Thermobacteriological characterization of Enterobacter sakazakii

In the present study the influence of various environmental and physiological factors on the heat resistance of Enterobacter sakazakii (Cronobacter) have been investigated. Our results demonstrated that the heat resistance of E. sakazakii depended on the strain studied, the growth conditions - phase and temperature - the characteristics of treatment medium and the recovery conditions. The strain STCC 858 (ATCC type strain 29544) showed maximum heat resistance among the strains tested and it was selected for the further study. Stationary-phase cells grown between 20 and 37 degrees C (mean D(60)=0.9 min) resulted to be more resistant than cells grown at 10 degrees C (D(60)=0.2 min). Resistance decreased when the treatment medium pH was lower than pH 6.0, and it increased with decreasing water activity of the treatment medium, with a 32-fold increase in resistance when lowering water activity to 0.96. z value at pH 4.0 (z=4.79 degrees C) was significantly higher than at pH 7.0 (z=4.06 degrees C), although E. sakazakii cells were approximately 10 times more heat resistant at pH 7.0 than at pH 4.0 within the range of temperatures tested. Contrary to pH, the magnitude of the influence of a(w) on heat resistance did not significantly change with treatment temperature. The proportion of sublethally damaged cells was similar regardless of the treatment medium pH, but it decreased when lowering the water activity. Nevertheless, increasing treatment temperature would not result in a decreased proportion of sublethally injured E. sakazakii cells within the surviving population. Thus, the design of a theoretical combined process that could take advantage of the occurrence of sublethally injured cells would be similarly effective at low and high temperatures. E. sakazakii proved to be more heat resistant in four different liquid food matrixes than in buffers at the same pH, and this disagreement was especially higher in orange juice, which resulted to be the product that induced a greater protective effect in E. sakazakii cells against heat.