Antibacterial Activity of D-Tryptophan against Salmonella enterica and Escherichia coli O157:H7 under Osmotic Stress and Its Application to Oyster Preservation

Introduction: The adaptive response of bacteria to osmotic stress includes accumulation of compatible solutes, such as glycine-betaine, to maintain homeostasis. This survival strategy has protective effects for bacterial cells and makes it difficult to control bacteria in low water activity (a_w) foods. In our previous study, D-tryptophan, an incompatible solute that is structurally similar to compatible solutes, had adverse effects on the growth of foodborne bacteria under high salt conditions.

Purpose: This study evaluatee the inhibitory effects of D-tryptophan on Escherichia coli O157:H7 and Salmonella enterica under various mixed NaCl and D-tryptophan stress conditions and, subsequently, test its antibacterial efficacy against E. coli O157:H7 in freshly shucked oysters.

Methods: We examined the antibacterial effect of D-tryptophan on E. coli O157:H7 and S. enterica in liquid media with various NaCl concentrations. A model identifying the boundary between growth/no growth of the bacteria as a function of NaCl and D-tryptophan concentration was developed by logistic regression with R statistical software. In addition, freshly shucked oysters were artificially contaminated with E. coli O157:H7 in artificial seawater (3.5% NaCl) with or without 40 mM D-tryptophan. Bacterial survival was enumerated using plate counts.

Results:  Growth/no growth boundary models were successfully developed for both pathogens. Salmonella enterica and E. coli O157:H7 growth was affected at NaCl concentrations >3% and 4%, respectively, in the presence of 20 mM D-tryptophan; but higher concentrations of both substrates were needed for efficient growth inhibition. In addition, adding D-tryptophan significantly inhibited E. coli O157:H7 growth in freshly shucked oysters.

Significance:  D-tryptophan may act as a novel, natural food preservative to control foodborne bacterial growth, protect low a_w food quality, and extend the shelf-life of processed seafood.