Studies of the growth-modifying actions for Escherichia coli, Saccharomyces cerevisiae, and Tetrahymena thermophila of helium, nitrogen, argon, krypton, xenon, and nitrous oxide led to the conclusion that there are two definable classes of gases. Class 1 gases, including He, N2, and Ar, are not growth inhibitors; in fact, they can reverse the growth inhibitory action of hydrostatic pressures. Class 2 gases, including Kr, Xe, and N2O, are potent growth inhibitors at low pressures. For example, at 24°C, 50% growth-inhibitory pressures of N2O were found to be ca. 1.7 MPa for E. coli, 1.0 MPa for S. cerevisiae, and 0.5 MPa for T. thermophila. Class 1 gases could act as potentiators for growth inhibition by N2O, O2, Kr, or Xe. Hydrostatic pressure alone is known to reverse N2O inhibition of growth, but we found that it did not greatly alter oxygen toxicity. Therefore, potentiation by class 1 gases appeared to be a gas effect rather than a pressure effect. The temperature profile for growth inhibition of S. cerevisiae by N2O revealed an optimal temperature for cell resistance of ca. 24°C, with lower resistance at higher and lower temperatures. Overall, it appeared that microbial growth modification by hyperbaric gases could not be related to their narcotic actions but reflected definably different physiological actions
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