114 BENZER ET AL. 



There are no means of introducing directly an exact and Jehiiwd 

 ^..^hPY of phages into individual Vihi i.-ii^- Txirecuuu is brought about 

 by mixing a sviapensioii of a known number of bacteria with a suspen- 

 sion of a known number of phages. The proportion of these two num- 

 bers determines roughly the multiplicity of infection. Since adsorption 

 of phages is never 100%, the actual multiplicity has to be determined 

 for each experiment, for instance by comparison of the plaque count 

 after adsorption with the total virus input. Each bacterium does not 

 receive exactly the same number of infecting phage particles. Because 

 of statistical fluctuations some will have more, some less. This kind 

 of fluctuation is governed by the Poisson Law p(r)^n'"e""/r!, which 

 gives the probability p(r) of r objects being present in a given sample, 

 when the average number of objects per sample is n. If, for example, 

 n=4, the proportion of bacteria which have been infected by exactly 

 r=4 phage particles is p(4)=4*e"V4!=o.2. Only 20% of all bacteria 

 have been infected with exactly 4 phage particles. In a similar way 

 the percentages for all values of r can be calculated. Besides infecting 

 bacteria singly or multiply with only one type of phage one can also 

 try to infect them with two or more different phage strains. This is 

 called mixed infection. The true conditions of such an experiment 

 again have to be determined and calculated in the same manner in- 

 dicated above. 



For the analysis of mixed infection experiments, indicator strains 

 of bacteria are used, which are resistant to certain phages. They are 

 derived as mutants from the E. coli strain B which is commonly used 

 in all these experiments and which is sensitive to all the seven T phages. 

 The resistance pattern of each mutant is indicated in the following 

 manner: B/2 means a strain resistant to T2; B/3, 4, 7 means a strain 

 resistant to T3, T4, and T7, etc. 



A mixture of T2 and T4 will show only the plaques of T4 when 

 plated on B/2 and only those of T2 on B/4. // B/2 and B/4 are mixed 

 and used for plating a mixture of T2 and T4, both phage strains will 

 form plaques on the same plate, but the plaques will generally be 

 turbid, since in each of them only one of the bacterial strains present 

 is eliminated by lysis. But where the areas of a T2 and a T4 plaque 

 overlap, a clear zone will be formed, since there both bacterial strains 

 are lysed. Completely clear plaques are formed on mixed indicators 

 by bacteria which have been mixedly infected and yield a mixed prog- 

 eny of phages, since in such a plaque both indicators are lysed. 



For experiments involving a phage and its host range mutants it 

 is useful to employ a mixture of B and an indicator strain. Here the 



