Sunlight-induced propagation of the lysogenic phage encoding cholera toxin

In toxigenic Vibrio cholerae, the cholera enterotoxin (CT) is encoded by CT X Phi, a lysogenic bacteriophage. The propagation of this filamentous phage can result in the origination of new toxigenic strains. To understand the nature of possible environmental factors associated with the propagation of CTX Phi, we examined the effects of temperature, pH, salinity, and exposur e to direct sunlight on the induction of the CTX prophage and studied the t ransmission of the phage to potential recipient strains. Exposure of cultur es of CTX Phi, lysogens to direct sunlight resulted in similar to 10,000-fo ld increases in phage titers. Variation in temperature, pH, or salinity of the culture did not have a substantial effect on the induction of the proph age, but these factors influenced the stability of CTX Phi particles. Expos ure of mixed cultures of CTX Phi lysogens and potential recipient strains t o sunlight significantly increased both the in vitro and in vivo (in rabbit ileal loops) transduction of the recipient strains by CTX Phi. Included in these transduction experiments were two environmental nontoxigenic (CTX Ph i(-)) strains of V. cholerae O139. These two O139 strains were transduced a t high efficiency by CTX Phi, and the phage genome integrated into the O139 host chromosome. The resulting CTX Phi lysogens produced biologically acti ve CT both in vitro and in rabbit ileal loops. This finding suggests a poss ible mechanism explaining the origination of toxigenic V. cholerae O139 str ains from nontoxigenic progenitors. This study indicates that sunlight is a significant inducer of the CTX prophage and suggests that sunlight-induced transmission of CTX Phi may constitute part of a natural mechanism for the origination of new toxigenic strains of V. cholerae.