A Novel Vibrio cholerae Type VI Secretion System Gene Cluster is Involved in Interbacterial Competition

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Williams, Kenneth G.
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The waterborne pathogen Vibrio cholerae inhabits microbiomes on chitinous crustaceans in marine environments and within the gastrointestinal tracts of animals, including humans, after ingestion of contaminated water. All sequenced isolates of V. cholerae contain genes encoding for a Type Six Secretion System (T6SS), a harpoon-like apparatus capable of delivering toxic effectors into neighboring cells and conferring a competitive advantage in dense microbial communities. The T6SS has been primarily studied in host-derived V. cholerae isolates, which have four T6 loci: a main cluster encoding structural and regulatory components, and four auxiliary (Aux1-3) clusters that each encode an effector toxin and cognate immunity protein to avoid kin or self-intoxication. We applied a predictive algorithm we developed to the genome sequences from a set of environmental isolates and identified in two isolates a novel Aux cluster which we named Aux5. Aux5 was predicted to encode a T6 effector with homology to T6-secreted lipase of Pseudomonas aeruginosa. Due to limited genetic tractability of the environmental isolate, the Aux5 cluster was horizontally transferred onto the chromosome of a host-derived, reference V. cholerae strain that is naturally transformable on chitin. The transformed strain killed the parental reference strain in a T6SS-dependent manner. Disruption of the Aux5 effector gene (Tse5) abolished the ability of the transformed strain to kill its parent. These results demonstrate 1) the V. cholerae T6 arsenal includes a novel Aux5 effector, and 2) non-native auxiliary clusters can be acquired by natural transformation and utilized for T6-mediated competition. We predict that horizontal exchange of active T6 loci allows rapid adaptation of bacteria in host and environmental microbiomes.
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