320 BACTERIOPHAGES 



materials for genetic research, and perhaps more important, the 

 fact that the research workers were not trained as classical 

 geneticists. 



1. Mixed Infection with Unrelated Phages — Mutual Exclusion 



The first clear-cut experiments involving the mixed infection 

 of bacterial cells with genetically unrelated phages were per- 

 formed by Delbriick and Luria (1942). They infected E. coli 

 strain B with the two unrelated phages Tl and T2, both of which 

 can grow on this host. The phage yields from such mixedly 

 infected bacteria were identified qualitatively by plating the 

 infected cells before lysis on agar plates seeded with indicator 

 bacteria: B/1 to recognize T2 yielders or B/2 to recognize Tl 

 yielders (Chapter IX). Similarly, the phage yield after lysis 

 could be assayed separately for its content of the two phage 

 types by plating samples on each of the two indicators. (Actu- 

 ally, in this instance, the two phages can also be distinguished 

 by the very different size of plaques.) 



Making use of these principles, Delbriick and Luria (1942) 

 performed one-step growth experiments (Chapter II), infecting 

 cells of strain B with the mixed phages Tl and T2, and plating 

 samples of the culture alternately on B/1 and B/2. The early 

 platings showed the proportion of the mixedly infected bacteria 

 that liberated each kind of phage, and the later platings showed 

 the average yield per infected bacterium for each phage type. 

 Following simultaneous mixed infection with an average multi- 

 plicity of 4 phage particles of each type, it was found that the 

 mixedly infected bacteria liberated only phage T2. Phage Tl 

 was unable to multiply although it adsorbed normally. If the 

 input ratio of phage T2 was decreased the proportion of infected 

 bacteria liberating T2 decreased and the proportion liberating 

 Tl increased correspondingly. The proportion of bacteria 

 liberating phage Tl corresponded exactly to the expected porpor- 

 tion that escaped infection by T2. This result means that 

 adsorption of a single T2 phage particle to a host bacterium 

 renders that bacterium incapable of serving as a host for phage 



