|Analysis of bacterial communities present in high-pressure treated, quick-frozen, and raw oysters was carried out independently during three different seasons: winter; summer; and fall of 2006. Oysters used in all experiments were supplied by Bon Secour Fisheries, Inc. Bon Secour, AL. Determination of bacterial numbers and species diversity in each sample was conducted at 0, 7, 14, and 21 days of storage at 4oC (high-pressure treated and raw oysters) and -20oC (quick-frozen oysters).
Results show that the numbers of total bacterial counts in treated oysters were significantly lower than in untreated oysters at day 0 in all samplings. Total numbers of aerobic bacteria in high-pressure treated oysters at day 0 were lower than 105 colony forming units (CFU)/g in every season whereas quick-frozen oysters maintained their levels between 104 and 106 CFU/g during the 21-day storage period regardless of the sampling season. However, total bacterial counts in quick-frozen oysters during this study were statistically different in all seasons (P<0.05). Season has significant influence on variation of total bacterial numbers in both treated and untreated oysters (P<0.05). An increase in the total number of bacterial counts in high-pressure treated oysters observed at day 7, 14, and 21 indicating that some bacteria can survive the treatment and can be proliferate during storage at 4oC. High-pressure treated oysters presented lower total bacterial counts than raw oysters at the time oysters get to market. However, the recommended shelf-life for this product (3 weeks) seems to be too long based on the number of bacteria present in the oysters after 2 weeks under strict refrigeration.
Sequencing of the 16S rDNA from bacterial isolates revealed seven different classes within the bacterial communities in oysters. The majority of the isolates were Gram-negative bacteria, with the Gammaproteobacteria class representing between 56% and 92% of all sequences. The remaining Gram-negative belonged the Alphaproteobacteria, Betaproteobacteria, Flavobacteria and Sphingobacteria classes. Gram-positive bacteria included two classes: Actinobacteria and Bacilli. The most common bacterial genera found in this study were Shewanella, Vibrio and Psychrobacter. Four species of human pathogenic bacteria were also identified: V. vulnificus, V. parahaemolyticus, V. alginolyticus, and Aeromonas hydrophila. Vibrio vulnificus was identified only from untreated (raw) oysters.