X-Ray Sensitivity 97 



effect of oxygen tension on X-ray sensitivity of cells can best 

 be illustrated by the work with bacteria. In all the bacterial 

 studies which will be discussed we used Escherichia coli, B/r, 

 a radiation-resistant strain (Witkin, 1946, 1947). This 

 organism was grown either aerobically in broth or anaero- 

 bically in the presence of glucose, washed after a 24-hour 

 growth period, and suspended in phosphate buffer solution 

 at pH 6-8. Suspensions were irradiated in vessels in which 

 the gas could be controlled during irradiation. Irradiations 

 were made with 250 kv X-rays at 30 ma with 3 mm. Al 

 filtration, after which the organisms were surface plated on 

 nutrient agar. Macroscopically visible colonies were counted 

 after 24 hours' incubation at 37 °C. 



Fig. 1 illustrates the results of a typical experiment. The 

 most sensitive cells were the ones grown and irradiated 

 aerobically; the most resistant ones were those grown and 

 irradiated anaerobically. The difference in energy used to 

 obtain an equivalent lethal effect is ten- to twelvefold between 

 the two extreme conditions. The difference in survival ratio 

 is as much as 10^-fold at 60,000 r; up to this dose, few bacteria 

 are killed under anaerobic conditions. Slopes of the killing 

 curves are determined by the oxygen tension of the suspension 

 during irradiation. The threshold dose is influenced by the 

 mode of growing the organisms, i.e. aerobically or anaero- 

 bically. Any inert gas can be used to replace oxygen. The 

 important point here is the presence or absence of oxygen 

 during irradiation. 



Very similar results in regard to X-ray sensitivity, but not 

 as striking as those with bacterial inactivation, have been 

 obtained with chromosome breakage in Tradescantia (Giles 

 and Riley, 1949), inhibition of growth of bean roots (Thoday 

 and Read, 1947, 1949), gene mutations in Drosophila 

 (Baker and Sgourakis, 1950), inhibition of mitosis in grass- 

 hopper neuroblasts (Gaulden and Nix, 1950). It is important 

 that oxygen has no effect if it is added after the irradiation has 

 been completed. The effect of depletion becomes pronounced 

 at concentrations of less than 21 per cent. Fig. 2 illustrates 



ISOTOPES 8 



