EFFECTS OF RADIATION ON BACTERIA 



393 



The physiological nature of these genetically determined differences 

 resulting in radiation resistance are as yet unknown. 



Po stir radiation Treatment. Hollaender and Claus (1937) found that 

 holding ultraviolet-irradiated cells in distilled water or physiological salt 

 solution resulted in significant increase in survival. Reference has 

 already been made to the work of Roberts and Aldous (1949) in which 

 they discovered that \'arious postirradiation treatments produced as 

 high as one-hundredfold increases in survival of E. coli B which had 

 been exposed to 2537 A ultraviolet. For example, significantly greater 

 survival was obtained when the irradiated cells were plated on syn- 

 thetic agar plates than when plated on 

 nutrient agar plates. Furthermore, 

 holding the irradiated cells in fluid 

 media resulted in striking increases 

 in survival. This recovery did not 

 depend to any great extent on the 

 presence of specific factors in the hold- 

 ing fluid; distilled water, saline, syn- 

 thetic medium, synthetic medium 

 without an energy source, and nutrient 

 broth gave approximately the same 

 results. Varying the pH from 5 to 9 

 or the concentration of bacteria from 

 10^ to 10^ per ml likewise had no effect 

 on the extent of recovery. Factors 

 which did influence the recovery in 

 fluid media were the radiation dose and 

 the temperature. As the radiation 

 dose was increased, the rate and final 

 level of recovery attained decreased. 

 The rate of recovery was found to be 

 directly correlated with temperature. 

 When the logarithm of the survival 

 ratio was plotted against dose of ultra- 

 violet for varj^ing periods of recovery, 

 significant changes in the slope and the shape of the survival curves 

 were observed. In Fig. 10-9 are shown the various survival curves 

 obtained. It will be observed that the survival curves change progres- 

 sively from a concave through a straight-line condition to a convex form 

 as the recovery approaches the maximum possible. Obviously, caution 

 must be exercised in the interpretations placed on the shape of survi\-al 

 curves. 



In discussing their results, Roberts and Aldous (1949) point out that 

 very careful control of technitiues must be exercised in order to obtain 



40 80 120 160 



DOSE, relative units 



Fig. 10-9. Survival curves of E. coli 

 B after 0-4.5 hours of recovery from 

 2537 A ultraviolet. {Adapted from 

 Roberts and Aldous, 1949.) 



