Heat Resistance and Population Stability of Lyophilized Bacillus subtilis Spores.

Bacillus subtilis 5230 spores were lyophilized in 0.067 M phosphate buffer and stored at 2 to 8 degrees C for 9 to 27 months. The lyophilized spores were reconstituted with buffer or 0.9% saline, and the heat resistance was determined in a thermoresistometer. Lyophilization had no effect on the heat resistance of the spores but did result in a slight decrease in population (</=0.3-logarithm reduction). The lyophilized spores maintained heat resistance and population levels over the test periods. The D-values ranged from 0.44 to 0.54 min at 121.1 degrees C, and the z-values ranged from 6.1 to 6.6 degrees C. Lyophilization was concluded to be an acceptable alternative for storage of bacterial spores that are to be used as biological indicators in sterilization processes.