and 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 in high-salt environments. In this study, we aimed to evaluate the antibacterial effect of d-Trp on and in culture media, artificial seawater, and shucked and live oysters. The effectiveness of d-Trp in growth inhibition of spp. was highly dependent on environmental NaCl concentrations. Higher levels of NaCl (>4.0%) with d-Trp (>20 mM) resulted in higher and more consistent growth inhibition of both spp. Treatment with 40 mM d-Trp significantly ( < 0.05) reduced viable cell counts in tryptic soy broth (TSB) with>4.0% NaCl at 25°C. In contrast, was more sensitive to d-Trp (20 mM) than d-Trp (40 mM) treatment with NaCl (>4.5%) significantly ( < 0.05) inhibited the growth of and in shucked oysters immersed in peptone water at 25°C throughout a 48-h incubation period. In artificial seawater, d-Trp exhibited a stronger growth-inhibitory effect on and at 25°C than in TSB at the same level of salinity and inhibited the growth of both and in live oysters at 25°C for 48 h. Furthermore, we tested the synergistic effect of d-Trp and salinity on the inhibition of total viable bacterial counts (TVC) at refrigeration temperature. d-Trp (40 mM) 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 alternative food preservative to control spp. in live oysters at ambient temperature and to extend the shelf-life of shucked oysters at refrigeration temperature. Oysters are the primary transmission vehicles for human infections. Raw oyster consumption is frequently associated with gastroenteritis. The current postharvest methods, such as high-pressure processing, used to control spp. in fresh oysters are still insufficient because of limited facilities, high cost, and potential adverse effects on production. We demonstrate that adding a small amount of d-tryptophan (d-Trp) inhibits the growths of and in a high-salt environment at even ambient temperature. We further investigated the d-Trp treatment conditions and clarified the relationship between salt and d-Trp concentrations for optimal growth-inhibitory effect of spp. The results will be useful for enhancing the effectiveness of d-Trp by increasing salinity levels. Furthermore, in a nutrientfree environment (artificial seawater), a stronger inhibitory effect could be observed at relatively lower salinity levels, indicating that d-Trp may be regarded as effective food preservation in terms of salinity reduction. Therefore, we suggest the use of exogenous d-Trp in a seawater environment as a novel and effective strategy not only for controlling in live oysters at even ambient temperature but also for effectively retarding spoilage bacterial growth and extending the shelf life of shucked oysters at refrigeration temperature.

We previously reported that d-tryptophan exhibits adverse effects on bacterial physiology under osmotic stress. However, the mechanism by which d-tryptophan acts as an inhibitor and/or incompatible solute for bacterial growth has not yet been investigated in detail. In this study, we aimed to determine how osmotic pressure and temperature affect the antimicrobial effect of d-tryptophan. Even at the same level of osmotic pressure, d-tryptophan in conjunction with sodium chloride (NaCl) had a stronger inhibitory effect on the growth of Escherichia coli than that obtained by incubation with potassium chloride (KCl) and sucrose. Because d-tryptophan with NaCl showed the strongest inhibitory effect, we determined the optimum concentration combination of d-tryptophan and NaCl. The growth inhibition boundary conditions as a function of d-tryptophan and NaCl concentrations were determined by a logistic regression model. We found that the minimum level of NaCl for E. coli growth inhibition was 2.5% (w/v) together with 40 mM d-tryptophan. Moreover, the higher the NaCl concentration, the lower the concentration of d-tryptophan that was needed to inhibit bacterial growth. The logistic regression model that we developed enabled us to predict the concentrations required to inhibit bacterial growth. Furthermore, we examined the effect of incubation temperatures ranging from 15 to 46°C on the antimicrobial effect of d-tryptophan. The higher the reaction temperature, the more rapid the decrease of viable E. coli that was observed. This trend is likely attributable to activation of physiological metabolism under the optimum growth temperature. Together, our findings should make a significant contribution to the development of a novel bacterial growth control strategy using d-tryptophan.