Sunlight-induced propagation of the lysogenic phage encoding cholera toxin

In toxigenic Vibrio cholerae, the cholera enterotoxin (CT) is encoded by CTXΦ, 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 CTXΦ, we examined the effects of temperature, pH, salinity, and exposure to direct sunlight on the induction of the CTX prophage and studied the transmission of the phage to potential recipient strains. Exposure of cultures of CTXΦ lysogens to direct sunlight resulted in ~10,000-fold increases in phage titers. Variation in temperature, pH, or salinity of the culture did not have a substantial effect on the induction of the prophage, but these factors influenced the stability of CTXΦ particles. Exposure of mixed cultures of CTXΦ lysogens and potential recipient strains to sunlight significantly increased both the in vitro and in vivo (in rabbit ileal loops) transduction of the recipient strains by CTXΦ. Included in these transduction experiments were two environmental nontoxigenic (CTXΦ^-) strains of V. cholerae O139. These two O139 strains were transduced at high efficiency by CTXΦ, and the phage genome integrated into the O139 host chromosome. The resulting CTXΦ lysogens produced biologically active CT both in vitro and in rabbit ileal loops. This finding suggests a possible mechanism explaining the origination of toxigenic V. cholerae O139 strains from nontoxigenic progenitors. This study indicates that sunlight is a significant inducer of the CTX prophage and suggests that sunlight-induced transmission of CTXΦ may constitute part of a natural mechanism for the origination of new toxigenic strains of V. cholerae.