332 F. JACOB AND E. L. WOLLMAN 



phage but is immune to its action, a mutant K12(A)/A may eventually be 

 obtained wliich will not adsorb phage A. 



Immmiity is the expression of the presence, in the bacterium, of a prophage 

 homologous to the infecting phage. When the prophage, either spontaneously 

 or after induction, enters the vegetative state, immunity disappears and the 

 superinfecting phage starts multiplying. This can be easily demonstrated 

 when lysogenic bacteria are superinfected with a mutant of the homologous 

 type (Bertani, 1953a; Jacob and Wollman, 1953). When, for instance, induc- 

 ible lysogenic bacteria are first induced and then infected with an adequate 

 multipUcity of a mutant of the homologous phage, each bacterium releases 

 particles of the prophage as well as of the mutant type. 



What is the fate of the phage material which has thus penetrated lysogenic 

 bacteria in their normal noninduced state? In the vast majority of the 

 infected cells, it does not either multiply or lysogenize: it is diluted out at 

 each cell division, as can be accurately measured in mducible systems 

 (Jacob, 1954). This material must thus be in a different state from either the 

 prophage or the vegetative phage, since it is neither rephcated as the former 

 nor does it multiply as the latter. Its probability of developing into vegeta- 

 tive particles is very small, since this event can only occur if the prophage 

 itself also develops. Its probability of becoming a prophage is also very 

 limited, as shown by Bertani (1953a). Whereas lysogenic bacteria can easily 

 be lysogenized by temperate phages unrelated to the prophage type, lyso- 

 genization with a phage homologous to the prophage is a rare event : there 

 is incompatibility, at the prophage level, between related phages. In some 

 cases, the uifecting type replaces the original prophage type. In rarer cases, 

 the infecting type is replicated together with the original prophage type. In 

 such double lysogenic bacteria, where the two prophages differ by two or 

 more genetic characters, genetic recombination in the prophage state has 

 been observed (Bertani, 1953b; Appleyard, 1954b). 



The presence of a given prophage therefore prevents both muItipUcation 

 and lysogenization by a homologous infecting phage. Immunity reflects a 

 block in the processes by which the infecting material of a phage enters the 

 vegetative state, but the nature of this block is as yet unknown. Incompat- 

 ibility, as win be seen in Section VIII, is an expression of the specific binding 

 of the existing prophage to a chromosomal site of the bacterium. 



B. Infection with Unrelated Phages 



Generally, a prophage does not interfere with the reproduction of un- 

 related phages, either virulent or temperate. For example, E. coli K12(A), 

 lysogenic for A, exhibits the same sensitivity as nonlysogenic E. coli K12 to 

 iiofection with virulent phages Tl or T5. In the same way, bacterial responses 

 to infection with unrelated temperate phages are generally not modified by 



