522 BACTERIOPHAGES 



the pH should be between 7.5 and 8 for maximum recovery of 

 plaque formers. These studies were carried out using phages 

 T2, T4, and lambda on their respective hosts strains B and K12 

 of E. coli. 



3. Estimation of intracellular phage development by use of 

 streptomycin. Symonds (1957) has developed a technique to 

 measure, at any time during the latent period, the number of 

 bacteria that contain infective phage. This technique involves 

 the use of a streptomycin-resistant mutant as indicator organism. 

 The infected bacteria are streptomycin sensitive. During the 

 latent period samples are plated in streptomycin-containing 

 agar with the resistant mutant as the indicator. The infected 

 bacteria are therefore killed by the streptomycin and no further 

 phage development proceeds. However, those of the infected 

 bacteria which contained, at the time of plating, infective phage, 

 now lyse and release this phage, thereby forming a plaque on 

 the streptomycin-resistant indicator.] 



4. Lysis by sonic vibration. This method of lysing infected 

 bacteria during the latent period is restricted to small phages 

 such as Tl, T3, and T7, which are relatively slowly inactivated 

 by the sonic treatment. The method is the same as the foregoing 

 except that samples withdrawn from the dilution tube at inter- 

 vals were diluted into tubes of growth medium which had been 

 prechilled to ° C. This diluted sample was placed in the sonic 

 vibrator and treated for 5 min. at 5 ° C. Appropriate samples 

 were then plated by the agar layer method for phage assay. 



Experiments in which separate samples from the same dilu- 

 tion tube were treated by lysis from without or by sonic vibra- 

 tion demonstrated an extremely close agreement between the 

 methods. 



