Background: Of the hundreds of Vibrio cholerae serogroups, O1 and O139 are the main epidemic-causing ones. Although non-O1/non-O139 serogroups rarely cause epidemics, the possibility exists for strains within them to have pathogenic potential.
Results: We selected 25 representative strains within 16 V. cholerae serogroups and examined their genomic and functional characteristics. We tentatively constructed a gene pool containing 405 homologous gene clusters, which is well organized and functions in O-antigen polysaccharide (O-PS) synthesis. Our network analysis indicate that great diversity exists in O-PS among the serogroups, and several serogroup pairs share a high number of homologous genes (e.g., O115 and O37; O170 and O139; O12 and O39). The phylogenetic analysis results suggest that a close relationship exists between serogroups O170, O89 and O144, based on neighbor-joining (NJ) and gene trees, although serogroup O159 showed an inconsistent phylogenetic relationship between the NJ tree and the gene tree, indicating that it may have undergone extensive recombination and horizontal gene transfer. Different phylogenetic structures were observed between the core genes, pan genes, and O-PS genes. The virulence gene analysis indicated that the virulence genes from all the representative strains may have their sources from four particular bacteria (Pseudomonas aeruginosa, V. vulnificus, Haemophilus somnus and H. influenzae), which suggests that V. cholerae may have exchanged virulence genes with other bacterial genera or species in certain environments. The mobile genetic element analysis indicated that O159 carries nearly complete VSP-II and partial VPI-1 and VPI-2, O170 carries partial VPI-1 and VPI-2, and several non-O1/non-O139 strains contain full or partial VPI-1 and VPI-2. Several genes showing evidence of positive selection are involved in chemotaxis, Na + resistance, or cell wall synthesis, suggestive of environmental adaptation.
Conclusions: This study reports on the newly sequenced O159 and O170 genomes and their comparisons with other V. cholerae serogroups. The complicated O-PS network of constituent genes highlights the detailed recombination mechanisms that have acted on the serogroups' genomes. The serogroups have different virulence-related gene profiles, and there is evidence of positive selection acting on other genes, possibly during adaptation to different environments and hosts.
Keywords: Selection pressure; Serogroup; Vibrio cholerae; Virulence-related gene.