652 



GROWTH OF PHAGE AND LYSIS 



count increases by a factor of twenty in 3 minutes and very slight inac- 

 curacies in timing will entail huge percentage deviations in the plaque count. 

 During the process of plating a bacterium may liberate the phage which it 

 contains and thus add the full number of a "burst" to the normal sample 

 value. This will place the point too high; it was probably the case in the 

 17 minute point of the first growth curve. 



7' 



10 



20 



30 40 



50 



Fig. 3. Phage growth on rapidly dividing bacteria. 



At time zero phage were added to the experimental culture of bacteria in its optimal 

 growth phase (3 hours after inoculation) and well below its maximum concentration 

 of bacteria (5 X 10^ against 2.5 X 10^). The initial phage concentration was 2 X 10"*, 

 so that even after the first step the bacteria were still much in excess and no multiple 

 infection occurred. After the second step the phage were in excess. The course of 

 events resulting from this situation will be discussed separately. 



Dates of experiments: • 9-13, O 9-15, X 9-22. 



Fig. 4 brings out very clearly one point which was not recognizable in the 

 logarithmic plots: Phage liberation starts suddenly after the latent period 

 of 17 minutes and continues at a constant rate for about 16 minutes, at which 

 point it ceases almost equally abruptly. In this interval from 17 to 33 

 minutes the plaque count increases by a factor 170. 



These characteristics of phage growth, namely the latent period, the 

 spread of the latent period, and the step size depend on the physiological 

 state of the bacteria. For example, if, instead of using the above defined 

 experimental culture after 3 hours when the bacteria are large and divide 

 rapidly we had taken the bacteria from the stock culture directly, the one 

 step phage growth curve under the same conditions (in broth at 37°C.) 



66 



