Survey of postharvest-processed oysters in the United States for levels of Vibrio vulnificus and Vibrio parahaemolyticus.

From June through October 2004, the U.S. Food and Drug Administration collected oysters (61 samples) that had been subjected to postharvest processing (PHP) methods, including mild heat treatment, freezing, and high hydrostatic pressure, from processors and retail markets in various states to determine Vibrio vulnificus and V. parahaemolyticus levels. Presence in a 25-g sample and most probable number (MPN) using standard enrichment and selective isolation procedures were utilized. Suspect colonies were isolated and identified using DNA probe colony hybridization. Neither species of vibrio was detected in 25-g portions of most samples regardless of the PHP. The lowest frequency of isolation of either pathogen (<10%) was observed with the mild heat process. Few (12 to 13%) frozen samples collected at the processor but not at retail contained >30 MPN/g of either pathogen. The mean levels of either organism in PHP oysters observed in the present study were 5 to 6 log less than in unprocessed raw Gulf Coast oysters. Of the 70 V. vulnificus isolates examined, only 5 possessed the putative virulence marker, type B 16S rRNA. Neither the thermostable direct hemolysin (tdh) nor the tdh-related hemolysin (trh) virulence gene was detected in any of the 40 V. parahaemolyticus isolates examined in the present study. These data suggest that if there is any selective advantage to pathogenic strains of V. vulnificus and V. parahaemolyticus, these differences are minimal. These results indicate that all PHP treatments greatly reduce exposure of V. vulnificus and V. parahaemolyticus to raw-oyster consumers. Consequently, these PHP oysters pose a much lower risk of illness to consumers due to these pathogens.