Radiation Killing ofE. ColiK1060: Role of Membrane Fluidity, Hypothermia and Local Anaesthetics

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Abstract
The enhancement of killing by γ-irradiation, which is seen when E. coli K 1060 are cooled below the transition temperature of their membrane lipids, is blocked by procaine-HCl. These data are consistent with the hypothesis that increased killing associated with irradiation at 0°C is the result of membrane microviscosity increases, since procaine is known to fluidize membranes. A cooling enhancement ratio (c.e.r.) is defined as the ratio of radiation D0 at 22°C to its value at 0°C. The c.e.r. for oxygen-bubbled cells is 1·5 and for nitrogen-bubbled cells is 2·1. In the presence of 25 mM procaine the respective c.e.r. values are 1·08 and 1·29. The oxygen enhancement ratio (o.e.r.) at 22°C is 3·43 and at 0°C is 2·45. The addition of procaine does not change the o.e.r. Thus, the temperature effect on o.e.r. does not appear to be related to membrane fluidity.