378 



GROWTH OF BACTERIOPHAGE 



If from a mixture containing many particles very small samples are 

 withdrawn, containing each on the average only about one or less 

 particles, then the fraction pr of samples containing r particles is given 

 by Poissons' (12) formula, 



Pr^ 



nTe' 



(1) 



where n is the average number of particles in a sample and e is the 

 Napierian logarithm base. If the average number n is unknown, it 

 can be evaluated from an experimental determination of any single 



TABLE II 



Distribution of Individual Particles among Small Samples 

 A suitably diluted phage preparation was added to 5 cc. of 18 hour bacteria 

 culture and 0.1 cc. samples of this mixture were plated. The distribution of 

 particles among the samples is that predicted by formula (1). 



one of the pr, for instance from a determination of Po, the fraction of 

 samples containing no particles: 



n = —Inpo (2) 



Let us now consider the following experiment. A small number of 

 phage particles is added to a suspension containing bacteria in high 

 concentration. Within a few minutes each phage particle has at- 

 tached itself to a bacterium. The mixture is then diluted with a large 

 volume of broth, in order to have the bacteria in low concentration so 

 that after the first burst a long time elapses before reinfection, as in 

 the one step growth curves. Samples (0.05 cc.) are removed from 

 this mixture to separate small vials and incubated at the desired 

 temperature. If these samples are plated separately (after adding a 

 drop of bacterial suspension to each vial) before the occurrence of 

 bursts, the fraction of the plates containing 0, 1,2, etc. plaques is found 



50 



