Growth Inhibition of Listeria monocytogenes, Salmonella enterica, and Escherichia coli O157:H7 by D-Tryptophan as an Incompatible Solute

Abstract

Under osmotic stress, bacterial cells uptake compatible solutes such as glycine-betaine to maintain homeostasis. It is unknown whether incompatible solutes exist that are similar in structure to compatible solutes but have adverse physiological effects on bacterial physiology. The objective of this study was to evaluate solute incompatibility of various amino acids against bacterial growth. Bacterial growth was evaluated by changes in optical density at 595 nm in peptone-yeast-glucose (PYG) broth. Twenty-three amino acids with l and/or d isomers were examined for the effect of bacterial growth inhibition. Among the various amino acids examined, d-tryptophan (~ 40 mM) in PYG broth supplemented with 0 to 4% (wt/vol) salt inhibited the growth of Listeria monocytogenes, Salmonella enterica, and Escherichia coli O157:H7 at 25°C. d-Tryptophan (30 to 40 mM) completely inhibited the growth of E. coli O157:H7 and Salmonella in the presence of <3% salt, but the growth of L. monocytogenes was not completely inhibited under the same conditions. Low concentrations of salt (0 to 2% NaCl) with d-tryptophan did not significantly inhibit the growth of all bacteria except L. monocytogenes, which was relatively inhibited at 0% NaCl. The effect of d-tryptophan differed depending on the bacterial species, illustrating the difference between gram-positive and gram-negative bacteria. These results indicate that the uptake of d-tryptophan as a compatible solute during osmotic stress may inhibit bacterial growth. The antibacterial effect of d-tryptophan found in this study suggests that d-tryptophan could be used as a novel preservative for controlling bacterial growth in foods.