Acquisition of classical CTX prophage from Vibrio cholerae O141 by El Tor strains aided by lytic phages and chitin-induced competence

The El Tor biotype of Vibrio cholerae O1, causing the current seventh pandemic of cholera, has replaced the classical biotype, which caused the sixth pandemic. The CTX prophages encoding cholera toxin in the two biotypes have distinct repressor (rstR) genes. Recently, new variants of El Tor strains that carry the classical type (CTX^class) prophage have emerged. These "hybrid" strains apparently originate through lateral gene transfer and recombination events. To explore possible donors of the CTX^class prophage and its mode of transfer, we tested environmental V. cholerae isolates for the presence of CTX^class prophage and mobility of the phage genome. Of the 272 environmental V. cholerae isolates tested, 6 were found to carry the CTX^class prophage; all of these belonged to the O141 serogroup. These O141 strains were unable to produce infectious CTX ^class phage or to transmit the prophage to recipient strains in the mouse model of infection; however, the CTX^class prophage was acquired by El Tor strains when cultured with the O141 strains in microcosms composed of filtered environmental water, a chitin substrate, and a V. cholerae O141-specific bacteriophage. The CTX^class prophage either coexisted with or replaced the resident CTXET prophage, resulting in El Tor strains with CTX genotypes similar to those of the naturally occurring hybrid strains. Our results support a model involving phages and natural chitin substrate in the emergence of new variants of pathogenic V. cholerae. Furthermore, the O141 strains apparently represent an alternative reservoir of the CTX^class phage genome, because the classical V. cholerae O1 strains are possibly extinct.