SUMMARY: The log survivor/time graph for mature cultures of one strain of was not always straight over its whole length. A preliminary period of slow death was frequently observed, and a final similar period less often. The log survivor/time graphs of young cultures varied in shape with age of culture and often showed an initial rapid death-rate, passing suddenly into a slower one.

The heat resistance of three strains was considerably influenced by the age of the culture. Resistance, judged either by the Decimal Reduction Time or the time taken to reach 99.9% mortality, was increased by transfer to fresh medium and fell during the lag phase of growth, reaching a minimum as rapid reproduction began. A second maximum in heat resistance occurred when growth became slow: thereafter the resistance gradually fell to that of mature cultures. The incubation temperature affected the heat resistance of young cultures, the effect of high temperatures being particularly marked and resulting in increased resistance, except with cultures 1 hr. old or less.


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