EMORY L. ELLIS AND MAX DELBRUCK 381 



average size, under our conditions, vary widely in individual size. A 

 burst occurs after a definite latent period following the adsorption of 

 the phage on susceptible bacteria, and visible lysis coincides only 

 with the last step-wise rise in the growth curve when the phage parti- 

 cles outnumber the bacteria present. It seemed reasonable to us to 

 assume that the burst is identical with the lysis of the individual 

 bacterium. 



Krueger and Northrop (3), in their careful quantitative studies 

 of an ant'i-staphylococcus phage came to an interpretation of their 

 results which differs in some important respects from the above: 



1. Their growth curves were smooth and gave no indication of steps; 

 they concluded therefore that the production of phage is a continuous 

 process. 



2. In their case, the free phage during the logarithmic phase of a 

 growth curve was an almost constant small fraction of the total phage. 

 This led them to the view that there is an equilibrium between intra- 

 cellular and extracellular phage. With an improved technique, 

 Krueger (10) found that the fraction of free phage decreased in propor- 

 tion to the growth of the bacteria, in conformity with the assumption 

 of an equilibrium between two phases. 



3. Krueger and Northrop (3) found that visible lysis occurred when 

 a critical ratio of total phage to bacteria had been attained, and they 

 assumed that there was no lysis in the earlier period of phage growth. 



To appreciate the nature of these differences it must be born in 

 mind that their method of assay was essentially different from ours. 

 They used, as a measure of the "activity" of the sample of phage 

 assayed, the time required for it to lyse a test suspension of bacteria 

 under standard conditions. This time interval, according to the 

 picture of the growth process given here, is the composite effect of a 

 number of factors: the average time required for adsorption of free 

 phage, its rate of growth in the infected bacteria, the time and size of 

 burst, and the average time required for repetition of this process 

 until the number of phage particles exceeds the number of bacteria 

 and infects substantially all of them. Then, after a time interval 

 equal to the latent period, lysis occurs. 



This lysis assay method tends to measure the total number of 

 phage particles rather than the number of infective centers as the 



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