376 



UAIMATIO.N lilOLOOY 



were more seiisiti\'e as their coiiceiitration in the irradiated suspeiision.s 

 was rechiced and that nutrient broth and certain amino acids afforded 

 some protection. All these observations indicate that an indirect action 

 of X rays is involved in bacterial inactivation, which may be similar to the 

 indirect action of X rays on dilute suspensions of enzymes and viruses 

 (for discussion, see liea, 1947). 



40 60 80 



X-RAY DOSE , kr 



120 



Fig. 10-3. Comparative sensitivity of aerobic and anaerobic cells irradiated in high and 

 low oxygen tensions. I, aerobic broth cells irradiated in oxygen-saturated buffer; 

 II, aerobic broth cells irradiated in nitrogen-saturated buffer; III, anaerobic glucose 

 cells irradiated in oxygen-saturated buffer; IV, anaerobic glucose cells irradiated in 

 nitrogen-saturated buffer. {HoUaender, Stapleton, and Martin, 1951.) 



Current experiments (Stapleton, personal communication) indicate 

 that since the cell suspensions in the earher studies were prepared at room 

 temperature and were not chilled until just before irradiation, the appar- 

 ent decrease in sensitivity with increased bacterial concentration may be 

 attributed to the greater decrease in oxygen concentration resulting from 

 endogenous oxygen utilization by the cells in the more concentrated 

 suspensions. The existence of a critical oi- threshold concentration of 

 oxygen l)elow which survival is greatly increased is indicated by Fig. 

 10-5 (from Morse, Burke, and Burnett — see HoUaender and Stai)leton, 



