The Sensitivity of Microorganisms to Irradiation under Controlled Gas Conditions

Abstract

Variation in radiosensitivity with oxygen concentration was examined for 3 strains of bacteria (Escherichia coli B, E. coli B/r, and Shigella flexneri Y6R), and for a haploid yeast (Saccharomyces cerevisiae). Gas mixtures were kept continuously bubbling through the suspensions during irradiation; under these conditions treatments other than change in oxygen concentration were ineffective in altering radiosensitivity. With X-irradiation the maximum ratio for enhancement of sensitivity by oxygen was about 3 for the bacteria and 2.4 for the haploid yeast; with neutron irradiation the ratio was 1.5 for one strain of bacteria examined and 1.6 for the yeast. In all cases the radiosensitivity was increased to half the maximum ratio with oxygen concentration of 4 to 7 [mu] [image]. It was found that with one microorganism the relative biological effectiveness (RBE) for fast neutrons to X-rays was greater than one under anoxic conditions, but less than one under full oxygenation. It is suggested that oxygen enhances the effect of radiation by reacting with ionized target molecules which in the absence of oxygen might be restored to normal functioning. In anoxic conditions a reaction leading to effective damage might take place between the ionized target molecule and another ionized molecule nearby, so that the enhancing effect of oxygen would be less with densely ionizing radiation; furthermore, the RBE of such radiation would depend on the oxygen concentration within the cell.