MIXED INFECTION 325 



infected with PI, the majority of the bacteria Hberate PI and 

 not P8, again demonstrating exclusion of endogenous induced 

 phage by infecting phage. The reverse experiment could not be 

 tried because lysogenic strain 13(1) is not inducible (Jacob, 

 1952c). 



It seems evident that the penetration hypothesis cannot 

 explain many cases of mutual exclusion. The exclusion must 

 depend on some property of intracellular vegetative phage that is 

 not shared by intracellular prophage. 



Although mixed infection with unrelated phages usually 

 results in mutual exclusion, there is at least one well documented 

 case in which unrelated phages can mature in the same host cell 

 (Collins, 1957). Phage Tl and phage BG8 have a common 

 host in strain Cullen of E. coli. The two phages are not sero- 

 logically related, are morphologically distinct, differ in their 

 requirement for calcium ion, and differ in the ability of the 

 ultraviolet inactivated particles to kill the host cell and manifest 

 multiplicity reactivation. Despite this strong evidence for lack 

 of any close relationship, mixed infection results in a large 

 proportion of mixed yielders. Evidence of genetic recombina- 

 tion was sought but not found. Any general mechanism for 

 mutual exclusion of unrelated phages must also consider the 

 fact that in certain cases mutual exclusion does not occur. At 

 the present time, no satisfactory mechanism can be suggested. 



2. Mixed Infection with Related Phages 



a. Absence of Mutual Exclusion 



In the early work with the T group of coliphages it had been 

 observed that both burst size and latent period were independent 

 of the multiplicity of infection. Logically it would seem reason- 

 able that infection of a bacterium with two phage particles 

 should give either a shorter latent period or a larger burst size 

 than infection with only one. When neither effect was observed 

 it was concluded that only one phage particle could be involved 

 in infection of a single bacterium regardless of how many par- 



