OXYGEN TENSION 



289 



for aerobically grown E. coli (B/r), whereas the relative sensitivity for 

 oxygen-treated samples, as compared with nitrogen-treated ones, is ap- 

 proximately three-fold. 



The same modification of sensitivity has been demonstrated on E. 

 coli, strain B, which has been reported to be more sensitive to radiation 



Anaerobic 



40 60 



X-ray dose, kiloroentgens 



Fig. 2. "Anaerobic" and "aerobic" refer to the mode of growing tbe organisms. 

 "Oxygen" and "nitrogen" refer to type of suspension in which bacteria were kept 



during irradiation. 



than B/r. Since strain B is more sensitive to x-rays, the rate of killing 

 is greater under both conditions studied; nevertheless the ratio of sensi- 

 tivity of oxygen-treated cells to nitrogen-treated cells is approximately 

 equal to that found for B/r. Nitrogen can be replaced with helium, 

 hydrogen, or carbon dioxide without significantly changing sensitivity. 



