SummaryA small fraction, about 0.05'%, of the microorganisms in milk are resistant to pressure to at least 150,000 p.s.i. The pressure-resistant microorganisms were shown to be mainly bacterial spores. Some of the pressure-resistant organisms were identified. The lethal effect of pressure is more pronounced in the solid phase than in the liquid phase of water. B. sitbtilis was found to be capable of surviving solid-phase transitions from Ice 11, 111, and 1 ' to Ice I. The lethal effect of high pressure in the liquid phase of water was found not to be counteracted by an increase in temperature. The pressure resistance of B. subtilis was found to be maximum around neutral or slightly basir pH. The damaging effect of low and high pH are counteracted by sodiilm chloride and glucose. It is suggested that the primary mechanism oE protein denaturation by pressures above 30,000 p.s.i. is ionization and formation of ionic bonds between charged groups on proteins, altering their solubility.

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