LYSOGENY 339 



VII. Lysogenization 



By definition, temperate phages are able to lysogenize. Sensitive bacteria 

 can respond to infection with temperate phages by different series of events. 

 In a certain fraction of the population, the infecting particles enter the vege- 

 tative state. They multiply and the bacteria lyse, releasing a burst of phages 

 {productive response). In another fraction, the infecting particles turn to the 

 prophage state. These bacteria survive and produce clones containing lyso- 

 genic cells (Z^/so^remc response). Exceptionally, in some rare cells the infecting 

 particle disappears. Such cells may either survive without becoming lysogenic 

 (re/rac^oryresponse),ordiewithoutreleasingphage(Ze^/iaZresponse)(Lieb,1953). 



Under standard conditions, any given temperate phage gives a constant 

 fraction of lysogenic responses upon infection of a given strain of sensitive 

 bacteria. This constant rate of lysogenization could be due to an intrinsic 

 heterogeneity in the phage population, of which only a fraction, generally 

 small, would be able to lysogenize at all (Boyd, 1952). Under another 

 hypothesis, the population of infecting phages would be essentially homo- 

 geneous with respect to their lysogenizing power, but each particle would 

 have a small but constant probabiHty, under a given set of conditions, of 

 estabhshing itself as a prophage instead of entering the vegetative state 

 (Lieb, 1953). As, in a given population of sensitive bacteria, any clone appears 

 identical as far as its behavior toward infection with a temperate phage is 

 concerned, the variability in the bacterial response would be of a phenotypic 

 and not of a genotypic nature. This is indeed what has been found in all 

 systems which have been analyzed, since the ratio "productive response : 

 lysogenic response" may be modified by changes in the conditions of infection. 



Nevertheless, the capacity of a phage to lysogenize is genetically controlled, 

 since mutants can be isolated whose lysogenizing abihty is altered or 

 even suppressed (see Section VI, C). 



A. Phenotypic Aspects of Lysogenization 

 The frequency of lysogenization can be modified by varying the conditions 

 of infection. The factors which bring about such modifications differ widely, 

 according to the system under study. 



After infection of Shigella dysenteriae with phage PI, the frequency of 

 lysogenization is strongly affected by temperature (Bertani and Nice, 1954). 

 At 37°C. most of the infected cells give a productive response, and only a 

 few become lysogenic. At 20°C. most of the cells give a lysogenic response. 

 When bacteria are infected and incubated at 37°C., the shift toward lyso- 

 genization can be observed even when the cells are brought to 20°C. as late 

 as 20 or 30 minutes after infection. This result suggests that, after infection, 

 the first steps leading to both lysogenization and phage production remain 

 identical for at least 20 minutes. 



