Purpose: The purpose of this study was to evaluate tryptophan as an antibacterial substance inhibiting the growth of pathogenic bacteria under ambient temperature.
Methods: Listeria monocytogenes, Salmonella, and Escherichia coli O157:H7 were evaluated. The bacterial growth was monitored at 25°C in peptone-yeast-glucose broth supplemented with 0 to 4% (w/v) salt and tryptophan (D/L/DL) in concentrations between 0 and 40 mM. After the broth was inoculated with pathogens, the optical density at 595 nm was continuously monitored every 10 min during the incubation period. Growth curves were generated, and time-to-detection and specific growth rates were calculated and compared between the tested conditions using the Tukey’s multiple comparison test.
Results: D-tryptophan greatly inhibited all bacterial growth tested in this study. In particular, D-tryptophan completely inhibited the growth of E. coli O157:H7 and Salmonella in the presence of more than 3% salt over a 3-day incubation period. The salt concentration in the environment greatly influenced the antibacterial effect of D-tryptophan. In the culture with no added salt (0% NaCl), all pathogens exhibited significant growth inhibition with 40 mM D-tryptophan. However, 1-2% NaCl did not significantly affect bacterial growth, while more than 3% NaCl conditions caused a significant impact on the inhibition of the bacterial growth.
Significance: The antibacterial effect of D-tryptophan demonstrated in this study could be an alternative approach to controlling bacterial growth in foods.