Effect of Ploidy on Vibrio parahaemolyticus and Vibrio vulnificus Levels in Cultured Oysters

Vibrio parahaemolyticus and V. vulnificus are naturally occuring human pathogenic bacteria commonly found in estuarine environments where oysters are cultured. The use of triploid oysters has increased, due to their rapid growth rate and that they maintain a high quality throughout the year. Previous work suggested levels of Vibrio spp. may be lower in triploid oysters, as compared to diploids. Therfore, this study aimed to determine if there is a difference in the abundances of V. parahaemolyticus and V. vulnificus between half-sibling diploid and triploid oysters. In four trials, 100 individual oysters (either iced or temperature abused) were analyzed for V. parahaemolyticus and V. vulnificus using direct plating followed by colony hybridization. Mean levels of V. parahaemolyticus in iced and abused diploid oysters were 3.55 and 4.21 log CFU/g, respectively. Mean levels in iced and abused triploid oysters were 3.49 and 4.27 log CFU/g, respectively. Mean levels of V. vulnificus in iced and abused diploid oysters were 3.53 and 4.56 log CFU/g, respectively. Mean levels in iced and abused triploid oysters were 3.54 and 4.55 log CFU/g, respectively. The differences in Vibrio spp. abundances between diploid and triploid oysters was not significant (p>0.05). However, the differences across treatments were significant (p<0.05), with the exception of V. parahaemolyticus levels in trial 3 (p=0.83). Variation between individual oysters was also observed, with 12 of 808 measurements being outside of the 95 th percentile. This phenomenon of occasional statistical outliers ("hot" or "cold" oysters) has been previously described and supports the appropriateness of composite sampling to account for inherent animal variability. In summary, the data indicate that abundances of V. parahaemolyticus and V. vulnificus are not dependent on the ploidy of cultured oysters, but vary with the type of handling.