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Paper I — 2024 December (Supplementary) (2019 Scheme) — Q9

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Paper I
2024 December (Supplementary) (2019 Scheme) · 100 marks · 180 min

Question

What is virulence. List the different mechanisms of virulence in bacteria. Explain one of them in detail. (1+ 3+2)

Q96 marksShort Essays

Answer

Virulence is the degree or intensity of pathogenicity of a microorganism — a quantitative measure of how severely a given pathogen can cause disease in a susceptible host, determined by its specific virulence factors.

Mechanisms of virulence in bacteria:

  1. Adherence factors — pili/fimbriae, adhesins, and biofilm formation, allowing the organism to attach to and colonize host tissue, the essential first step in most infections.
  2. Invasion factors — enzymes (hyaluronidase, collagenase) and mechanisms enabling the organism to penetrate host tissue barriers and spread.
  3. Toxin production — exotoxins (potent, specific, often gene-encoded proteins) and endotoxin (LPS, in Gram-negative bacteria), directly damaging host cells/tissue or disrupting normal physiological function.
  4. Anti-phagocytic mechanisms — capsules and surface proteins that resist phagocytic uptake/killing by host immune cells.
  5. Antigenic variation — alteration of surface antigens to evade the developing host antibody response, permitting persistence/relapse.

Detailed explanation — Toxin production Bacterial toxins are broadly classified as exotoxins and endotoxin:

  • Exotoxins are potent, specific proteins actively secreted by living bacteria (mostly Gram-positive, some Gram-negative), acting on specific host cell targets/receptors with high potency (active in very small doses) — examples include diphtheria toxin (inhibits protein synthesis by ADP-ribosylating elongation factor-2), tetanus toxin (blocks inhibitory neurotransmitter release, causing spastic paralysis), and cholera toxin (raises intracellular cAMP, causing massive fluid secretion). Exotoxins are strongly antigenic and can be converted into toxoids (formalin-inactivated, retaining immunogenicity but not toxicity) for use in vaccines.
  • Endotoxin is the lipopolysaccharide (LPS) component, specifically its Lipid A moiety, of the outer membrane of Gram-negative bacteria, released upon bacterial lysis/death; it activates host macrophages via Toll-like receptor 4 (TLR4), triggering release of pro-inflammatory cytokines (TNF-α, IL-1), producing fever, hypotension, and, in severe Gram-negative sepsis, disseminated intravascular coagulation and septic shock. Unlike exotoxins, endotoxin is heat-stable, weakly immunogenic, and cannot be converted into a toxoid.

Together, these toxin-mediated mechanisms illustrate how a bacterium’s virulence can derive from a potent, highly specific molecular effector (exotoxin) or from a structural component that triggers a damaging host inflammatory response (endotoxin), each contributing distinctly to the severity of the resulting disease.

  1. 1General Properties of Viruses and Virus Host Interaction

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