Effect of prior growth conditions on the thermal inactivation of 13 strains of Listeria monocytogenes in two heating menstrua.

The thermal tolerance of 13 Listeria monocytogenes strains was tested using a submerged heating coil apparatus. The strains were grown individually for 18 h at 37 degrees C in acidogenic tryptic soy broth (without dextrose) supplemented with 1% glucose and 1% glutamine (TSB+G) or nonacidogenic tryptic soy broth supplemented with 1% glutamine but containing no glucose (dextrose) (TSB-G). The former medium results in cells induced for pH-dependent, stationary-phase acid resistance, whereas the latter medium allows L. monocytogenes to grow to high numbers in the absence of glucose, yielding cells that are not induced for pH-dependent, stationary-phase acid resistance. The average final pH values of the 18-h TSB+G and the TSB-G cultures were 4.7 and 6.7, respectively. The cells grown in the acid resistance-inducing and non-acid resistance-inducing media were then tested in two heating menstrua that consisted of brain heart infusion broth adjusted to pH 3.0 and water activity (a(w)) of 0.987 or pH 7.0 and a(w) 0.970. In 14 of the 26 menstruum-strain combinations tested, the acid resistance-induced strains were more heat resistant then the equivalent noninduced cultures. No difference in the pattern of thermal resistance in response to induction of acid resistance was apparent among the different serovars tested. The results suggest that the ability of prior induction of acid resistanceto enhance thermal resistance can vary substantially among L. monocytogenes strains.