Reduction of the concentration of oxygen in bacterial suspensions has been shown to result in a several-fold reduction in the X-ray sensitivity of Escderidia coli (Hollaender, Stapleton, and Martin, 1951) . Addition of a wide variety of chemical compounds to supensions of E. coli prior to X irradiation afforded similar protection (Burnett, Stapleton, Morse, and Hollaender, 1951; Hollaender, Burnett, and Stapleton, 1951) . Pyruvate, added to suspensions of this organism before irradiation, has

more »

... en found to reduce the lethal and mutagenic effect of X rays (Thompson el al., 1951). Several explanations have been offered for the protection of bacterial cells by either the removal of oxygen from the suspension or by the action of chemical agents, namely: (1) reduction of the ionic yield of oxidizing radiodecomposition products of water in the aqueous suspension; (2) protection of sulfhydryl groups of cellular proteinr; and (3) modification of the reductive state of the organism. Some basis for the first explanation is available from the investigations of Dale (1947 ), Weiss (1947 , and Barron et al. (1949) which suggest that X-ray production of NO, and H20" in aqueous solutions, is dependent on the presence of dissolved oxygen. If these highly reactive producta are toxic agents, then the removal of dissolved oxygen, by any means, should reduce the lethal effect per unit dose. Too little experimental evidence is available to allow an assessment of the relative importance of the various suggested mechanisms. We have investigated the conditions which affect the protection given by some of the chemical compounds in an effort to elucidate the mechanism of chemical protection. It was found that removal of oxygen either by cellular enzyme action or autooxidation in the presence of these compounds can explain the protection afforded againt X-ray inactivation of E. coli. METHODS Cultures of E. coli, stran B/r, were grown for 18 hours under constant aeration at 37 C in Difco 0.8 per cent nutrient broth. Suspensions, harvested by centrifugation and washed in M/15 phosphate buffer (pH 6.8), were brought to one-tenth the broth volume in phosphate buffer. Final suspensions contained approximately 2 X 1010 cells per ml. In the irradiation studies 0.1 ml of the suspension was added to 0.9 ml of buffered solution of each chemical to be studied. Preirradiation treatment consisted of holding cells with or without the chemical for 30 minutes prior to irradiation at either ice bath temperature or at 37 C, as indicated in table 1.