382 GROWTH OF BACTERIOPHAGE 



following considerations show. Let us take a sample of a growth 

 mixture in which is suspended one infected bacterium containing 

 fifty phage particles. If this sample is plated, it can show but a 

 single plaque. However, if the sample is assayed by the lysis method, 

 this single infective center soon sets free its fifty particles (or more, if 

 multiplication is still proceeding) and the time required to attain 

 lysis will approximate that for fifty free particles rather than that for a 

 single particle. 



Since the burst does not lead to an increase in the number of phage 

 particles, but only to their dispersion into the solution, the lysis method 

 cannot give any steps in the concentration of the total phage in a growth 

 curve. On the other hand one might have expected a step-wise in- 

 crease in the concentration of free phage. However, the adsorption 

 rate of the phage used by Krueger (10) is so slow that the infection 

 of the bacteria is spread over a time longer than the presumed latent 

 period, and therefore the bursts would be similarly spread in time, 

 smoothing out any steps which might otherwise appear. Moreover, 

 their measurements were made at 30 minute intervals, which even 

 in our case would have been insufficient to reveal the steps. 



The ratio between intracellular and extracellular phage would be 

 determined, according to this picture of phage growth, by the ratio of 

 the average time of adsorption to the average latent period. The 

 average time of adsorption would decrease as the bacteria increased, 

 shifting the ratio of intracellular to extracellular phage in precisely 

 the manner described by Krueger (10). 



As we have indicated in the description of our growth curves, lysis 

 of bacteria should become visible only at a late time. Infection of a 

 large fraction of the bacteria is possible only after the free phage has 

 attained a value comparable to the number of bacteria, and visible 

 lysis should then set in after the lapse of a latent period. At this time 

 the total phage (by activity assay) will be already large compared to 

 the number of bacteria, in agreement with Krueger and Northrop's 

 findings. 



It appears therefore that while Krueger and Northrop's picture 

 does not apply to our phage and bacteria, their results do not exclude 

 for their phage the picture which we have adopted. It would be of 

 fundamental importance if two phages behave in such a markedly 

 dilEferent way. 



54 



