Heat shock-induced thermotolerance inListeria monocytogenes13-249 is dependent on growth phase, pH and lactic acid

The effect of heat shock on the thermotolerance ofListeria monocytogenes13-249 was compared as a function of growth phase (log and late stationary phase), 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 found 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 only marginal differences in thermotolerance in TPB+and beef regardless of pH. For log phase cultures, the increase in heat shock induced thermotolerance (HSIT) depended on pH in both TPB+and beef with gradually larger increases at rising pH. At the highest pH tested (pH 7.050 in TPB+and pH 6.2 in beef) the increase in HSIT was 5.7-fold and 2.4-fold, respectively. These increases were highly significant (P< 0.001). If no lactic acid was present, the HSIT increased at both pH 5.4 for 7.0 for log phase cultures. This indicates that the presence of lactic acid in naturally occurring levels will have a specific effect on the response ofL. monocytogenesto heat shock. The D60-values ofL. monocytogenes13-249 were generally higher (2—6-fold) in minced beef than in TPB+.