Abstract
Vibrio parahaemolyticus is a gram-negative marine bacterium. A limited population of the organisms causes acute gastroenteritis in humans. Almost all of the clinical V. parahaemolyticus isolates exhibit beta-type hemolysis on Wagatsuma agar, known as the Kanagawa phenomenon (KP). KP is induced by the thermostable direct hemolysin produced by the organism and has been considered a crucial marker to distinguish pathogenic strains from nonpathogenic ones. Since 1996, so-called "pandemic clones," the majority of which belong to serotype O3:K6, have caused worldwide outbreaks of gastroenteritis. In this study, we used a DNA microarray constructed based on the genome sequence of a pandemic V. parahaemolyticus strain, RIMD2210633, to examine the genomic composition of 22 strains of V. parahaemolyticus, including both pathogenic (pandemic and nonpandemic) and nonpathogenic strains. More than 86% of the RIMD2210633 genes were conserved in all of the strains tested. Many variably present genes formed gene clusters on the genome of RIMD2210633 and were probably acquired through lateral gene transfer. At least 65 genes over 11 loci were specifically present in the pandemic strains compared with any of the nonpandemic strains, suggesting that the difference between pandemic and nonpandemic strains is not due to a simple genetic event. Only the genes in the 80-kb pathogenicity island (Vp-PAI) on chromosome II, including two tdh genes and a set of genes for the type III secretion system, were detected only in the KP-positive pathogenic strains. These results strongly suggest that acquisition of this Vp-PAI was crucial for the emergence of V. parahaemolyticus strains that are pathogenic for humans.