Pandemic Vibrio cholerae acquired competitive traits from an environmental Vibrio species.

is a human pathogen that thrives in estuarine environments. Within the environment and human host, uses the type VI secretion system (T6SS) to inject toxic effectors into neighboring microbes and to establish its replicative niche. strains encode a wide variety of horizontally shared effectors, but pandemic isolates encode an identical set of distinct effectors. Effector set retention in pandemic strains despite mobility between disparate strains suggests that horizontal acquisition of these effectors was crucial for evolving pandemic We attempted to locate the donor of the pandemic effectors to To this end, we identified potential gene transfer events of the pandemic-associated T6SS clusters between a fish pathogen, , and We supported the likelihood of interaction between these species by demonstrating that homologous effector-immunity pairs from and can cross-neutralize one another. Thus, constitutes an environmental reservoir of pandemic-associated T6SS effectors that may have initially facilitated competition between pre-pandemic and for an environmental niche.