EFFECTS OF RADIATION ON BACTERIA 375 



communication). Although strain B/r shows about the median resist- 

 ance in this group, strain B is significantly more sensitive than any of the 

 others. 



Relation of Oxygen Concentration to X-ray Effects. The current interest 

 in the relation of oxygen tension to the effects of X rays on living cells 

 stems largely from the work of Thoday and Read (1947j, although earlier 

 investigators had made similar ol)servations. The relation of oxygen 

 concentration to cytogenetic effects is discussed by Giles (Chap. 10, 

 volume I of this series) along with possible interpretations to be placed 

 upon the observations. 



A similar relation between oxygen concentration and bactericidal effec- 

 tiveness of X rays has been observed for E. coli B/r by Hollaender and 

 coworkers (Hollaender, Stapleton, and Martin, 1951; Hollaender, Baker, 

 and Anderson, 1951: Hollaender, Stapleton, and Burnett, 1951; Hol- 

 laender and Stapleton, 1953). This same group has obtained interesting 

 results on the closely allied problem of chemical protection against X rays. 



Except when studying temperature effects, all irradiations were per- 

 formed at 2°C with washed cells suspended in M/lb phosphate buffer. 

 Reduction of oxygen tension was accomplished by partial evacuation fol- 

 lowed by saturation with nitrogen, helium, hydrogen, or carbon dioxide. 

 The particular gas used to replace oxygen was of no importance. 



Figures 10-3 and 4 from Hollaender, Stapleton, and Martin (1951) show 

 that lowering the oxygen tension changes not only the slope of the sur- 

 vival curve but also the shape, although in a later publication (Burnett 

 et al., 1951) exponential survival curves are shown for both oxygen-satu- 

 rated and nitrogen-saturated suspensions as well as for suspensions con- 

 taining 0.04 M concentrations of sodium hydrosulfite (Na2S204), British 

 anti-Lewisite (BAL), and ethanol. Equally apparent in Figs. 10-3 and 4 is 

 the lower sensitivity of cells grown in glucose broth and of cells grown 

 anaerobically, in contrast to the sensitivity of aerobically grown cells. 

 Although the ultimate slope of the nitrogen-saturation survival curves is 

 less steep than the slope of those for oxygen saturation, cells grown in 

 glucose broth or nutrient broth, either aerobically or anaerobically, 

 exhibit parallel survival curves once the threshold dosage is exceeded. 

 The threshold dose is a function of the method of culturing. 



Hollaender, Stapleton, and Martin (1951) observed that cells irradiated 

 in oxygen-saturated suspensions were more sensitive at 2°C, whereas cells 

 irradiated in the absence of oxygen were more sensitive at 37°C. As dis- 

 cussed earlier, Hercik (1934a) and Lea et al. (1936) failed to detect an 

 effect of temperature. The apparent disagreement is probably attribut- 

 able to the solubility of oxygen at different temperatures and to differ- 

 ences in the techni(iue of irradiation, since the latter workers exposed the 

 cells either on the surface of dried agar plates or in dried films of gelatin. 

 Hollaender, Stapleton, and Martin (1951 ) further observed that the cells 



