370 BACTERIOPHAGES 



4. Lysogenization 



Upon infection with a temperate phage, a sensitive bac- 

 terium may respond in at least two different ways. In a certain 

 fraction of the population, the infecting phage enters the vegeta- 

 tive state. The bacteria lyse and release phage particles {pro- 

 ductive response). In another fraction the phage enters the pro- 

 phage state. The bacterium survives and gives rise to a progeny 

 containing lysogenic bacteria {lysogenic response). The relative 

 frequency with which these two responses occur depends on 

 both the conditions of infection and the genetic constitution of 

 the phage. 



For a given temperate phage, the frequency of lysogenization 

 can be drastically modified by changing the conditions of infec- 

 tion. The factors which determine such alterations appear to 

 vary widely according to the phage-bacterium system involved. 

 In a phage acting on Sh. dysenteriae, 90 per cent of the infected 

 cells produce phage when incubated at 37 ° after infection, 

 whereas 80 per cent become lysogenic at 20° C. (Bertani and 

 Nice, 1954). With a phage of S. typhimurium, the type of re- 

 sponse depends upon the multiplicity of infection. When the 

 multiplicity of infection is less than one, most of the infected 

 bacteria lyse and produce phage. If the multiplicity of infec- 

 tion is gradually increased, the frequency of lysogenic response 

 increases and reaches almost 100 per cent with 10 phages per 

 bacterium (Boyd, 1953a). Moreover, in the same system the 

 frequency of lysogenization can be influenced by such treatments 

 as starvation of the cells or addition of various antimetabolites 

 to the cultures (Lwoff, Kaplan, and Ritz, 1954). 



It appears, therefore, that the lysogenic response does not re- 

 sult from the selection of preexisting mutants, either of the 

 phage or the bacterium. As first demonstrated by Lieb (1953) 

 in E. coli, the decision between lysogenization and lysis must be 

 made after infection. Lieb's work suggests that, in those bac- 

 teria that are to become lysogenic, the genetic material of the 

 infecting phage for some time behaves as a cytoplasmic particle. 

 While in this condition it is particularly sensitive to heat, does 



