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

Horizontal transfer of genes encoding virulence factors has played a centra l role in the evolution of many pathogenic bacteria. The unexpected discove ry that: the genes encoding cholera toxin (ctxAB), the main cause of the pr ofuse secretory diarrhea characteristic of cholera, are encoded on a novel filamentous phage named CTX Phi, 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 in to nontoxigenic V. cholerae isolates, including strains that lack both the CTX Phi receptor, the toxin coregulated pilus (TCP), and attRS, the chromos omal attachment site for CTX Phi, integration. A temperature-sensitive muta nt of the V. cholerae generalized transducing bacteriophage CP-TI (CP-T1ts) was used to transfer a genetically marked derivative of the CTX prophage i nto four nontoxigenic V. cholerae strains, including two V. cholerae vaccin e strains. We demonstrate that CTX Phi, transduced by CP-T1ts can replicate and integrate into these nontoxigenic V. cholerae strains with high effici ency. In Fact, CP-T1ts transduces the CTX prophage preferentially when comp ared with other chromosomal markers. These results reveal a potential mecha nism hy which CTX Phi(+) V. cholerae strains that lack the TCP receptor may have arisen. Finally, these findings indicate an additional pathway for re version of Live-attenuated V. cholerae vaccine strains.