374 BACTERIOPHAGES 



lesions which, as suggested by the very nature of inducing agents, 

 could be an alteration in the nucleic acid economy of the host. 



6. Immunity 



Lysogenic bacteria possess the remarkable property of re- 

 sistance against infection with the homologous phage. As 

 already observed by Bail (1925), Bordet (1925), Burnet and Lush 

 (1936), Wollman and Wollman (1936b), lysogenic bacteria are 

 not sensitive to the phage they release although they may 

 adsorb it. Upon exposure to homologous phage particles, 

 lysogenic bacteria survive and the superinfecting phage does 

 not multiply. This property is called immunity. Lysogenic 

 bacteria are immune against the homologous phage and most 

 of its mutants, with the exception of a special class of virulent 

 mutants (Lederberg and Lederberg, 1953; Bertani, 1953a; 

 Jacob and Wollman, 1953). Immunity of lysogenic bacteria 

 involves a mechanism of resistance to phage infection completely 

 different from inability to adsorb. 



That the genetic material of the infecting particles is injected 

 inside an immune bacterium may be demonstrated by infecting 

 lysogenic bacteria with a mutant of the homologous phage 

 (Bertani, 1953a, b; Jacob and Wollman, 1953). The infecting 

 particle is able to multiply in those bacteria in which the pro- 

 phage develops, either spontaneously or after induction by ultra- 

 violet light. If, for example, lysogenic bacteria are first induced 

 and then infected with a mutant of the homologous phage, they 

 release particles of the superinfecting as well as of the prophage 

 type. Moreover, the ratio between the two types found in the 

 progeny depends upon the multiplicity of infection. A ratio 

 of 1 : 1 is found when the multiplicity of infection is 3 or 4, 

 equal to the average number of nuclei per cell, a result which 

 suggests the presence of one prophage per nucleus in the lyso- 

 genic bacterium (Jacob and Wollman, 1953). 



The genetic material of the phage which has entered an im- 

 mune bacterium does not multiply and is diluted out through 

 the course of bacterial multiplication. Immunity reflects a 



