Growth and survival of Vibrio parahaemolyticus in postharvest American oysters

Oysters at the retail stage of distribution generally contain greater densities of Vibrio parahaemolyticus than do oysters at harvest. The objective of this study was to determine the effects of postharvest storage at 26 and 3 degrees C on the growth and survival of naturally occurring V. parahaemolyticus in shellstock American oysters (Crassostrea virginica). Oysters were collected monthly from May 1998 through April 1999 from Mobile Bay, Alabama, and their V. parahaemolyticus densities were determined after 0, 5, 10, and 24 h of postharvest storage at 26 degrees C. After 24 h of storage at 26 degrees C, oysters were transferred to a refrigerator at 3 degrees C and analyzed 14 to 17 days later. V. parahaemolyticus numbers were determined by a direct plating method involving an alkaline-phosphatase-labeled DNA probe that targets the species-specific thermolabile hemolysin gene (tlh-AP) to identify suspect isolates. From April to December, when water temperatures at harvest were >20 degrees C, the geometric mean harvest density of V. parahaemolyticus was 130 CFU/g. When water temperatures were <20 degrees C, the geometric mean harvest density was 15 CFU/g. After harvest, V. parahaemolyticus multiplied rapidly in live oysters held at 26 degrees C, showing a 50-fold increase (1.7 log CFU/g) at 10 h and a 790-fold increase (2.9 log CFU/g) at 24 h (April through December). Average V. parahaemolyticus numbers showed a sixfold decrease (0.8 log CFU/g) after approximately 14 days of refrigeration. These results indicate that V. parahaemolyticus can grow rapidly in unrefrigerated oysters.