Thermal inactivation parameters of spores from different phylogenetic groups of Bacillus cereus

The thermal inactivation kinetics of spores from 39 Bacillus cereus strains belonging to six different phylogenetic groups (group II to VII) was studied. Fresh spore suspensions in glass capillaries were heated in an oil bath at three or more different temperatures for five different times. Survival curves and thermal death curves were established and the kinetic parameters DT (decimal reduction time at temperature T) and z (temperature dependence of DT) were derived by linear regression. Most strains (38/39) had survival curves without a pronounced shoulder or tail, as reflected by linear regression coefficients R2 generally higher than 0.95. The heat resistance of the strains and groups of strains was then compared by determining the temperature (°C) at which logD = 0.8 (TlogD = 0.8). Spores from group VI strains showed significantly lower heat resistance than all other groups except group II, with TlogD = 0.8 ranging between 82.7 °C and 92.8 °C. Spores from groups III and VII, on the other hand, were generally most heat resistant, with TlogD = 0.8 between 91.9 °C and 101.8 °C. Further analysis revealed a positive correlation between spore heat resistance and both minimal and maximal growth temperatures of the strains. In contrast, the z value was negatively correlated with the minimal and maximal growth temperatures. The availability of genetic group-specific heat resistance data will contribute to a more accurate risk assessment of B. cereus.