Virulence genes in environmental strains of Vibrio cholerae

The virulence of a pathogen is dependent on a discrete set of genetic deter minants and their well-regulated expression. The ctxAB and tcpA genes are k nown to play a cardinal role in maintaining virulence in Vibrio cholerae, a nd these genes are believed to be exclusively associated with clinical stra ins of O1 and O139 serogroups. In this study, nle examined the presence of five virulence genes, including ctx;LB and tcpA, as well as toxR and toxT, which are involved in the regulation of virulence, in environmental strains of V. cholerae cultured from three different freshwater lakes and ponds in the eastern part of Calcutta, India. PCR analysis revealed the presence of these virulence genes or their homologues among diverse serotypes and ribo types of environmental V. cholerae strains. Sequencing of a part of the tcp A gene carried by an environmental strain showed 97.7% homology to the tcpA gene of the classical biotype of V. cholerae O1. Strains carrying the tcpA gene expressed the toxin-coregulated pilus (TCP), demonstrated by both aut oagglutination analysis and electron microscopy of the TCP pill, Strains ca rrying ctx FB genes also produced cholera toxin, determined by monosialogan glioside enzyme-linked immunosorbent assay and by passage in the ileal loop s of rabbits. Thus, this study demonstrates the presence and expression of critical virulence genes or their homologues in diverse environmental strai ns of V. cholerae, which appear to constitute an environmental reservoir of virulence genes, thereby providing gem insights into the ecology of V. cho lerae.