EFFECTS OF RADIATION ON BACTERIA 401 



(Mizyme formation was proportional to dose for exposures less than 00,000 

 r, the amount of enzyme synthesized in such irradiated cell suspensions 

 was much greater than could be attributed to the cells still capal)le of 

 forming colonies. These studies suggest that inhil)ition of enzyme syn- 

 thesis may be more important in the bactericidal effect of radiation than 

 inactivation of enzyme molecules already present in the cells, as suggested 

 by Dale (1940, 1942). 



It appears that irradiated cells rendered incapable of forming visible 

 colonies are still able to perform many if not most of the normal metabolic 

 functions. Complex processes such as growth and phage synthesis would 

 seem to indicate that no gross disturbances of metal)olism result from 

 irradiation and that the cell membrane remains essentially intact. In this 

 latter connection, however, Loofbourow and associates (for review^ see 

 Loofbourow, 1948) have shown the accumulation in the suspending 

 medium of growth-promoting factors from irradiated cells, and Billen 

 (personal communication) has shown a leakage of adenosinetriphosphate 

 from E. coli cells following exposure to X radiation. 



The bactericidal effects of radiation have long been recognized more as a 

 specific inhibition of cell division than as a general inhibition of metabo- 

 lism, and the results discussed indicate greater radiosensitivity of cell 

 division as compared to the radiosensitivity of respiration and of certain 

 synthetic processes. Morse and Carter's investigations of nucleic acid 

 synthesis are an example of further studies of specific metabolic functions 

 intimately related to cell division which are needed to elucidate the par- 

 ticular metabolic processes involved in the inactivation of bacterial cells 

 by radiations. 



SUBLETHAL EFFECTS OF RADIATION 



The genetic changes are the most widely studied sublethal effects of 

 radiations. Before considering the genetic phenomena, however, some 

 other sublethal effects will be considered. 



The extension of the lag phase observed by numerous workers has been 

 studied in some detail by Hollaender and Duggar (1938). Working with 

 E. coll, they had observed a delay in the appearance of colonies following 

 irradiation with ultraviolet of 2650 A wave length. During their study 

 of the delay in growth of irradiated organisms in liquid cultures, they 

 observed a second effect, an apparent initial increase in the number of 

 cells. Thus, although the control suspensions showed no increase in 

 numbers of cells for the first 2 hr of incubation, the irradiated suspensions 

 showed a significant increase. The apparent early increase in rmmbers of 

 cells did not eliminate the extended lag phase, the duration of which was 

 estimated by extrapolating the logarithmic portion of the growth curve 

 back to zero growth. The extension of the lag phase is evident with 

 survivals as high as 70 per cent, whereas the apparent initial increase does 



