Radiation killing of E. coli K1060: role of membrane fluidity, hypothermia and local anaesthetics.

The enhancement of killing by gamma-irradiation, which is seen when E. coli K1060 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 degree 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 degrees C to its value at 0 degree 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 degree C is 3.43 and at 0 degree 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.