330 BACTERIOPHAGES 



plies but not both. If one phage is clearly dominant under 

 conditions of simultaneous mixed infection it is possible to 

 transfer the advantage to the second strain by giving it a few 

 minutes head start. The mechanism of mutual exclusion is not 

 known but it clearly does not involve interference with adsorp- 

 tion, interference with penetration, or competition for a unique 

 key enzyme. There is one well authenticated case in which two 

 unrelated phages can grow simultaneously in the same host cell. 



In contrast to the cases of mutual exclusion, simultaneous 

 mixed infection with related phages permits both to multiply. 

 The mixed growth is usually accompanied by genetic recombina- 

 tion of suitable markers. If the pair of infecting phages is not 

 very closely related, partial exclusion often results. 



Partial exclusion between identical phages occurs under cer- 

 tain conditions. Primary infection of a bacterium with phage 

 T2 results in very rapid changes in the properties of the infected 

 cell. Superinfecting T2 phage is excluded from multiplication 

 and half of its DNA is rapidly broken down by deoxyribonuclease. 

 At the same time the bacterium becomes increasingly resistant 

 to lysis from without by high multiplicities of superinfecting 

 phage. 



Partial exclusion may occur in all instances of multiple infec- 

 tion. Thus a bacterium infected with many particles of T2, 

 one of which is genetically marked, is likely not to yield any 

 marked particles. Results of this kind suggest that only a 

 limited number of particles, about 10 in the case of T2, can 

 participate genetically in multiplication within a single bacterial 

 cell. The mechanisms of exclusion of all kinds are unknown. 



