Inactivation of bacterial spores subjected to sub-second thermal stress

Rapid heat pulse is the primary method for neutralizing large quantities of spores. Characterizing heat inactivation on a millisecond time scale has been limited by the ability to apply ultrafast, uniform heating to spores. Using our system for immobilization of spores on metal surfaces, bacterial spores were subjected to high temperatures (200-800 degrees C) and heating rates (similar to 10(3) degrees C/s to similar to 10(5) degrees C/s). Spore inactivation increased with temperature and fit a sigmoid response. We observed the critical peak temperature (T-c) which caused a 2-fold reduction in spore viability was 382 degrees C and 199 degrees C for heating rates of similar to 10(4) degrees C/s and similar to 10(5) degrees C/s, respectively. Repetitive heating to the same peak temperature had little effect on viability. In contrast, stepwise heating to elevated peak temperatures inactivated spores in a manner similar to a single pulse heating to the same peak temperature. These results indicate that the maximum temperature rather than the overall heating time is primarily responsible for spore neutralization at similar to 10(4) degrees C/s heating rate. The mechanism of spore inactivation was further investigated at two heating rates (similar to 10(4) degrees C/s and similar to 10(5) degrees C/s). Viability reduction was mainly due to DNA damage at the heating rate of similar to 10(4) degrees C/s as mutant strains defective for sspA sspB and recA were more sensitive to heat than the wide-type strains. At the higher heating rate (similar to 10(5) degrees C/s), spore inactivation was correlated with physical damage from ultrafast vapor pressurization inside spores. This new approach of pulse heating generates a temperature, time, and kill relationship for Bacillus spores at sub-second timescales. (C) 2015 Elsevier B.V. All rights reserved.