Influence of Time and Temperature of Incubation on Heat Resistance of Escherichia coli

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Abstract
Cultures of E. coli, grown in sterile reconstituted skim milk, exhibited a decided increase in heat resistance, as evidenced by percentage survival of cells during heat treatment, while in the initial stationary phase of growth. The increase in heat resistance was more marked in cultures incubated at 28[degree] than in those incubated at 38.5[degree] C. The time and temp. of incubation of the culture used for inoculum decidedly influenced the degree of increase in heat resistance during the initial stationary phase of growth of the subculture. The heat resistance of all of the cultures decreased as reproduction commenced and their resistance fell to a minimum during the period of most active reproduction. The resistance then increased again to a 2d peak as the rate of reproduction decreased and the culture entered the maximum stationary phase of growth. Growth at and above the optimum temp. resulted in cultures whose heat resistance during the maximum stationary phase was distinctly greater than was true of cultures incubated at temps. below the optimum for growth. Possible reasons for the variations between heat resistance of cultures grown at high and low temps. and the practical significance of the results are discussed.