Impacts of Water Salinity and Type and Size of Oysters on Depuration for Reducing Vibrio parahaemolyticus in Raw Pacific Oyster (Crassostrea gigas)

Vibrio parahaemolyticus is a leading cause of gastroenteritis associated with consumption of seafood, particularly oysters. Depuration of raw oysters in seawater (30 ppt) at temperatures below 15°C for 5 days has been reported capable of reducing this organism by >3.0 log MPN/g. This study investigated effects of water salinity (10, 20, 25 and 30 ppt), oyster types (diploid and triploid) and oyster sizes (extra-small and medium) on depuration for reducing V. parahaemolyticus contamination in raw Pacific oysters (Crassostrea gigas). Raw oysters were inoculated with a mixture of five clinical strains of V. parahaemolyticus (10290, 10292, 10293, BE 98-2029 and 027-1c1) to levels of 104-6 MPN/g. The inoculated oysters were held in UV-sterilized artificial seawater in a recirculating depuration system at 12.5±1ºC for 5 days. Reductions of V. parahaemolyticus in oysters by depuration in seawater with a salinity of 10 ppt (2.2-2.6 log MPN/g) were significantly (P < 0.05) smaller than those (>3.0 log reductions) observed in oysters depurated in the water with higher salinities (20-30 ppt). Seawater with 25 ppt was chosen for studying effects of type and size of oysters on depuration for decontaminating V. parahaemolyticus as it was more commonly observed for water in the area where the oysters were cultivated. Significant two-way interactions involving depuration time and oyster type or type and size of oysters were observed. Slightly greater reductions (P < 0.05) of V. parahaemolyticus were observed in diploid (3.1 log MPN/g) than in triploid (2.8 log MPN/g) oysters after 5-day of depuration. Reductions of V. parahaemolyticus were greater in extra-small than medium diploid oysters while the pattern was reversed for triploid oysters. Water salinity and type and size of oysters may have impacts on the efficacy of depuration and need to be considered when developing a depuration process for removing V. parahaemolyticus from oysters.