Pandemic Vibrio cholerae Acquired Competitive Traits from an Environmental Vibrio Species

BackgroundVibrio cholerae, the causative agent of cholera, is a human pathogen that thrives in estuarine environments. V. cholerae competes with neighboring microbes by the contact-dependent translocation of toxic effectors with the type VI secretion system (T6SS). Effector types are highly variable across V. cholerae strains, but all pandemic isolates encode the same set of distinct effectors. It is possible that acquisition of these effectors via horizontal gene transfer played a role in the development of pandemic V. cholerae. ResultsWe assessed the distribution of V. cholerae T6SS loci across multiple Vibrio species. We showed that the fish-pathogen V. anguillarum encodes all three V. cholerae core loci as well as two of the four additional auxiliary clusters. We further demonstrated that V. anguillarum shares T6SS effectors with V. cholerae, including every pandemic-associated V. cholerae effector. We identified a novel T6SS cluster (Accessory Aux1) that is widespread in V. anguillarum and encodes the pandemic V. cholerae effector TseL. We highlighted potential gene transfer events of Accessory Aux1 from V. anguillarum to V. cholerae. Finally, we showed that TseL from V. cholerae can be neutralized by the V. anguillarum Accessory Aux1 immunity protein and vice versa, indicating V. anguillarum as the donor of tseL to the V. cholerae species. ConclusionsV. anguillarum constitutes an environmental reservoir of pandemic-associated V. cholerae T6SS effectors. V. anguillarum and V. cholerae likely share an environmental niche, compete, and exchange T6SS effectors. Further, our findings highlight the fish as a potential reservoir of pandemic V. cholerae.