Origin of Vibrio parahaemolyticus O3:K6 pandemic clone.

O3:K6 pandemic clone of Vibrio parahaemolyticus has caused outbreaks in coastal countries since 1996. Mutilocus sequence typing (MLST) is an important tool to trace the source and analysis the evolution of bacteria. Based on MLST, the first pandemic clonal complex (CC) of V. parahaemolyticus has been confirmed. In this study, 57 pandemic strains, 27 pathogenic strains (tdh or trh positive) and 36 nonpathogenic strains isolated from China were analyzed with MLST. Forty-seven unique sequence types, one clonal complex (CC) and one doublet (D) were identified by eBURST and Mega4 analyses. CC corresponded to not only the known O3:K6 pandemic clone (including ST-3, ST-192, ST-227) but nonpathogenic clone (including ST-3, S-T2, ST-196, ST-220, ST-226). ST-3 was the founder of the complex. STs of the isolates were not inevitably associated with the presence or number of the accessory genes or the serotypes of the isolates. The ancestor strain of O3:K6 pandemic clone was originated from an environmental nonpathogenic O3:K6, ST-3 strain. The pandemic O3:K6 clone was developed from this strain in approximately 1996 by laterally transferring large fragments of genes including systematic functional genes and genomic islands.