Genetics of type VI secretion and natural transformation in Vibrio cholerae

The facultative waterborne pathogen Vibrio cholerae transitions between its human host and the environment where it colonizes chitinous surfaces in aquatic settings. Growth on chitin coordinates the induction of sets of genes for 1) chitin utilization; 2) a type VI secretion system that allows contact-dependent killing of neighboring bacteria; and 3) DNA uptake by natural transformation, which is a mechanism for horizontal gene transfer. This thesis describes the regulatory network controlling these behaviors in V. cholerae and the consequences of their coordinate regulation. Results from high-throughput RNA sequencing (RNA-seq) show that transcription factor CytR is one of four positive regulators comprising the chitin-induced regulatory network. A combination of genetic and phenotypic assays reveal the four regulators TfoX, HapR, QstR and CytR control each behavior in a distinct manner in a commonly used clinical reference strain of V. cholerae. Whole genome sequencing and bioinformatics analyses of a set of strains isolated from diverse sources reveal novel type VI secretion system components present in environmental, but not clinical isolates. Finally, I show that chitin-induced natural transformation can facilitate horizontal gene transfer of distinct type VI secretion system genes between strains. Horizontally acquired effector-immunity proteins are functional in the new genetic background and can be employed in antibacterial antagonism against parental cells and simultaneously protect against attacks by the donor cells. This thesis sheds light on diverse behavioral adaptations that allow this important human pathogen to spread and persist in the environment.Ph.D