Sunlight-induced propagation of the lysogenic phage encoding cholera toxin

In toxigenic Vibrio cholerae, the cholera enterotoxin (CT) is encoded by CTX 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 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 CTX Phi, lysogens to direct sunlight resulted in similar to 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 CTX Phi particles. Exposure of mixed cultures of CTX Phi 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 CTX Phi. Included in these transduction experiments were two environmental nontoxigenic (CTX Phi(-)) strains of V. cholerae O139. These two O139 strains were transduced at high efficiency by CTX Phi, and the phage genome integrated into the O139 host chromosome. The resulting CTX Phi 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 CTX Phi may constitute part of a natural mechanism for the origination of new toxigenic strains of V. cholerae.