Lysogenic conversion of environmental Vibrio mimicus strains by CTX Phi

The filamentous bacteriophage CTX Phi, which encodes cholera toxin (CT) in toxigenic Vibrio cholerae, is known to propagate by infecting susceptible s trains of V. cholerae by using the toxin coregulated pilus (TCP) as its rec eptor and thereby causing the origination of new strains of toxigenic V, ch olerae from nontoxigenic progenitors. Besides V. cholerae, Vibrio mimicus s trains which are normally TCP negative have also been shown to occasionally produce CT and cause diarrhea in humans, We analyzed nontoxigenic V, mimic us strains isolated from surface waters in Bangladesh for susceptibility an d lysogenic conversion by CTX Phi, and studied the expression of CT in the lysogens by using genetically marked derivatives of the phage, Of 27 V. mim icus strains analyzed, which were all negative for genes encoding TCP but p ositive for the regulatory gene toxR, 2 strains (7.4%) were infected by CTX -Km Phi, derived from strain SM44(P27459 ctx::km), and the phage genome int egrated into the host chromosome, forming stable lysogens, The lysogens spo ntaneously produced infectious phage particles in the supernatant fluids of the culture, and high titers of the phage could be achieved when the lysog ens were induced with mitomycin C. This is the first demonstration of lysog enic conversion of V. mimicus strains by CTX Phi. When a genetically marked derivative of the replicative form of the CTX Phi genome carrying a functi onal ctxAB operon, pMSF9.2, was introduced into nontoxigenic V. mimicus str ains, the plasmid integrated into the host genome and the strains produced CT both in vitro and inside the intestines of adult rabbits and caused mild -to-severe diarrhea in rabbits. This suggested that in the natural habitat infection of nontoxigenic V, mimicus strains by wild-type CTX Phi may lead to the origination of toxigenic V. mimicus strains which are capable of pro ducing biologically active CT. The results of this study also supported the existence of a TCP-independent mechanism for infection by CTX Phi and show ed that at least one species of Vibrio other than V. cholerae may contribut e to the propagation of the phage.