Abstract

Summary. Damage induced in bacterial spores by exposure to reduced pressures of the order of 10‐8 torr, has been assessed in terms of the differences in the heat resistance of the dried spores. The response of the spores has been assessed as a function of (a), the drying temperature from 0‐65°; (b) the duration and level of rehydration; (c) the presence or absence of oxygen during heating. Comparison has also been made between spores dried to a given water level and spores rehydrated to the same water level after prolonged drying. Log survivors/heating time curves for treated spore samples have been constructed and have been shown to exhibit a shoulder at high survival levels and a linear portion below a surviving fraction of 0·1. These curves have been explained on the basis of the shoulder representing the time during which necessary structural changes occur in the spore, before the lethal mechanism responsible for the linear portion of the curve becomes operative. The heat response has been shown to be a function of the temperature of drying, and of the presence of oxygen during heating, the structural change itself being reversible by water.

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