Kinetic evaluation of physiological heterogeneity in bacterial spores during thermal inactivation.

Bacterial spores are in cryptobiosis during the dormant period, which can change vegetative cells upon exposure to favorable conditions and eventually cause serious illness through ingestion of spore-contaminated food (Leuschner et al., 1999; Reissbrodt et al., 2002; Roszak and Colwell, 1987). The most common spore-forming bacteria causing foodborne disease are aerobic Bacillus spp. and anaerobic Clostridium spp. such as B. cereus, C. perfringens, and C. botulinum (Cho et al., 2007; Lee et al., 2007; Shin et al., 2008). Other Bacillus spores such as B. subtilis, B. coagulans, B. licheniformis are the causes of food spoilage (Cazemier et al., 2001). However, spores are extremely resistant to heat and other external stresses, which is primarily attributed to their ultra-structural characteristics (Nicholson et al., 2002; Oomes et al., 2007). Heat treatment triggers an activation, germination, and destruction of dormant spores through a complex multistage process (Collado et al., 2006; Iciek et al., 2006). The activated spores are more susceptible to heat than the dormant spores. However, the inactivation of dormant spores during thermal processing is not clearly explained by general inactivation processes due to the nature of the heterogeneous populations (Davey and Kell, 1996). Some spores are directly heatinactivated without an activation stage, while others are heat-injured or remain in a dormant state for a longer heating time (Abraham et al., 1990; Esty and Meyer, 1922; Vary and Halvorson, 1965). Therefore, understanding the heat-induced inactivation of bacterial spores with special regard to physiological, biochemical, and molecular properties is essential for developing an appropriate kinetic model elucidating the thermal effect on spore inactivation. Mathematical predictive modeling is used as a tool for quantifying the effi cacy of food preservation technologies for the inactivation of microorganisms. Heterogeneous populations are present during thermal treatment, including an injured, activated, germinated, or inactivated state of bacterial spores (Iciek et al., 2006; Sapru et al., 1993). There have been few studies of mathematical models for heterogeneous bacterial spores (Abraham et al., 1990; Sapru et al., 1993). It is important to describe the kinetic patterns of spore inactivation including variability in thermal resistance. Therefore, the objective of this study was to quantitatively describe the population heterogeneity based on a kinetic approach to thermal inactivation of bacterial spores. In this study, B. amyloliquefaciens TMW 2.479 Fad 82 and B. subtilis KACC 10854 were used to represent heat-resistant and heat-sensitive spores, respectively. J. Gen. Appl. Microbiol., 55, 295‒299 (2009)