Induction of the lysogenic phage encoding cholera toxin in naturally occurring strains of toxigenic Vibrio cholerae O1 and O139

In toxigenic Vibrio cholerae, the CTX genetic element which carries the genes for cholera toxin (CT) is the genome of a lysogenic bacteriophage (CTX-phi). Clinical and environmental strains of V. cholerae O1 or O139 and stools that were culture positive for cholera were analyzed to study the induction and transmission of CTX-phi. To our knowledge, this is the first report of the examination of CTX-phi in clinical materials and in naturally occurring strains. DNA probe analysis revealed that 4.25% (6 of 141) of the isolated V. cholerae strains spontaneously produced a detectable level of extracellular CTX-phi particles in the culture supernatants whereas another 34.04% (48 of 141) produced CTX-phi particles when induced with mitomycin C.CTX-phi isolated from 10 clinical or environmental strains infected a CT-negative recipient strain, CVD103, both inside the intestines of infant mice and under laboratory conditions. All culture-positive stools analyzed were negative for the presence of CTX-phi both in the DNA probe assay and by in vivo assay for the infection of the recipient strain in infant mice. These results suggested that naturally occurring strains of toxigenic V. cholerae are inducible lysogens of CTX-phi but that cholera pathogenesis in humans is not associated with the excretion of CTX-phi particles in stools, indicating that induction of the phage may not occur efficiently inside the human intestine. However, in view of the efficient transmission of the phage under conditions conducive to the expression of toxin-coregulated pili, it appears that propagation of CTX-phi in the natural habitat may involve both environmental and host factors.