Determination of lethality rate constants and D-values for Bacillus atrophaeus (ATCC 9372) spores exposed to dry heat from 115 degrees C to 170 degrees C.

Dry heat microbial reduction is the NASA-approved sterilization method to reduce the microbial bioburden on spaceflight hardware for missions with planetary protection requirements. The method involves heating the spaceflight hardware to temperatures between 104 degrees C and 125 degrees C for up to 50 hours, while controlling the humidity to very low values. Collection of lethality data at temperatures above 125 degrees C and with ambient (uncontrolled) humidity conditions would establish whether any microbial reduction credit can be offered to the flight project for processes that occur at temperatures greater than 125 degrees C. The goal of this research is to determine the survival rates of Bacillus atrophaeus (ATCC 9372) spores subjected to temperatures higher than 125 degrees C under both dry (controlled) and room ambient humidity (36-66% relative humidity) conditions. Spores were deposited inside thin, stainless steel thermal spore exposure vessels (TSEVs) and heated under ambient or controlled humidity conditions from 115 degrees C to 170 degrees C. After the exposures, the TSEVs were cooled rapidly, and the spores were recovered and plated. Survivor ratios, lethality rate constants, and D-values were calculated at each temperature. At 115 degrees C and 125 degrees C, the controlled humidity lethality rate constant was faster than the ambient humidity lethality rate constant. At 135 degrees C, the ambient and controlled humidity lethality rate constants were statistically identical. At 150 degrees C and 170 degrees C, the ambient humidity lethality rate constant was slightly faster than the controlled humidity lethality rate constant. These results provide evidence for possibly modifying the NASA dry heat microbial reduction specification.