Heat shock-induced thermotolerance in Listeria monocytogenes 13-249 is dependent on growth phase, pH and lactic acid

The effect of heat shock on the thermotolerance of Listeria monocytogenes 1 3-249 was compared as a function of growth phase (log and late stationary p hase) pH (5.4, 5.8, 6.2) and heating medium (broth vs minced beef). Lactic acid was added to tryptic phosphate broth (TPB+) to simulate the level foun d in beef. Furthermore, the specific effect of lactic acid was investigated , comparing results obtained in broth with and without lactic acid with pH adjusted to 5.4 and 7.0. in late stationary cultures, heat shock produced o nly marginal differences in thermotolerance in TPB+ and beef regardless of pH. For log phase cultures, the increase in heat shock induced thermotolera nce (HSIT) depended on pH in both TPB+ and beef wish gradually larger incre ases at rising pH At the highest pH tested (pH7.0 in TPB+ and pH 6.2 in bee f) the increase in HSIT was 5.7-fold and 2.4-fold, respectively. These incr eases were highly significant (P < 0.001). if no lactic acid was present, t he HSIT increased at both pH5.4 for 7.0 for log phase cultures. This indica tes that the presence of lactic acid in naturally occurring levels will hav e a specific effect on the response of L. monocytogenes to heat shock. The D-60-values of L. monocytogenes 13-249 were generally higher (2-6-fold) in minced beef than in TPB+ (C) 1999 Academic Press.