HEAT-RESISTANCE OF DIFFERENT SEROVARS OF LISTERIA-MONOCYTOGENES

Twelve Listeria monocytogenes strains representing seven serovars were heat-treated in physiological saline by a glass capillary tube method. Five strains were treated at 58 degrees, 60 degrees and 62 degrees C, three at 60 degrees, 62 degrees and 64 degrees C and four at 60 degrees C. Heat-treated bacteria were recovered on blood agar in two ways: (I) incubation at 37 degrees C for 7 d; and (2) preincubation at 4 degrees C for 5 d, followed by incubation at 37 degrees C for 7 d. D and x values were determined. Better average recovery and higher D values were obtained when the preincubation procedure was used. The final evaluations of the heat resistance properties of the strains were therefore based on values for preincubated samples. D values recorded at 58 degrees, 60 degrees, 62 degrees and 64 degrees C for preincubated samples were 1.7-3.4, 0.72-3.1, 0.30-1.3 and 0.33-0.68 min, respectively. z values determined were 5.2-6.9 degrees C. D values were compared statistically. Significant differences in heat resistance were noted both between serovars and between strains belonging to the same serovar.