The Effect of Temperature on the Growth of Bacterium coli at pH 7{middle dot}0 with a Constant Food Supply

Abstract

SUMMARY: The effect of temperature on the growth of Bact. coli under accurately controlled conditions has been examined in an apparatus which permits food to be supplied at any desired rate by means of an automatic syringe mechanism. Six temperatures were used, from 15° to 40° at 5° intervals, with a single rate of food supply. Total and viable cell counts were frequently made and growth curves constructed. In some cases the changes in numbers were followed after stopping the food supply. At all stages the total counts greatly exceeded the viable. There was always first an ‘initial’ phase in which the daily increments in total cells varied with time. Very early in this phase cell division lagged behind cell growth. At 15° this condition probably persisted throughout the whole experiment. The initial phase was longest when the temperature was lowest, but the time taken to reach 300 × 106 total cells/ml. did not vary greatly, except at 15° when the time was markedly increased. The yield of total cells at the end of the initial phase was highest when the temperature was lowest and depended on the amount of food added, i.e. on the duration of the phase, but relatively more cells were formed in a long than in a short phase. The viable cell count at the end of the initial phase was also enhanced by low temperature, except at 15° where the viable cells were fewer than expected in comparison with the other experiments. The ratio of the total to the viable count at the end of the initial phase was often close to 2·0. The initial phase was followed by a ‘steady’ phase in which there were, on the whole, constant daily increments in both total and viable counts, although there were indications of stepwise increments. In the steady phase the calculated rates of increase in both total and viable cells were highest when the temperature was low, and bore a linear relation to the temperature. The rate of increase in viable cells was almost zero at 35°, which was apparently a critical temperature for viability. At 40° there was only a slow decline in viable cells, but nevertheless at both 35° and 40° growth of the cultures occurred since the total counts continually increased, but approximately half the cells formed were non-viable. To express the difference between the total and viable counts a non-viability index was calculated. This was constant throughout the steady phase and was smallest when the temperature was lowest. The amount of food previously shown to be required for the formation of a new cell at 35° was found to be not inconsistent with the experimental data at any of the other temperatures, but if this amount were constant the food used per cell for maintenance and wastage must have declined with decreasing temperature. After cessation of the food supply the total count declined at both 15° and 30°, being slightly faster at the lower temperature. The viable count also declined during starvation at 15°; the rate of decrease of the viable count was greater at 30°.