Thermal inactivation and injury of Bacillus stearothermophilus spores

Aqueous spore suspensions of Bacillus stearothermophilus ATCC 12980 were heated at different temperatures for various time intervals in a resistometer, spread plated on antibiotic assay medium supplemented with 0.1% soluble starch without (AAMS) or with (AAMS-S) 0.9% NaCl, and incubated at 55 degrees C unless otherwise indicated. Uninjured spores formed colonies on AAMS and AAMS-S; injured spores formed colonies only on AAMS. Values of D, the decimal reduction time (time required at a given temperature for destruction of 90% of the cells), when survivors were recovered on AAMS were 62.04, 18.00, 8.00, 3.33, and 1.05 min at 112.8, 115.6, 118.3, 121.1, and 123.9 degrees C, respectively. Recovery on AAMS-S resulted in reduced decimal reduction time. The computed z value (the temperature change which will alter the D value by a factor of 10) for spores recovered on AAMS was 8.3 degrees C; for spores recovered on AAMS-S, it was 7.6 degrees C. The rates of inactivation and injury were similar. Injury (judged by salt sensitivity) was a linear function of the heating temperature. At a heating temperature of less than or equal to 118.3 degrees C, spore injury was indicated by the curvilinear portion of the survival curve (judged by salt sensitivity), showing that injury occurred early in the thermal treatment as well as during logarithmic inactivation (reduced decimal reduction time). Heat-injured spores showed an increased sensitivity not only to 0.9% NaCl but also to other postprocessing environmental factors such as incubation temperatures, a pH of 6.6 for the medium, and anaerobiosis during incubation.