70 F. HERCIK 



stopped by chloramjihenicol either before or after infection of the cells 

 with phage (Hercik, 1959, 1960). 



The bacteria were irradiated on broth agar, from which they were 

 washed off either immediately after irradiation or 24 hr later. They 

 were then tested for the ability to form phage by adding phage T 3 and 

 determining the titre of the phages developed. The survival of the 

 capacity after irradiation was determined from the ratio of phage 

 yield of irradiated and of non-irradiated controls. To avoid formation 

 of colonies out of surviving cells, the bacteria destined for the observa- 

 tion of the delayed effect were kept for 24 hr at a temperature of 6°C. 

 This procedure was in accordance with that of Stapleton et al. (1953), 

 who found that if cells of E. coli were cultured at temperatures between 

 4°C and 37 °C after irradiation, the inhibition of division was the same 

 at 4°C as at 37°C (with a maximum of recovery at 18°C.) By using this 

 method with doses up to 20,000 r it was possible to avoid errors that 

 would otherwise have occurred as a result of capacity in bacteria that 

 had developed by proliferation of surviving cells. 



The bacteria were irradiated with a tube of 60kV, 4 mA and a dose 

 rate of 4,000 r/min. The range of doses used was from 4,000 r to 

 960,000 r. The survival of E. coli B corresponded to the customary value 

 of 3,500 r for the half-value dose (one-hit curve). After a dose of 

 120,000 r only one cell in 10'^ is capable of forming colonies. 



The results of this series of experiments are shown in Fig. 1. The 

 inactivation of the capacity by ionizing radiation differs according to 

 whether it is determined immediately after irradiation or 24 hr later. 

 Immediately after irradiation the decrease in capacity in relation to 

 the dose, on a semilogai'ithmic scale, is only approximately linear. This 

 is in agreement with the results obtained by Pollard et al. (1958) for 

 phage T 1. This is even more manifest after 24 hr. After small doses 

 (4,000 r and 8,000 r) a sharp linear decrease in capacity occurs. The 

 response to higher doses is quite dissimilar. There is also a linear de- 

 crease, but the drop in the capacity is much slower. 



The explanation of this phenomenon is difficult. It should be taken 

 into account that the inactivation of the bacterial capacity to produce 

 phage is the result of the irradiation reaction which has its own course. 

 It appears that recovery processes develop in the case of the immediate 

 effect of high radiation doses (as a result of the long exposure period) 

 which reduces the degree of inactivation of the capacity. On the other 

 hand with the delayed effect the change in the capacity follows the 

 course of the completed irradiation reaction and, therefore, relatively 

 small doses of irradiation produce a large drop in the capacity. With 

 doses higher than 80,000 r the bacterial capacity becomes more radio- 



