506 



BACTERIOPHAGES 



The mutant is given the symbol r (rapid lysis) and the corre- 

 sponding wild type is designated r"^, e. g., T2r+, and T2r. The 

 wild type r+ particles form small plaques with very turbid halos 

 when plated by the agar layer technique, whereas the mutant r 

 variants form larger plaques with clear halos. Another char- 

 acteristic distinguishing /•+ from r is the time required for lysis of 



Figure 15. Phage T2, showing lr+ plaque, \r plaqi 

 Dark, field iUumination. 



and 2 mottled plaques. 



visibly turbid cultures. The latent period of phage growth is about 

 21 min. for members of the T2, T4, T6 family regardless of 

 whether the virus is the r+ or the r variant. Visibly turbid cultures 

 of bacteria will be lysed by the r mutants between 21 and 30 min. 

 after all the bacteria have been infected. However, in the case 

 of the r+ phages, the visibly turbid cultures remain turbid for 

 several hours after all the bacteria have been infected. This 

 phenomenon of lysis inhibition was investigated by Doermann 

 (1948), who concluded that when a bacterial cell infected with 



