The lethality of γ-radiation doses of 0.2 to 1.0 kGy for Salmonella typhimurium ATCC 14028 was measured in the presence of air, N2 and N2 O and with the hydroxyl radical scavengers formate and polyethylene glycol (PEG), M r 8,000. Saturation of cell suspensions with either N2 O or${\rm N}_{2}/{\rm N}_{2}{\rm O}$ (1:1, v/v) gas was expected to double the number of hydroxyl radicals (${\rm OH}^{\bullet}$) and to produce an equivalent increase in lethality, but this did not occur. Adding 10% (v/v) O2 to either N2 or N2 O gas produced approximately the same γ-irradiation lethality for S. typhimurium as did air. Addition of hydroxyl radical scavengers, 40 mM formate and 1.5% (w/v) PEG, significantly reduced the lethality of γ radiation for S. typhimurium in the presence of air but not in the presence of N2 or N2 O gases. Membrane-permeable formate provided slightly better protection than nonpermeable PEG. Cells of S. typhimurium grown under anaerobic conditions were more sensitive to radiation, and were less protected by hydroxyl radical scavengers, especially formate, than when cells grown under aerobic conditions were irradiated in the presence of oxygen. Hydroxyl radical scavengers provided no further protection during irradiation in the absence of oxygen. These results indicated that the increased radiation sensitivity of cells grown under anaerobic conditions may be related to superoxide radicals which could increase intercellular damage during irradiation in the presence of oxygen. However, endogenous superoxide dismutase and catalase activities did not protect cells from the radiation-induced lethality of S. typhimurium. Cytoplasmic extracts protected bacterial DNA in vitro in either the presence or absence of oxygen, and no radiation-induced lipid peroxidation of the cellular components was identified by measuring the levels of 2-thiobarbituric acid. These results suggest that most radiation-induced cell lethality was related to the cooperative effects of extracellular${\rm OH}^{\bullet}$ and O2 on the cell surface as the radiation dose increased.

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