Evaluation of the pH-dependent, stationary-phase acid tolerance in Listeria monocytogenes and Salmonella Typhimurium DT104 induced by culturing in media with 1% glucose:: a comparative study with Escherichia coli O157:H7

Aims: To comparatively evaluate the adaptive stationary-phase acid tolerance response (ATR) in food-borne pathogens induced by culturing in glucose-containing media, as affected by strain variability and antibiotic resistance, growth temperature, challenge pH and type of acidulant. Methods and Results: Antibiotic resistant or sensitive strains of Listeria monocytogenes, Salmonella including S. Typhimurium DT104, and Escherichia coli O157:H7 were cultured (30degreesC for 24 h; 10degreesC for up to 14 days) in trypticase soya broth with yeast extract (TSBYE) with 1% or without glucose to induce or prevent acid adaptation, respectively. Cultures were subsequently exposed to pH 3.5 or 3.7 with lactic or acetic acid at 25degreesC for 120 min. Acid-adapted cultures were more acid tolerant than nonadapted cultures, particularly those of L. monocytogenes and Salmonella. No consistent, positive or negative, influence of antibiotic resistance on the pH-inducible ATR or acid resistance (AR) was observed. Compared with 30degreesC cultures, growth and acid adaptation of L. monocytogenes and S . Typhimurium DT104 at 10degreesC markedly reduced their ATR and AR in stationary phase. E. coli O157:H7 had the greatest AR, relying less on acid adaptation. A 0.2 unit difference in challenge pH (3.5-3.7) caused great variations in survival of acid-adapted and nonadapted cells. Conclusions: Culturing L. monocytogenes and Salmonella to stationary phase in media with 1% glucose induces a pH-dependent ATR and enhances their survival to organic acids; thus, this method is suitable for producing acid-adapted cultures for use in food challenge studies. Significance and Impact of the Study: Bacterial pathogens may become acid-adapted in foods containing glucose or other fermentable carbohydrates. Low storage temperatures may substantially decrease the stationary-phase ATR of L. monocytogenes and S . Typhimurium DT104, but their effect on ATR of E. coli O157:H7 appears to be far less dramatic.