Sialic Acid Catabolism and Transport Gene Clusters Are Lineage Specific in Vibrio vulnificus

Sialic or nonulosonic acids are nine-carbon alpha ketosugars that are present in all vertebrate mucous membranes. Among bacteria, the ability to catabolize sialic acid as a carbon source is present mainly in pathogenic and commensal species of animals. Previously, it was shown that severalVibrio species carry homologues of the genes required for sialic acid transport and catabolism, which are genetically linked. InVibrio cholerae on chromosome I, these genes are carried on theVibrio pathogenicity island-2 region, which is confined to pathogenic isolates. We found that among the three sequencedVibrio vulnificus clinical strains, these genes are present on chromosome II and are not associated with a pathogenicity island. To determine whether the sialic acid transport (SAT) and catabolism (SAC) region is universally present withinV. vulnificus , we examined 67 natural isolates whose phylogenetic relationships are known. We found that the region was present predominantly among lineage I ofV. vulnificus , which is comprised mainly of clinical isolates. We demonstrate that the isolates that contain this region can catabolize sialic acid as a sole carbon source. Two putative transporters are genetically linked to the region inV. vulnificus , the tripartite ATP-independent periplasmic (TRAP) transporter SiaPQM and a component of an ATP-binding cassette (ABC) transporter. We constructed an in-frame deletion mutation insiaM , a component of the TRAP transporter, and demonstrate that this transporter is essential for sialic acid uptake in this species. Expression analysis of the SAT and SAC genes indicates that sialic acid is an inducer of expression. Overall, our study demonstrates that the ability to catabolize and transport sialic acid is predominately lineage specific inV. vulnificus and that the TRAP transporter is essential for sialic acid uptake.