Growth-Inhibitory Effect of d -Tryptophan on Vibrio spp. in Shucked and Live Oysters

Abstract

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.

Occurrence and D-Tryptophan Application for Controlling the Growth of Multidrug-Resistant Non-O157 Shiga Toxin-Producing Escherichia coli in Dairy Products

“…Our findings revealed that D-Trp revealed a reduction in E. coli viability in contaminated cheese containing 1.5% and 3% NaCl by 1 and 2.5 log CFU/g during a 4-week incubation period at 4 °C, respectively. A similar finding was reported by Chen et al [ 55 ], who found that increasing concentrations of NaCl (3–5%) with 40 mM D-Trp elicited bactericidal effects on Vibrio spp. growth in oysters.…”

“…Such a reduction in the bacterial population could be attributed to the freezing stress, which enhances the inhibitory effect of D-Trp in ice cream. This finding was similar to that given by Chen et al [ 55 ], but the authors used D-Trp combined with osmotic stress to control the growth of Vibrio spp. in oysters, while Elafify et al [ 20 ] tested the effect of D-Trp combined with chilled stress to inhibit L. monocytogenes growth in milk.…”

Prevalence of Multidrug-Resistant Listeria monocytogenes in Dairy Products with Reduction Trials Using Rosmarinic Acid, Ascorbic Acid, Clove, and Thyme Essential Oils

“…e obtained results indicated that the used antimicrobial additives exerted the greatest overall suppression of L. monocytogenes when combined with NaCl 3%. Similarly, Chen et al [55] found that using D-tryptophan as an antimicrobial amino acid for suppression of the growth of V. vulnificus and V. parahaemolyticus at a high salt content (5% NaCl) exerted more bacteriostatic effect than low salt content (3.5% NaCl). Also, the same authors found that the total bacterial population in freshly shucked oysters in artificial seawater treated with 40 mM D-tryptophan with a high salt content revealed a significant decrease in the bacterial counts than that recorded at low salt content.…”

“…Also, the same authors found that the total bacterial population in freshly shucked oysters in artificial seawater treated with 40 mM D-tryptophan with a high salt content revealed a significant decrease in the bacterial counts than that recorded at low salt content. e high NaCl content led to an increase in the withdrawal of water from the bacterial cells, followed by a more gradual reduction in bacterial populations [55].…”

Antimicrobial activity of D-amino acid in combination with photo-sonoactivated hypericin nanoparticles against Acinetobacter baumannii

“…ELafify et al [ 53 ] reported D-Trp has an inhibitory effect on the growth of Listeria monocytogenes under different stress conditions. Also, the results of the study by Chen et al [ 54 ] displayed that adding a small amount of D-Trp inhibits the growths of Vibrio parahaemolyticus and V. vulnificus in a high-salt environment. The antimicrobial effect of D-Trp might be involved in the inhibition of microbial growth by reducing the initial adhesion between cells and by changing the properties of the extracellular matrix, which has a considerable role in protecting the microorganisms from environmental insults [ 55 ].…”