Interbacterial competition and anti-predatory behavior of environmental Vibrio cholerae strains

Abstract

Vibrio cholerae isolates responsible for cholera pandemics represent only a small portion of the diverse strains belonging to this species. Indeed, most V. cholerae are encountered in aquatic environments. To better understand the emergence of pandemic lineages, it is crucial to discern what differentiates pandemic strains from their environmental relatives. Here, we studied the interaction of environmental V. cholerae with eukaryotic predators or competing bacteria and tested the contributions of the hemolysin and the type VI secretion system (T6SS) to those interactions. Both of these molecular weapons are constitutively active in environmental isolates but subject to tight regulation in the pandemic clade. We showed that several environmental isolates resist amoebal grazing and that this anti-grazing defense relies on the strains T6SS and its actin-cross-linking domain (ACD)-containing tip protein. Strains lacking the ACD were unable to defend themselves against grazing amoebae but maintained high levels of T6SS-dependent interbacterial killing. We explored the latter phenotype through whole-genome sequencing of fourteen isolates, which unveiled a wide array of novel T6SS effector and (orphan) immunity proteins. By combining these in silico predictions with experimental validations, we showed that highly similar but nonidentical immunity proteins were insufficient to provide cross-immunity among those wild strains. Originality-Significance StatementThis work contributes to the understanding of phenotypic consequences that differentiate diverse Vibrio cholerae strains. We focused on the type VI secretion system (T6SS) and the pore forming toxin hemolysin, which are tightly regulated in pandemic strains but remain constitutively active in non-pandemic isolates. We unveiled diverse arrays of T6SS effector/immunity modules in a set of environmental strains by long-read whole genome sequencing and de novo assembly. These modules determine whether the strains are able to evade amoebal predation and dictate their level of compatibility or competitiveness with one another.