Effects of Thermal Stress on Viability and Ribonucleic Acid of Aerobacter aerogenes in Aqueous Suspension

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Abstract
The death-rate of washed Aerobacter aerogenes in aqueous suspension at 47[degree] depended on the nature of the growth medium, the composition of the liquid used to wash and resuspend the bacteria, the bacterial growth phase, the bacterial concentration in heated suspensions, the pH value, the oxygen tension and the composition of the diluent in which bacteria were heated. The relative resistance of bacteria in different growth phases differed according to the growth medium and the washing fluid; stationary phase bacteria were not more resistant than exponential phase organism under all conditions. Starvation increased the thermal resistance of exponential and stationary phase bacteria. High bacterial concentration favored survival at 47[degree] under more conditions; cell-free filtrate from a heated dense suspension (1010 bacteria/ml) protected a sparser popu-lation of fresh bacteria (107[long dash]109/ml) heated in it. Protective material in filtrate was heat-stable (100[degree]/15 min.) and diffused through cello-phane. The optimum pH value for survival at 47[degree] was near pH 6.5. Aerobic conditions favored survival in distilled water but not in salt solutions or phosphate saline (pH 6.5). The effects of various concentrations of NaCl and KC1 on the survival of bacteria at 47[degree] under aerobic conditions were different, K+ concentrations above 0.1 M being more lethal than equivalent concentrations of Na+; the lethal effect of heating in mixtures of these salts (total M > 0[center dot]1) increased with K+ concentration. Growth medium, Mg2+ (0[center dot]01-5m[image]) and, to a lesser extent, Mn2+ (0.5m[image]) or Co2+ (5 m[image]) decreased the death-rate, whereas ethylene-diamine tetraacetic acid (m[image]) or various sugar increased it. Mg2+ but not Mn2+ reversed the lethal effect of sugars. Generally, conditions which accelerated the death-rate of Aerobacter aerogenes at 47[degree] also increased the rate of degradation of endogenous ribonucleic acid (RNA). This was accompanied by an increase in the ultraviolet absorption of cold acid-extracts of bacteria and of the suspending fluid. Bacterial protein was degraded to a smaller extent. Depletion of RNA is probably not the primary cause of death at 47[degree] but the effect on bacterial metabolism of a rapid increase in endogenous pool constituents resulting from RNA degradation may contribute to the lethal effect.