Intracellular pH and survival of Listeria monocytogenes scott a in tryptic soy broth containing acetic, lactic, citric, and hydrochloric acids

To study the effect of citric, acetic, lactic, and hydrochloric acids on Listeria monocytogenes Scott A, growth, survival, and intracellular pH (pH_in) values were determined during growth in a pH-controlled fermentation vessel. Under the experimental conditions, L. monocytogenes Scott A grown in tryptic soy (plus yeast extract) broth survived even when the pH was reduced to 3.5. For most acids, L, monocytogenes maintained a pH gradient (intracellular pH-external pH) of about 1.0 pH unit and a pH_in near 5.0. When the citric and lactic acid-treated cells at pH values 3.5, 4.0, and 4.5 were incubated for a longer time (24 h), both the pH gradient and the pH_in values decreased. Although citric and lactic acids were more effective in lowering the pH_in, acetic acid had the greatest effect on cell survival. A greater than 4-log reduction in cell number occurred when L. monocytogenes was held in acetic acid-treated broth for 24 h at pH 3.5 even though the pH_in was 5.0. The results suggest that inhibition of L. monocytogenes by acids is caused not by a decrease in the intracellular pH, per se, but rather by specific effects of undissociated acid species on metabolic or other physiological activities.