes that determine the degree of pathogenicity — enabling attachment, invasion, evasion of host defences, and tissue damage.
Adherence factors — pili/fimbriae (E. coli P-fimbriae bind uroepithelium, causing pyelonephritis), other adhesins (Streptococcus pyogenes M protein and lipoteichoic acid bind pharyngeal epithelium), and biofilm formation (Pseudomonas aeruginosa on catheters/prosthetic devices, Staphylococcus epidermidis on implants) resist clearance and antibiotic penetration.
Anti-phagocytic factors — polysaccharide capsules (Streptococcus pneumoniae, Klebsiella pneumoniae, Neisseria meningitidis) resist complement deposition and phagocytic uptake; Protein A of Staphylococcus aureus binds the Fc portion of IgG, blocking opsonization.
Invasive enzymes — hyaluronidase (“spreading factor,” breaks down connective-tissue matrix), collagenase (Clostridium perfringens, tissue destruction in gas gangrene), coagulase (S. aureus, forms a protective fibrin wall around the organism), streptokinase/fibrinolysin (dissolves clots, promotes spread), IgA protease (N. gonorrhoeae, N. meningitidis, H. influenzae — cleaves mucosal secretory IgA).
Exotoxins — potent, gene-encoded proteins with specific mechanisms: diphtheria toxin (ADP-ribosylates EF-2, halts protein synthesis), cholera toxin (ADP-ribosylates Gs protein, raises cAMP, causes secretory diarrhoea), tetanus toxin/botulinum toxin (block neurotransmitter release), pyrogenic superantigens of S. aureus/S. pyogenes (nonspecific, massive T-cell activation — toxic shock).
Endotoxin (LPS) — the lipid A component of the Gram-negative outer membrane, released on bacterial lysis, activates macrophages via TLR4 to release TNF-α and IL-1, producing fever, hypotension, and disseminated intravascular coagulation in Gram-negative sepsis.
Antigenic variation — Neisseria gonorrhoeae pilin and Borrelia recurrentis surface proteins alter their antigenic structure to evade the developing antibody response, permitting relapse/persistence.
Iron-acquisition systems — siderophores (e.g. enterobactin) scavenge host-bound iron, essential for bacterial growth in the iron-restricted host environment.