The effect of hydrostatic pressures as high as 1,700 atm at 25 C on the heat and radiation resistance of Bacillus pumilus spores was studied. Phosphate-buffered spores were more sensitive to compression than spores suspended in distilled water. Measurements of the turbidity of suspensions, the viability, refractility, stainability, dry weight, and respiratory activity of spores, and calcium and dipicolinic acid release were made for different pressures and times. Initiation of germination occurred at pressures exceeding 500 atm and was the prerequisite for inactivation by compression. The rate of initiation increased with increasing pressure at constant temperature. This result is interpreted as a net decrease in the volume of the system during initiation as a result of increased solvation of the spore components.

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The kinetics of initiation of germination and inactivation by hydrostatic pressure of phosphate-buffered Bacillus pumilus spores is shown to be a consecutive firstorder process at 25 C. The effect of increasing pressure at constant temperature was studied, and rate constants were derived by using the criteria of heat resistance, refractility, and stainability. The calculated volume change of activation (AVt*) was-139 + 6 cm3/mole for loss of heat resistance,-158 i 8 cm'/mole for the loss of refractility, and-153 i 4 cm3/mole for the change in permeability to dilute stains for the pressure range 800 to 1,010 atm at 25 C. It is suggested that the spore exists as a Donnan phase and that pressure triggers germination by influencing the equilibrium.

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