| Summary: | The expression of the two major virulence genes of <i>Vibrio cholerae</i>—<i>tcpA</i> (the major subunit of the toxin co-regulated pilus) and <i>ctxAB</i> (cholera toxin)—is regulated by the ToxR regulon, which is triggered by environmental stimuli during infection within the human small intestine. Special culture methods are required to induce the expression of virulence genes in <i>V. cholerae</i> in the laboratory setting. In the present study, induction of the expression of virulence genes by two point mutations (65th and 139th amino acids) in <i>toxT</i>, which is produced by the ToxR regulon and activates the transcription of the virulence genes in <i>V. cholerae</i>, under laboratory culture conditions has been investigated. Each of the four <i>toxT</i> alleles assessed displayed different transcriptional activator functions in a given <i>V. cholerae</i> strain. Although the ToxR regulon has been known to not be expressed by El Tor biotype <i>V. cholerae</i> strains cultured under standard laboratory conditions, the variant <i>toxT</i> alleles that we assessed in this study enabled the expression virulence genes in El Tor biotype strains grown under simple culture conditions comprising shake culture in LB medium, suggesting that the regulation of virulence gene expression may be regulated more complexly than previously thought and may involve additional factors beyond the production of ToxT by the ToxR regulon.
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