MIXED INFECTION 327 



The generalizations just stated refer to the lytic cycle of phage 

 growth. During the lysogenic phase diametrically opposite 

 rules seem to hold (Chapter XIX). Bacteria can often be 

 lysogenized by several unrelated temperate phages, but seldom 

 by two or more related ones. 



b. Partial Exclusion 



Mixed infection with somewhat distantly related phage pairs 

 such as T2-T4, T2-T6, T4-T6 (Delbriick and Bailey, 1946), 

 or T5-PB (Adams, 1951a) results in one of the pair of infecting 

 phages being partially excluded. Thus less than half of the 

 bacteria to which both T5 and PB particles attach liberate both 

 phages. Using single cell burst experiments, Adams demon- 

 strated that phage PB tends to exclude T5. In mixed infections 

 each bacterium liberates PB-like particles, but only a fraction 

 liberates T5-like particles as well. The number of T5-like 

 particles appearing in mixed bursts is small but variable. The 

 similar example of T2 and T4, in which T2 tends to be ex- 

 cluded, has been studied by Streisinger and Weigle (1956). 

 They find that the ability of T4-like phages to partially exclude 

 T2 is correlated with several characteristics, including high 

 DNA-glucose content, none of which segregates in crosses as do 

 the usual genetic markers. The examples cited above evidently 

 depend on heritable differences between the phages, which may 

 be taken as the diagnostic criterion for partial exclusion of a 

 first kind. 



A second kind of partial exclusion is seen when bacteria are 

 infected first with one and then with another of a pair of very 

 closely related phages. It differs from partial exclusion of the 

 first kind in that it does not depend (except for convenience of 

 demonstration) on genetic differences between the two phages. 

 It has been observed only withT2 and, as the following discussion 

 suggests, may be limited to the T2 and T5 species and very few 

 others. Needless to say, if a bacterium is infected first with T2 

 and then with T4, both kinds of partial exclusion presumably 

 operate. 



