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

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, D 70 values of 0AE53 min at 5°C, 0AE74 min at 10°C, 0AE83 min at 20°C, 0AE79 min at 30°C, 0AE63 min at 37°C, 0AE48 min at 40°C and 0AE41 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 D 70 values of 1AE73 min for cells incubated at 5°C for 50 days and 1AE04 min for those grown at 10°C for 16 days. At other temperatures assayed heat resistance became stable for late stationary phase cells, reaching D 70 values of 1AE05, 1AE08 and 1AE01 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 D 70 values of 0AE76 and 0AE67 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 0AE33 to 1AE73 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 4AE50 ± 0AE39°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.

Diversity and biofilm-forming capability of bacteria recovered from stainless steel pipes of a milk-processing dairy plant

“…However, as concerns the recovery of isolates, those of E. faecalis and B. cereus increased their numbers after pasteurization. The heat resistance of enterococci from milk and dairy products has been reported before by several authors (Martinez et al 2003;McAuley et al 2012). …”

Influence of pH and temperature on the growth of Enterococcus faecium and Enterococcus faecalis

“…The ability to grow at 10 °C and 45 °C, to initiate growth in 6.5% NaCl broth at pH 9.6 and to survive at 60°C for 30 min, are commonly used traits to segregate enterococci from other streptococci [11,26].…”

Scientific Opinion on Hatchery Waste as animal by‐products

“…However, z values calculated were not significantly influenced by these factors. A mean value of 4·50 ± 0·39°C was found (Martinez et al, 2003). (Schalk et al, 1935) There are several reports of the isolation of Mycobacterium avium from different European countries (Finland, Norway, Denmark, Germany and UK) (Fulton and Sanchez, 2008).…”

“…The processing conditions prevailing are equivalent to a pasteurization treatment, where the vegetative forms of pathogenic microorganisms are killed, while most spores remain viable. Enterococcus faecium is considered to be the most heat-tolerant non-sporeforming bacterium (Fernandez et al, 2009;Martinez et al, 2003). Its high tolerance to adverse conditions and its ubiquitous nature makes E. faecium a reference microorganism appropriate to evaluate the efficacy of pasteurization processes (Smith et al, 1990) and disinfection methods (Spicher et al, 2002).…”