Thermal inactivation of Enterococcus faecium: effect of growth temperature and physiological state of microbial cells

Aims: To provide data on the effects on culture temperature and physiological state of cells on heat resistance of Enterococcus faecium, which may be useful in establishing pasteurization procedures. Methods and Results: The heat resistance of this Ent. faecium (ATCC 49624 strain) grown at different temperatures was monitored at various stages of growth. In all cases, the bacterial cells in the logarithmic phase of growth were more heat sensitive. For cells which had entered in the stationary phase, D-70 values of 0.53 min at 5degreesC, 0.74 min at 10degreesC, 0.83 min at 20degreesC, 0.79 min at 30degreesC, 0.63 min at 37degreesC, 0.48 min at 40degreesC and 0.41 min at 45degreesC were found. By extending the incubation times cells were more heat resistant as stationary phase progressed, although a different pattern was observed for cells grown at different temperatures. At the lower temperatures heat resistance increased progressively, reaching D-70 values of 1.73 min for cells incubated at 5degreesC for 50 days and 1.04 min for those grown at 10degreesC for 16 days. At other temperatures assayed heat resistance became stable for late stationary phase cells, reaching D-70 values of 1.05, 1.08 and 1.01 min for cultures incubated at 20, 30 and 37degreesC. Heat resistance of cells obtained at higher temperatures, 40 and 45degreesC, was significantly lower, with D-70 values of 0.76 and 0.67 min, respectively. Neither the growth temperature nor the growth phase modified the z-values significantly. Conclusions: D-70 values obtained for Ent. faecium (ATCC 49624) varies from 0.33 to 1.73 min as a function of culture temperature and physiological state of cells. However, z values calculated were not significantly influenced by these factors. A mean value of 4.50 +/- 0.39degreesC was found. Significance and Impact of the Study: Overall results strongly suggest that, to establish heat processing conditions of pasteurized foods ensuring elimination of Ent. faecium, it is advisable to take into account the complex interaction of growth temperature and growth phase of cells acting on bacterial thermal resistance.