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RADIATION HIOLOGY 



appaixMilly roduccd. The second method is to draw tangents to the two 

 survival curves, in darkness and after photoreactivation, at points of 

 same survival; the pair of tangents drawn to the two curves for any value 

 of the survival must intersect on the ordinate. Novick and Szilard have 

 shown that both conditions are fulfilled in their experiments with E. coli 

 B r (Fig. 12-6). 



200 



4 00 



500 



300 

 D , sec 

 Fig. 12-6. Survival curves for Escherichia coli B/r in darkness (curve I) and after max- 

 imum photoreactivation (curve II). Curve III gives the ultraviolet dose correspond- 

 ing to a given survival after photoreactivation versus the dose corresponding to the 

 same survival in darkness. The dotted lines are a pair of tangents to the survival 

 curves at two points of same survival (see text). For survival curves, the logarithm of 

 surviving bacteria (left-hand scale) is plotted versus the idtraviolet dose in seconds of 

 exposure (bottom scale). For the lower curve, the light-vdtraviolet dose in seconds of 

 exposure (right-hand scale) is plotted versus the dark-ultraviolet dose (bottom scale). 

 {From Novick and Szilard, 1949.) 



In E. coli B the relation between the survival in the dark and after 

 photoreactivation depends on the physiological conditions of the bacteria 

 (Xovick, personal communication). In resting bacteria, obtained with 

 the method previously described, multiple-hit type survival curves are 

 observed both in darkness and after maximum photoreactivation; these 

 curves show constant dose reduction by the light. The survi\-al curve of 

 bacteria from a growing culture shows a first period of rapid inactivation, 

 the slope of the curve then decreasing; after maximum photoreactivation 

 a curve of multiple-hit type is obtained. Novick suggests that in growing 

 bacteria the .survival curve in darkness is the result of two types of inacti- 

 vation, of which the first, affecting all but a small fraction of the popula- 



