SUMMARY: The death-rate of washed in aqueous suspension at 47° 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 organisms under all conditions. Starvation increased the thermal resistance of exponential and stationary phase bacteria. High bacterial concentration favoured survival at 47° under most conditions; cell-free filtrate from a heated dense suspension (10 bacteria/ml.) protected a sparser population of fresh bacteria (10-10/ml.) heated in it. Protective material in filtrate was heat-stable (100°/15 min.) and diffused through cellophan. The optimum pH value for survival at 47° was near pH 6·5. Aerobic conditions favoured survival in distilled water but not in salt solutions or phosphate saline (pH 6·5). The effects of various concentrations of NaCl and KCl on the survival of bacteria at 47° under aerobic conditions were different, K concentrations above 0·1 being more lethal than equivalent concentrations of Na; the lethal effect of heating in mixtures of these salts (total > 0·1) increased with K concentration. Growth medium, Mg (0·01–5 m) and, to a lesser extent, Mn (0·5 m) or Co (5 m) decreased the death-rate, whereas ethylenediamine tetraacetic acid (m), or various sugars, increased it. Mg but not Mn reversed the lethal effect of sugars.

Generally, conditions which accelerated the death-rate of at 47° also increased the rate of degradation of endogenous 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° but the effect on bacterial metabolism of a rapid increase in endogenous pool constituents resulting from RNA degradation may contribute to the lethal effect.


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