Oysters are often consumed raw or undercooked; however, they are known to harbor Vibrio parahaemolyticus, a foodborne bacterial pathogen which causes gastrointestinal illness. Shellfish producers have proposed using depuration systems to support the oyster’s natural biological function to reduce pathogen loads. For this approach to be suitable for industrial use, the efficacy of depuration must be validated in relevant species and at commercial scale using appropriate conditions (time, temperature). Validation of a new process requires several stages to verify assumptions and demonstrate method performance. This study describes the initial experimental phase to i) compare the efficacy of depuration in various oyster species, ii) to demonstrate the suitability of non-pathogenic V. parahaemolyticus as a surrogate, and iii) optimize depuration conditions at pilot-scale to identify process variables likely to achieve >3.0-log reduction of V. parahaemolyticus.
Three oyster species (Crassostrea gigas, C. sikamea, C. virginica) were placed in individual containers of artificial seawater containing a cocktail of either non-pathogenic (NP) or pathogenic (P) strains (n = 5 per cocktail) of V. parahaemolyticus. Inoculated oysters were then placed in a pilot-scale recirculating depuration system. Oysters were sampled every 24 hours from days 0 through 7 and V. parahaemolyticus in oyster tissue was enumerated using standard serial dilution and spread plating techniques on TCBS agar. Three replicate depuration trials were conducted, with 5 oysters of each species and cocktail sampled at each time point.
The inoculation procedure used in this study achieved at least a 5 log CFU/g V. parahaemolyticus in C. gigas and C. sikamea; however, V. parahaemolyticus accumulation in C. virginica was more variable and failed to achieve target density within 24 hrs of exposure to contaminated seawater. Accumulation of NP and P cocktails was similar for all oyster species. Depuration at 11°C achieved a >3 log CFU/mL reduction of the P cocktail in C. gigas and C. sikamea tissues within five days, whereas C. virginica averaged a 2.8 log CFU/mL reduction. V. parahaemolyticus clearance rate was rapid during the first 24-48 hours of depuration. NP V. parahaemolyticus was reduced at a comparable or lower rate than P V. parahaemolyticus indicating its suitability as a surrogate for commercial validations. Results from this study confirm that depuration can effectively reduce V. parahaemolyticus in live oysters; however, identical processes may not be suitable to achieve targeted log-reduction goals in all species.