Regulation of the type VI secretion system in environmental isolates of vibrio cholerae
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Vibrio cholerae is a human pathogen that causes the severe diarrheal disease cholera, but can also inhabit aquatic environments. The type-VI secretion system (T6SS) is a macromolecular contractile machine that injects neighboring cells with cytotoxic effector proteins. Clinical strains of V. cholerae express the T6SS only when exposed to high cell density and starvation conditions in the presence of chitin, a process regulated by the master regulator QstR. The atypical clinical strain V52 expresses its T6SS constitutively, a trait shared by many V. cholerae strains isolated from the environment. Recently it was discovered that the TfoY regulator controls T6SS expression independent of QstR in V52. In examining strains from environmental sources, I found that one constitutive environmental strain is also under TfoY control. However, I also uncovered that T6SS-mediated constitutive killing in four additional environmental strains was unaffected by a tfoY deletion. Furthermore, I demonstrated that other known regulators (TfoX, QstR, OscR) also played no role in T6SS expression in these strains. For example, the environmental strain BGT69 remains capable of T6SS-mediated killing when these four known T6SS regulators were deleted. These results suggest the presence of a novel regulatory pathway(s) for type-VI secretion in this and other environmental strains of V. cholerae.