LET as a Determinant of Bacterial Radiosensitivity, and Its Modification by Anoxia and Glycerol

Changes in radiosensitivity and in the radioprotection afforded by anoxia, glycerol, and other compounds have been studied as a function of LET in Shigella flexneri Y6R. The radiations used were 5-MeV electrons, 250-kVp X-rays, neutrons of mean energy 6 MeV, and beams of deuterons and helium nuclei in the range 60 to 800 MeV gm⁻¹ cm². The bacteria were irradiated in suspension or on cellophane carriers. With the latter, an anomalously high value of oxygen enhancement ratio (o.e.r.) was observed for X-rays, this value being higher also than for bacteria irradiated in suspension. Except for this, o.e.r. decreased with increasing LET. Protection ratios were the same for neutrons and radiations of low LET on suspensions, and, with the latter, the ratios were lower than when cellophane carriers were used. While the aerobic protection ratio was lower for helium ions at 800 MeV gm⁻¹ cm² than for other radiations, there was little change up to 500 meV ${ gm}⁻¹ {...