Genetic Diversity of Clinical and Environmental Vibrio parahaemolyticus Strains from the Pacific Northwest

Since 1997, cases ofVibrio parahaemolyticus -related gastroenteritis from the consumption of raw oysters harvested in Washington State have been higher than historical levels. These cases have shown little or no correlation with concentrations of potentially pathogenicV. parahaemolyticus (positive for the thermostable direct hemolysin gene,tdh ) in oysters, although significant concentrations oftdh + V. parahaemolyticus strains were isolated from shellfish-growing areas in the Pacific Northwest (PNW). We compared clinical and environmental strains isolated from the PNW to those from other geographic regions within the United States and Asia for the presence of virulence-associated genes, including the thermostable direct hemolysin (tdh ), the thermostable-related hemolysin (trh ), urease (ureR ), the pandemic group specific markersorf8 andtoxRS , and genes encoding both type 3 secretion systems (T3SS1 and T3SS2). The majority of clinical strains from the PNW were positive fortdh ,trh , andureR genes, while a significant proportion of environmental isolates weretdh + buttrh negative. Hierarchical clustering grouped the majority of these clinical isolates into a cluster distinct from that including the pandemic strain RIMD2210633, clinical isolates from other geographical regions, andtdh + ,trh -negative environmental isolates from the PNW. We detected T3SS2-related genes (T3SS2β) in environmental strains that weretdh andtrh negative. The presence of significant concentrations oftdh + ,trh -negative environmental strains in the PNW that have not been responsible for illness and T3SS2β intdh - andtrh -negative strains emphasizes the diversity in this species and the need to identify additional virulence markers for this bacterium to improve risk assessment tools for the detection of this pathogen.