SYLLABUS 



137 



TIME (MIN.) 

 Fig. 4. Increase in radiation resistance of infective cen- 

 ters during intracellular development of T2r phage at 

 37C. The log of the fraction of infective centers surviving 

 a constant dose of UV irradiation as function of the time 

 subsequent to addition of broth to a suspension of infected 

 cells in buffer (Curve I). A control is shown to which no 

 broth has been added (Curve II). 



in radiation resistance of infective centers with time. Suppose a stand- 

 ard dose is chosen which gives a survival of 1%. Then, as long as 

 no nutrients are supplied and intracellular development thus does not 

 take place, the survival of a sample subjected to the standard dose 

 continues to be 1 % . If broth is added at any time, the survival begins 

 to rise in the same manner as observed if the infection is initially 

 made in broth. (See Figure 4.) The adsorption time of the phage is 

 effectively reduced to zero in this way, since all infective centers start 

 to develop at the same time. After seven minutes in the presence of 

 broth at 37°C, the survival becomes 40 times as high as initially for 

 the same dose. 



The radiation resistance of an infective center is thus a very sen- 



