Vibrio cholerae OmpR represses the ToxR regulon in response to membrane intercalating agents that are prevalent in the human gastrointestinal tract

Multidrug efflux systems belonging Resistance-Nodulation-Division (RND) superfamily are ubiquitous in Gram-negative bacteria. RND efflux systems are often associated with multiple antimicrobial resistance but also contribute to the expression of diverse bacterial phenotypes including virulence, as documented in the intestinal pathogen Vibrio cholerae, the causative agent of the severe diarrheal disease cholera. Transcriptomic studies with RND efflux-negative V. cholerae suggested that RND-mediated efflux was required for homeostasis, as loss of RND efflux resulted in the activation of transcriptional regulators, including multiple environmental sensing systems. In this report we investigated six RND efflux responsive regulatory genes for contributions to V. cholerae virulence factor production. Our data showed that V. cholerae gene VC2714, encoding a homologue of Escherichia coli OmpR, was a virulence repressor. The expression of ompR was elevated in an RND-null mutant and ompR deletion partially restored virulence factor production in the RND-negative background. Virulence inhibitory activity in the RND-negative background resulted from OmpR repression of the key ToxR regulon virulence activator aphB, and ompR overexpression in WT cells also repressed virulence through aphB. We further show that ompR expression was not altered by changes in osmolarity, but instead was induced by membrane intercalating agents that are prevalent in the host gastrointestinal tract, and which are substrates of the V. cholerae RND efflux systems. Our collective results indicate that V. cholerae ompR is an aphB repressor and regulates the expression of the ToxR virulence regulon in response to novel environmental cues.