392 



RADIATION HIOLOGY 



samples of strain B. Tlic mctluxl consists in j)lutiM{i; samples of the bac- 

 teria on agar, irradiating with oO ergs/mm-, and incuhating at 37°C' for 3 

 hr. At the end of this time, all the surviving sensitive cells will have 

 formed long filaments. The resistant cells, however, will have divided 

 normally and formed a microcolony of 50 1 00 cells. The incubated plates 

 are then given a <ios(> of 700 ergs/mm'-. This exposure will leave fiom 10 

 to 20 resistant cells in each microcolony of strain U/v, whereas, if the fila- 

 ments of sensitive cells behave as single bacteria rather than as chains of 

 bacteria, all the sensitive bacteria will be killed by the second heavy irra- 

 diation. This was found to We the case and corresponds to the observa- 

 tion of Lea et al. (1937) that the filamentous forms were, like individual 

 cells, killed by a single hit and w^ere equally sensitive to radiation. Uti- 



300 



600 



900 



1200 1500 1800 



2 



Ergs per mm^ 



Fig. 10-8. Sensitivity of E. coli B and B/r to ultraviolet radiation. 

 Witkin, 1947.) 



(Adapted from 



lizing this quantitative double-irradiation technique, Witkin (1947) 

 demonstrated the mutational origin of the resistant cells and estimated 

 the mutation rate from radiation sensitivity to resistance to be about 10^^. 

 By concurrently testing the resistance of the radiation-resistant mutants 

 to penicillin and sodium sulfathiazole, at least four different types of 

 radiation-resistant mutations were demonstrated. Bryson (1947, 1948) 

 has extended Witkin's observations to include mustard and nitrogen 

 mustard. 



Survival curves obtained with E. coli B and B/r exposed to 2537 A 

 ultraviolet radiation are shown in Fig. 10-8. Although strain B is killed 

 exponentially with a change in the slope of the curve at about 1 per cent 

 survival, strain B/r follows a sigmoidal survival curve which, interpreted 

 within the framework of the target theory, would indicate that the muta- 

 tion to resistance causes a change from a single hit to multiple hits to be 

 necessary for lethality. Both strains are killed exponentially by X rays. 



