Alternative mechanism of cholera toxin acquisition by Vibrio cholerae: generalized transduction of CTXPhi by bacteriophage CP-T1.

Horizontal transfer of genes encoding virulence factors has played a central role in the evolution of many pathogenic bacteria. The unexpected discovery that the genes encoding cholera toxin (ctxAB), the main cause of the profuse secretory diarrhea characteristic of cholera, are encoded on a novel filamentous phage named CTXΦ, has resulted in a renewed interest in the potential mechanisms of transfer of virulence genes among Vibrio cholerae. We describe here an alternative mechanism of cholera toxin gene transfer into nontoxigenic V. cholerae isolates, including strains that lack both the CTXΦ receptor, the toxin coregulated pilus (TCP), and attRS, the chromosomal attachment site for CTXΦ integration. A temperature-sensitive mutant of the V. cholerae generalized transducing bacteriophage CP-T1 (CP-T1ts) was used to transfer a genetically marked derivative of the CTX prophage into four nontoxigenic V. cholerae strains, including two V. cholerae vaccine strains. We demonstrate that CTXΦ transduced by CP-T1ts can replicate and integrate into these nontoxigenic V. cholerae strains with high efficiency. In fact, CP-T1ts transduces the CTX prophage preferentially when compared with other chromosomal markers. These results reveal a potential mechanism by which CTXΦ+ V. cholerae strains that lack the TCP receptor may have arisen. Finally, these findings indicate an additional pathway for reversion of live-attenuated V. cholerae vaccine strains.