In Vibrio cholerae, the genes encoding cholera toxin are carried on which genetic element?

Introduction:
Cholera toxin (CTX) is the major virulence factor of toxigenic Vibrio cholerae, responsible for secretory diarrhea via increased cyclic AMP in enterocytes.

Given Data / Assumptions:

• We identify the genetic element that carries ctxA/ctxB genes.

Concept / Approach:
The ctxAB genes are encoded by a lysogenic filamentous bacteriophage known as CTXφ, which integrates into the V. cholerae chromosome but is phage-derived in origin.

Step-by-Step Solution:
Recall virulence regulation in V. cholerae. Recognize CTXφ as the carrier of ctxAB. Select the option indicating a bacteriophage.

Verification / Alternative check:
Molecular studies show phage-mediated horizontal transfer of ctx genes among vibrios.

Why Other Options Are Wrong:

• Plasmid / Transposon: Not the primary vehicle for ctxAB in V. cholerae.
• Chromosome (native genes): Although integrated, the origin is phage, not native chromosomal coding.
• Mitochondrial-like element: Bacteria lack mitochondria.

Common Pitfalls:
Confusing integration site (chromosome) with the genetic origin (phage).

Final Answer:
Cholera toxin genes are carried by a bacteriophage (CTXφ).