Why Does Cronobacter sakazakii Survive for a Long Time in Dry Environments? Contribution of the Glass Transition of Dried Bacterial Cells

Abstract

The mechanical glass transition temperature ( T g ) of dried Cronobacter sakazakii cells varied depending on differences in drying methods and water activity (a w ) levels. Because the T g of the dried bacterial cells varied depending on the drying method and a w , the T g will play an important role as an operational factor in the optimization of dry food processing for controlling microbial contamination in the future. To investigate the mechanism of adaptation of Cronobacter sakazakii to desiccation stress, the present study focused on the glass transition phenomenon of dried bacterial cells, using a thermomechanical technique. The mechanical glass transition temperature ( T g ) of dried C. sakazakii cells per se , prepared by different drying methods (air drying and freeze-drying) and with different water activity (a w ) levels (0.43, 0.57, 0.75, and 0.87), were determined. In addition, we investigated the survival of two strains of C. sakazakii (JCM 1233 and JCM 2127) prepared by different drying methods under different storage temperatures (4, 25, and 42°C) and a w conditions (0.43 and 0.87). While the T g of the air-dried C. sakazakii cells increased as the a w decreased, the freeze-dried C. sakazakii cells showed an unclear a w dependency of the T g . Air-dried C. sakazakii cells showed a higher T g than freeze-dried C. sakazakii cells at an a w of <0.57. Freeze-dried C. sakazakii cells were more rapidly inactivated than air-dried cells regardless of the difference in a w and temperature. The difference between the T g and storage temperature was used as an index that took into consideration the differences in the drying methods and a w levels. As the difference between the T g and storage temperature increased to >20°C, the dried C. sakazakii cells survived stably regardless of the drying method. In contrast, when the difference between the T g and storage temperature was reduced to <10°C, the viable cell numbers in dried C. sakazakii cells were quickly decreased. Thus, the T g is a key factor affecting the desiccation tolerance of C. sakazakii . IMPORTANCE The mechanical glass transition temperature ( T g ) of dried Cronobacter sakazakii cells varied depending on differences in drying methods and water activity (a w ) levels. Because the T g of the dried bacterial cells varied depending on the drying method and a w , the T g will play an important role as an operational factor in the optimization of dry food processing for controlling microbial contamination in the future. Furthermore, the differences between the T g and storage temperature were introduced as an integrated index for survival of bacterial cells under a desiccation environment that took into consideration the differences in the drying methods and a w levels. As the difference between the T g and storage temperature decreased to <10°C, the dried C. sakazakii cells were inactivated quickly, regardless of the drying methods. The relationship between T g and storage temperature will contribute to understanding the desiccation tolerance of bacterial cells.