Vibrio cholerae is a Gram-negative pathogen that causes a debilitating diarrheal disease that affects an estimated 5-10 million people each year. The ability of V. cholerae to cause epidemic disease is dependent on the coordinated expression of genes in vivo. Recent work has shown that many V. cholerae genes are temporally expressed in vivo. For example the genes encoding for the production of cholera toxin and the toxin coregulated pilus are induced early in infection, but repressed prior to exiting the host. Other genes are repressed early in infection and induced late in infection. One of the key questions in cholera research is how these genes are regulated in vivo. Quorum sensing has been proposed as a model for coordinate regulation of genes in vivo. However, a major shortcoming of the quorum-sensing model is that it fails to explain late gene regulation in widespread quorum sensing negative epidemic strains. In this application we present preliminary data to show that cyclic peptides function as quorum sensing independent signals that effect virulence gene regulation. We propose two specific aims to expand on these preliminary findings. In Aim 1 we will identify potent cyclic peptide inhibitors of virulence factor production and characterize their effects on virulence gene expression. In Aim 2 we will test whether the peptide inhibitors function in vivo. The completion of these aims will confirm the role of cyclic peptides in virulence gene expression and provide a novel mechanism for gene regulation during infection.