Heat Resistance and Population Stability of Lyophilized Bacillus subtilis Spores

Abstract

Bacillus subtilis 5230 spores were lyophilized in 0.067 M phosphate buffer and stored at 2 to 8°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°C, and the z -values ranged from 6.1 to 6.6°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.