EFFECTS OF RADIATION ON BACTERIA 'iOo 



(1951) isolated mutations affecting the nutritional requirements of Aero- 

 hacter aerogenes when the cells were exposed to X rays in the dry state. 



DELAYED EXPRESSION OF INDUCED MUTATIONS 



Demerec (1946) observed a delay in the phenotypic expression of 

 ultraviolet-induced mutations to resistance to Tl bacteriophage. Only 

 about 1 per cent of the induced phage-resistant mutations were observed 

 as mutants when irradiated cells were plated upon the surface of agar 

 plates containing an excess of phage. The peak expression of induced 

 mutations occurred after one or two generations of growth, and mduced 

 mutations were observed to continue to appear until 12 or 13 generations 

 of growth had occurred. The technique which made these observations 

 possible consisted in infecting the bacteria, after varying periods of 

 growth on agar plates, by exposing the plates to an aerosol of the bacterio- 

 phage, a method which does not disturb the distribution of cells on the 

 plates'. Later, Newcombe (1948) observed a similar delay in the expres- 

 sion of spontaneous mutations to phage resistance although the appear- 

 ance of mutations did not continue through as many generations of growth. 

 Newcombe and Scott (1949), in studies of the factors responsible for the 

 delayed appearance of radiation-induced mutants, concluded that a phe- 

 notypic lag in expression of mutations, similar to that occurring m spon- 

 taneous mutations, plus a variable delay in the onset of division of the 

 irradiated cells are responsible. 



Demerec, Dollinger, and FUnt (1951) found quite different patterns m 

 the phenotypic expression of induced phage-resistant and streptomycin- 

 resistant mutations. Witkin (1951) employed a double-screemng tech- 

 nique to obtain evidence that induced mutations in E. coli were sometimes 

 expressed in only part of a clone derived from a surviving irradiated cell. 

 These results indicate that nuclear segregation, a physiological lag, and a 

 variable onset of division are not sufficient to explain the delay in expres- 

 sion of radiation-induced mutations. 



A complete discussion of delayed expression of mutations is beyond 

 the scope of this chapter. It is obvious, however, that this delay m 

 expression is an important factor which must be considered in quanti- 

 tative studies of radiation-induced mutations in bacteria. 



QUANTITATIVE STUDIES OF RADIATION-INDUCED MUTATIONS 



In an important paper, Demerec and Latarjet (1946) have reported 

 studies on ultraviolet- and X-ray-induced mutations to resistance to Tl 

 bacteriophage in E. coli B and B/r. Both zero-point mutations (i.e. 

 mutations expressed immediately) and end-point mutations (i.e., total 

 mutations induced including those expressed immediately and after a 

 delay) were assayed. In Fig. 10-1 1 are shown the results for ultraviolet 

 irradiation of strain B/r. 



