Growth Inhibitory Effect of D-tryptophan on Vibrio spp. in Shucked and Live Oysters.

Vibrio vulnificus and Vibrio parahaemolyticus are important human pathogens that are frequently transmitted via consumption of contaminated raw oysters. A small amount of D-tryptophan (D-Trp) inhibits some foodborne pathogenic bacteria under high-salt environments. In this study, we aimed to evaluate the antibacterial effect of D-Trp on V. vulnificus and V. parahaemolyticus inoculated in culture media, artificial seawater, shucked and live oysters. The effectiveness of D-Trp in growth inhibition of Vibrio spp. was highly dependent on environmental NaCl concentrations. Higher levels of NaCl (>4.0%) with D-Trp (>20 mM) resulted in higher and consistent growth inhibition of both Vibrio spp. Treatment with 40 mM D-Trp significantly (P < 0.05) reduced viable V. parahaemolyticus cell counts in tryptic soy broth (TSB) with >4.0% NaCl at 25°C. In contrast, compared to V. parahaemolyticus, V. vulnificus was more sensitive to D-Trp (20 mM). D-Trp (40 mM) treatment with NaCl (>4.5%) significantly (P < 0.05) inhibited the growth of V. parahaemolyticus and V. vulnificus in shucked oysters immersed in peptone water at 25°C throughout a 48 h incubation period. In artificial seawater, D-Trp exhibited stronger growth inhibitory effect on V. vulnificus and V. parahaemolyticus at 25°C than in TSB at same level of salinity and inhibited the growth of both V. parahaemolyticus and V. vulnificus in live oysters at 25°C for 48 h. Furthermore, we tested the synergistic effect D-Trp and salinity on the inhibition of total viable bacterial counts (TVC) at refrigeration temperature. 40 mM D-Trp inhibited the growth of TVC in shucked oysters immersed in artificial seawater at 4°C. Therefore, these results revealed that D-Trp will serve as a novel and an alternative food preservative to control Vibrio spp. in live oysters at ambient temperature and extending the shelf-life of shucked oysters at refrigeration temperature. [ABSTRACT FROM AUTHOR]