EMORY L. ELLIS AND MAX DELBRUCK 375 



37°, all of which gave the 70-fold burst size, i.e. an average of 70 phage 

 particles per infected bacterium, occurring quite accurately at the time 

 shown, 30 minutes. Indeed, one of the most striking features of these 

 experiments was the constancy of the time interval from adsorption 

 to the start of the first burst. The magnitude of the rise (70-fold) 

 was likewise readily reproducible by all phage preparations which 

 had been stored under proper conditions to prevent deterioration (see 

 above). 



Multiple Infection 



The adsorption measurements showed that a single bacterium can 

 adsorb many phage particles. The subsequent growth of phage in 

 these "multiple infected" bacteria might conceivably lead to (a) an 

 increase in burst size; (b) a burst at an earlier time, or (c) the same 

 burst size at the same time, as if only one of the adsorbed particles 

 had been effective, and the others inactivated. In the presence of 

 very great excesses of phage, Krueger and Northrop (3) and Northrop 

 (2) report that visible lysis of the bacteria occurs in a very short time. 

 It was possible therefore, that in our case, the latent period could be 

 shortened by multiple infection. To determine this point, we have 

 made several experiments of which the following is an example. 0.8 

 cc. of a freshly prepared phage suspension containing 4 X 10^ particles 

 per cc. (assay corrected for efficiency of plating) was added to 0.2 cc. 

 of bacterial suspension containing 4 X 10^ bacteria per cc. The ratio 

 of phage to bacteria in this mixture was 4 to 1. 5 minutes were 

 allowed for adsorption, and then the mixture was diluted 1 to 12,500 

 in broth, incubated at 25°, and the growth of the phage followed by 

 plating at 20 minute intervals, with a control growth curve in which 

 the phage to bacteria ratio was 1 to 10. No significant difference was 

 found either in the latent period or in the size of the burst. The 

 bacteria which had adsorbed several phage particles behaved as if 

 only one of these particles was effective. 



Effect of Temperature on Latent Period and Burst Size 



A change in temperature might change either the latent period, i.e. 

 the time of the burst, or change the size of the burst, or both. In 

 order to obtain more accurate estimates of the burst size it is desirable 



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