Molecular mechanisms of virstatin resistance by non-O1/non-O139 strains of Vibrio cholerae

Virstatin is a previously described small molecule inhibitor of Vibrio cholerae virulence. We have demonstrated that the molecule inhibits the activity of the transcriptional activator ToxT, thereby preventing elaboration of the toxin co-regulated pilus (TCP) and cholera toxin in vitro and in vivo in O1 strains of V. cholerae. While strains of the O1 and O139 serogroups are the cause of most epidemic and endemic cholera currently seen globally, sporadic disease caused by strains of non-O1/non-O139 serogroups suggests that understanding the pathogenic mechanisms of these unusual strains is relevant for disease. Although some non-O1/non-O139 strains have acquired the pathogenicity island that encodes the TCP, the role that this essential colonization factor of O1/O139 strains plays in the virulence of non-O1/non-O139 strains has not been determined. In this study, we utilize virstatin in a 'chemical genetic approach' to examine the role of ToxT, and thus by inference TCP, in the colonization of a panel of predominantly non-O1/non-O139 tcp+ strains. We identified nine strains whose colonization was resistant to virstatin inhibition in the infant mouse model. These strains presumably colonize by a TCP-independent mechanism or contain a naturally occurring virstatin-resistant ToxT. Four strains contained the typical toxT gene found in O1/O139 strains (toxT(EPI)) isolated from cholera epidemics. Interruption of toxT in one of these strains did not affect colonization of the infant mouse small intestine. The remaining five strains were found to contain a sequence divergent toxT gene that has been previously designated toxT(ENV) because of its occurrence in isolates of V. cholerae from the environment. We show that ToxT(ENV) is resistant to virstatin in two separate heterologous systems and is necessary for efficient colonization of the infant mouse small intestine. These results support the new concept that chemical genetic probes for the in vivo function or expression of virulence genes can be used to identify strains that express alternative virulence factors or novel regulatory systems that are functional in vivo.