Heat and Acid Tolerance Responses of Listeria monocytogenes as Affected by Sequential Exposure to Hurdles during Growth

This study aimed to evaluate the effects of the level and sequence of hurdles, applied during growth, on the subsequent heat and acid tolerances of a 10-strain composite of Listeria monocytogenes. Individual strains were grown in glucose-free tryptic soy broth with 0.6% yeast extract (TSBYE–G). Then cultures were mixed and inoculated in fresh TSBYE–G (0.5% NaCl, pH 7.42; control), TSBYE–G that was supplemented with 3% NaCl (3.5% NaCl in total), or TSBYE–G with pH adjusted to 6.01 or 5.04 with lactic acid and incubated at 30°C for 24 h. Furthermore, the culture composite was exposed to the following five combinations of double sequential hurdles (12 h in each at 30°C): NaCl then pH 6.01, NaCl then pH 5.04, pH 7.42 then NaCl, pH 5.04 then NaCl, and pH 6.01 then NaCl. The heat and acid tolerances of the culture were assessed at 57°C (for 2 h) and at pH 3.5 (for 7 h), respectively, in TSBYE–G. No significant (P ≥ 0.05) differences in thermotolerance were observed among cultures exposed to various stresses. In contrast, the acid resistance followed the order: pH 6.01 = NaCl > NaCl then pH 5.04 > pH 6.01 then NaCl = pH 5.04 > pH 5.04 then NaCl > pH 7.42 then NaCl > control. The results suggest that exposure of L. monocytogenes to NaCl and low pH during growth may not affect its heat (57°C) tolerance, but it may increase its acid (pH 3.5) resistance, depending on the sequence and intensity of the applied stresses.