Genome plasticity in Vibrio species – how lateral gene transfer directs niche specialisation and pathogenicity
Bacteria of the Vibrio genus are abundant in aquatic environments, fulfil important nutrient cycling roles and are often found in association with marine animals such as corals, molluscs, sponges, crustaceans and fish. This diversity in niche occupation is a feature of Vibrio species and is largely (or at least in significant part) driven by lateral gene transfer (LGT). LGT is a two-step process firstly requiring physical transfer of DNA from one bacterial cell to another followed by subsequent integration of the transferred DNA into the genome thus allowing stable inheritance and expression. Numerous mechanisms for integration exist such as homologous recombination and a wide-range of diverse genetic elements such as transposons, integrative conjugative elements, prophages and integrons. Using two Vibrios species, V. rotiferianus and V. cholerae, research in our laboratory has sought to understand how mobile DNA contributes to vibrio evolution, niche specialisation and pathogenicity. In V. rotiferianus, we have been researching the integron, a genetic element that contributes up to 3% of a vibrios genome in laterally acquired mobile DNA. This region is dynamic and evolves at a faster rate than mutation. Our research has shown that this region provides the organism a mechanism for reworking surface polysaccharide affecting biofilm formation and potentially interactions with higher organisms. Furthermore, we have been researching evolution in V. cholerae isolated from Sydney estuarine waters. These isolates are pathogenic in animal models with all lacking the usual virulence factors of cholera toxin and colonisation factor tcpA and most lacking type III secretion indicating the presence of novel virulence factors. One isolate we are focussing on contains a novel 32-kb mobile element inserted into the indigenous recA but carries a unique recA with only 80% identity to other V. cholerae recA genes. Overall, our research demonstrates the plasticity of vibrio genomes and the significant contribution that LGT makes to the continuing evolution to the Vibrio genus.