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

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Abstract
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, D70 values of 0·53 min at 5°C, 0·74 min at 10°C, 0·83 min at 20°C, 0·79 min at 30°C, 0·63 min at 37°C, 0·48 min at 40°C and 0·41 min at 45°C 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 D70 values of 1·73 min for cells incubated at 5°C for 50 days and 1·04 min for those grown at 10°C for 16 days. At other temperatures assayed heat resistance became stable for late stationary phase cells, reaching D70 values of 1·05, 1·08 and 1·01 min for cultures incubated at 20, 30 and 37°C. Heat resistance of cells obtained at higher temperatures, 40 and 45°C, was significantly lower, with D70 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 ± 0·39°C 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.

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