MIXED INFECTION 323 



of the first and second phages, the less is the depressor effect on 

 the yield of the first phage. Presumably the depressor effect 

 requires the injection of the nucleic acid of the excluded phage 

 into the bacterium because treatment of the culture with either 

 antibacterial serum or antiserum against the second phage 

 markedly decreases the depressor effect. Both types of anti- 

 serum interfere with penetration of phage nucleic acid into the 

 host cell (Delbrlick, 1945b; Adams and Wassermann, 1956; 

 Chapter VIII). 



In a brief report Delbriick (1945d) noted that of 14 pairs of 

 unrelated phages tested in mixed infection all demonstrated the 

 phenomenon of mutual exclusion. In contrast two pairs of 

 related phages, T2-T4 and T4-T6, gave clear evidence that some 

 mixedly infected bacteria would permit multiplication of both 

 infecting types. 



Several mechanisms were proposed to explain mutual exclu- 

 sion, the most generally accepted for some time being the "pene- 

 tration hypothesis" (Delbriick, 1945c). According to this 

 hypothesis the dominant phage in a given mixedly infected 

 bacterium excludes the other phage by altering the bacterial 

 surface so that the second phage cannot penetrate. There is 

 some evidence that this hypothesis may be applicable to certain 

 examples of exclusion but quite convincing evidence that it does 

 not apply to most cases. In fact, the existence of the depressor 

 effect and the prevention of this eflfect by both antibacterial and 

 antiphage sera suggest that the second phage penetrates and de- 

 creases the rate of multiplication of the dominant phage by an 

 intracellular mechanism. Further evidence that the excluded 

 virus penetrates is derived from isotopic data indicating that 

 chemical components of the DNA of the excluded phage are 

 utilized for synthesis of the dominant phage (see Chapter XIII). 



Direct evidence that the penetration hypothesis does not apply 

 to all cases of mutual exclusion was obtained by Weigle and 

 Delbriick (1951) while studying the properties of a lysogenic 

 strain of E. coli K12. This bacterium is a satisfactory host for 

 the virulent phage T5, indicating that the carried prophage 



