486 BACTERIOPHAGES 



teria. The method was originaUy used by Burnet (1929a) to de- 

 termine the phage yield from single cells. Delbriick (1 945c) used 

 it in analysis of the phage yields of bacteria simultaneously in- 

 fected with 2 different types of phage. This technique is very 

 useful in study of the genetics of bacterial viruses (see pp. 501 ff.). 

 The distribution of particles among a number of samples fol- 

 lows the Poisson formula if the average number of particles per 

 sample is of the order of 10 or less. The Poisson formula is 



P(r) = "-^ 



in which P(r) is the proportion of samples containing" r particles 

 when the average number of particles per sample is ti. For the 

 single-cell method the value of n should be made so low that the 

 proportion of samples containing 2 or more infected bacteria 

 will be small in relation to the proportion containing 1 . Usually 

 the value of n is made about 0.3-0.4. A sample experiment is 

 described to illustrate the principle. 



Procedure. Coliphage T2 will be used to infect a broth-grown 

 culture of E. coli at an input ratio such that essentially all bacteria 

 will be infected during a 5 min. adsorption period. The free 

 phage will be neutralized by antiserum. The infected bacteria 

 will be about 0.6 bacterium/ml. This dilution will be distrib- 

 uted into 50 samples of 0.5 ml. each. After incubation until 

 all bursts have occurred the samples will be plated by the agar 

 layer method. 



The adsorption rate for T2 is known to be about 80 per cent in 

 5 min. at 37 °C. for strain B grown in broth to a concentration of 

 5X10'^ /ml. An input ratio of 4 T2/B would then result in an 

 adsorption of 4 X 0.8 = 3.2 T2/B in 5 min. The proportion of 

 uninfected bacteria would be equal to e~^-^ = 4 per cent, or 

 96 per cent of the bacteria would be infected . The concentration 

 of unadsorbed phage in the adsorption tube would be 0.8 X 5 X 

 10" = 4 X 10" ml., which means less than 1 free phage particle/ 

 bacterium. This would be negligible in comparison with the 



