On the origin and function of an insertion element VPaI-1 specific to post-1995 pandemic Vibrio parahaemolyticus strains.

Since 1995, new virulent strains of Vibrio parahaemolyticus have emerged and spread throughout the world. These "pandemic" strains have four strain specific genomic islands (GIs), which are considered to be potential factors of the pandemicity. We investigated the origin and function of 24 genes in the so-called VPaI-1, one of the four GIs, by searching homologs in various species in Bacteria and Archaea. Of these 24 genes, two are found only in Vibrio vulnificus CMCP6 and Shewanella sp. MR-7. The genomic segment (- 8 kb) encompassing the two genes shows the synteny among the three species. Since many of the Shewanella species can grow at 4 degrees C, these two genes may be candidates of adaptation to temperature stress. Further, we found a candidate for a swarming gene, which is reported as the V. cholerae virulence gene. Based on these findings, we hypothesized the emergence of pandemicity and discuss the mechanism for how these strains spread throughout the world.