Effects of thermoradiation on bacteria

Abstract

A 60Co source was used to determine the effects of thermoradiation on Achromobacter aquamarinus, Staphylococcus aureus, and vegetative and spore cells of Bacillus subtilis var. globigii. The rate of inactivation of these cultures, except vegetative cell populations of B. subtilis, was exponential and in direct proportion to temperature. The D10 (dose that inactivates 90% of the microbial population) value for A. aquamarinus was 8.0 Krad at 25°C and 4.9 Krad at 35°C. For S. aureus, D10 was 9.8 and 5.3 Krad at 35 and 45°C, respectively. Vegetative cells of B. subtilis demonstrated a rapid initial inactivation followed by a steady but decreased exponential rate. The D10 at 25°C was 10.3 Krad but at 35 and 45°C this value was 6.2 and 3.8 Krad, respectively. Between 0 and 95 Krad, survival curves for B. subtilis spores at 75°C showed slight inactivation, increasing in rate at and above 85°C. The D10 values for spores at 85 and 90°C were 129 and 92 Krad, respectively. Significant synergism between heat and irradiation was noted at 35°C for A. aquamarinus and 45°C for S. aureus. The presence of 0.1 mM cysteine in suspending media afforded protection to both cultures at these critical temperatures. On the other hand, cysteine sensitized B. subtilis spores at radiation doses greater than 100 Krad. The combined effect of heat and irradiation was more destructive to bacteria than either method alone.