Heat activation and inactivation of bacterial spores. Is there an overlap

Heat activation at a sublethal temperature is widely applied to promote Bacillus species spore germination. This treatment also has potential to be employed in food processing to eliminate undesired bacterial spores by enhancing their germination, and then inactivating the less heat resistant germinated spores at a milder temperature. However, incorrect heat treatment could also generate heat damage in spores, and lead to more heterogeneous spore germination. Here, the heat activation and heat damage profile of Bacillus subtilis spores was determined by testing spore germination and outgrowth at both population and single spore levels. The heat treatments used were 40-80 degrees Celcius, and for 0-300 min. The results were as follows. 1) Heat activation at 40-70 degrees Celcius promoted L-valine and L-asparagine-glucose-fructose-potassium (AGFK) induced germination in a time dependent manner. 2) The optimal heat activation temperatures for AGFK and L-valine germination via the GerB plus GerK or GerA germinant receptors were 65 and 50-65 degrees Celcius, respectively. 3) Heat inactivation of dormant spores appeared at 70 degrees Celcius, and the heat damage of molecules essential for germination and growth began at 70 and 65 degrees Celcius, respectively. 4) Heat treatment at 75 degrees Celcius resulted in both activation of germination and damage to the germination apparatus, and 80 degrees Celcius treatment caused more pronounced heat damage. 5) For the spores that should withstand adverse environmental temperatures in nature, heat activation seems functional for a subsequent optimal germination process, while heat damage affected both germination and outgrowth.