El Tor biotype of Vibrio cholerae O1 differs from the Classical biotype in several respects. Which of the following is the most clinically relevant virulence factor carried by the El Tor strain?
- A CTX phage-encoded cholera toxin (CT) — identical in both biotypes
- B Zonula occludens toxin (ZOT) increasing intestinal permeability exclusively in El Tor
- C TCP (toxin-coregulated pili) — present only in El Tor biotype as colonisation factor
- D El Tor haemolysin (El Tor HlyA) — a pore-forming cytolysin contributing to diarrhoea ✓
Explanation
The El Tor biotype of V. cholerae O1 differs from the Classical biotype in several ways: El Tor is more environmentally hardy, persists in water, and caused the 7th pandemic. El Tor produces haemolysin (HlyA, or 'El Tor haemolysin') — a pore-forming cytolysin that lyses sheep RBCs but not goat RBCs (Greig's test positive for El Tor, negative for Classical). This haemolysin contributes to intestinal colonisation and diarrhoea pathogenesis, though cholera toxin (CT) — encoded by CTXφ — is the primary virulence factor in both biotypes and is identical. TCP (toxin-coregulated pili) is present in both biotypes as the essential colonisation factor and receptor for CTXφ. Variant El Tor strains (Mozambique, Haiti) now produce classical CT associated with more severe disease.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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