Pandemic Vibrio cholerae shuts down site-specific recombination to retain an interbacterial defence mechanism

Vibrio cholerae is an aquatic microbe that can be divided into three subtypes: harmless environmental strains, localised pathogenic strains, and pandemic strains causing global cholera outbreaks. Each type has a contact-dependent type VI secretion system (T6SS) that kills neighbouring competitors by translocating unique toxic effector proteins. Pandemic isolates possess identical effectors, indicating that T6SS effectors may affect pandemicity. Here, we show that one of the T6SS gene clusters (Aux3) exists in two states: a mobile, prophage-like element in a small subset of environmental strains, and a truncated Aux3 unique to and conserved in pandemic isolates. Environmental Aux3 can be readily excised from and integrated into the genome via site-specific recombination, whereas pandemic Aux3 recombination is reduced. Our data suggest that environmental Aux3 acquisition conferred increased competitive fitness to pre-pandemic V. cholerae, leading to grounding of the element in the chromosome and propagation throughout the pandemic clade.Vibrio cholerae uses a type VI secretion system (T6SS) to kill neighbouring competitors. Here, Santoriello et al. show that a T6SS gene cluster (Aux3) exists as a mobile, prophage-like element in some environmental strains, and as a stable truncated form in pandemic isolates. They propose that Aux3 acquisition increased competitive fitness of pre-pandemic V. cholerae.