Postharvest multiplication of Vibrio parahaemolyticus and Vibrio vulnificus in molluscan shellfish may increase human health risks. Growth and survival of V. parahaemolyticus and V. vulnificus were determined in postharvest oysters stored at 26 degrees Centigrade for 0, 5, 10, and 24 hours. Following 24 hours at 26 degrees Centigrade, oysters were transferred to 3 degrees Centigrade and analyzed 14 to 17 days later. V. parahaemolyticus densities were determined by three methods: (1) a modified most probable number procedure (MPN) using an alkaline phosphatase-labeled DNA probe (VPAP) to identify suspect isolates (MPN-VPAP); (2) a direct plating method using the same AP-labeled DNA probe (Direct-VPAP) and; (3) another direct plating method utilizing a digoxigenin-labeled probe (Direct-VPDig) to identify suspect colonies. Both DNA probes targeted the thermolabile hemolysin (tlh) gene in V. parahaemolyticus. V. vulnificus counts were determined by two direct plating methods, utilizing AP- and Dig-labeled DNA probes (Direct VVAP and Direct-VVDig), which targeted the cytolysin-hemolysin gene in V. vulnificus. Qualitative changes (opaque/translucent and lipopolysaccharide (LPS) colony types) in V. vulnificus were also monitored over time as a function of season and/or time of incubation at 26 degreesCentigrade to determine changing trends.

V. parahaemolyticus multiplied rapidly in live oysters held at 26 degrees Centrigrade; numbers increased 1.9 log10 CFU/g after 10 hours (Direct-VPAP counts, April to October). During the months of June through October, these 10 hour oyster samples exceeded FDA's limit of 10,000 total V. parahaemolyticus counts in retail shellfish. V. vulnificus densities increased 1.2 log10 CFU/g between harvest and 10 hours at 26 degrees Centigrade (Direct-VVAP counts, April to October). Experiments on qualitative changes in V. vulnificus isolates showed that opaque colonies predominated throughout the year, and LPS serotype 2 was prevalent in Gulf Coast oysters. There appeared to be a greater percentage of LPS untypeable V. vulnificus strains during the winter and after refrigerated storage. The 1/5 LPS type (most common in clinical strains) was not detected during the winter months.

Limiting the amount of time postharvest oysters remain unrefrigeerated may decrease Vibrio multiplication and possibly reducec the human health risk associated with raw oyster consumption by lowering the dose consumed.