1952] 



ULTRAVIOLET LIGHT INDEX IN BACTERIOPHAGE 



63 



T2r is adsorbed to the extent of over 90 per cent in 10 minutes in buffer. At 

 the same buffer concentration, T7 adsorption is extremely slow, but the rate is 

 greatly increased by using buffer diluted 10 times with distilled water (Watson, 

 personal communication). Therefore, throughout the experiments with T7 diluted 

 buffer was used. The number of infective centers obtained (with either phage) 

 was usually found to be less, by one-third to one-half, than the number of phages 

 adsorbed. The cause of this "abortive adsorption" is not understood. To elimi- 

 nate unadsorbed phage, the cells are washed by centrifugation and resuspended 

 in fresh buffer. 



100- 



> 



u 



"^0 10 20 30 40 



TIME IN MINUTES AFTER ADDING BROTH 

 Figure 3. One-step growth curve at 37 C for T2r complexes formed in buffer. Broth 

 added at time zero. "Infective center" signifies a plaque-forming unit, either an infected 

 cell or a free phage particle. 



2. Growth. To the suspension of infected cells in buffer at 37 C, an equal volume 

 of broth at the same temperature is added, and time is reckoned from this 

 moment. In the case of phage T2r the radiation resistance promptly begins to 

 rise (figure 2). From the one step growth curve in figure 3, it may be seen that 

 the (minimum) latent period for T2r is 19 minutes, and the average burst size 

 is 60. For T7, the latent period is 12 minutes, and the burst size is 150. To stop 

 development at any chosen time during the latent period, a sample is removed 

 and diluted rapidly (by blowing out of a pipette) into buffer chilled in an ice 

 bath. The chilling brings the change in radiation resistance to an immediate 



302 



