APPENDIX 519 



and assayed for surviving infective centers by plating before the 

 end of the latent period. The killing curve for singly infected 

 bacteria immediately after infection is a single hit curve very 

 close to that for free phage. It differs only in that the infective 

 centers are slightly more resistant to ultraviolet light than free 

 phage, as might be expected owing to the screening effect of the 

 bacterium. The killing curve for multiply infected bacteria 

 immediately after infection was a multiple hit curve. This 

 indicates that the killing of infective centers is due to destruc- 

 tion of the reproducibility of the phage itself rather than to 

 damage to the host cell. 



[The U V method for following early stages of phage develop- 

 ment has subsequently been used and refined by Benzer (1952) 

 andSymonds (1957).] 



This experimental approach to the mechanism of virus 

 growth was modified by Latarjet (1948), using X-rays as the in- 

 activating agent to avoid the screening effect of intracellular 

 nucleic acid. The experimental procedure was the same as 

 that already described except that X-radiation of 0.95 A wave- 

 length was used at doses as high as 250 kiloroentgens. 



Methods Involving Interruption of Growth During the Latent Period 

 Followed by Disruption of Host Cells 



These methods, developed by Doermann (1948b), constitute 

 an important contribution to phage methodology because they 

 permit assay of intracellular phage particles during the latent 

 period of phage growth. The methods involve an interruption 

 of phage growth at intervals during the latent period by means 

 of cell poisons or low temperature, followed by release of intra- 

 cellular phage by means of lysis from without (Delbriick, 1 940b) 

 or sonic vibration (T. F. Anderson, Boggs, and Winters, 1948). 



7. Lysis from without. In a typical experiment the growth 

 medium consisted of acid-hydrolyzed casein, tryptophan, salts, 

 and glycerol. Phage T4r was the infecting virus, 0.01 Af KCN 



