372 BACTERIOPHAGES 



generation to produce phage, is constant. For different strains 

 of the same bacterial species, it varies widely (lO"^ to 10~^) 

 according to the type of prophage carried. The factors yet 

 unknown that determine phage production in lysogenic cultures 

 appear to select at random the small number of cells that lyse 

 in each generation. 



b. Induction 



In some lysogenic systems the probability of phage produc- 

 tion can be increased nearly to one by exposing the cultures to 

 the action of various agents such as ultraviolet light (Lwoff, 

 Siminovitch, and Kjeldgaard, 1950). After irradiation, bac- 

 terial growth proceeds for a time corresponding to one or two 

 divisions, then mass lysis occurs and a burst of phage is released 

 by almost every bacterium. 



This phenomenon, called induction, allows one to compare 

 phage production by lysogenic bacteria and by sensitive bacteria 

 infected with the identical phage. The characteristics of phage 

 development appear to be similar in both systems, a result taken 

 to mean that after induction and after infection phage multiplica- 

 tion occurs in a common vegetative state (Lwoff, 1953; Jacob 

 and Wollman, 1953). 



During the latent period following induction by ultraviolet 

 light, lysogenic bacteria are still able to synthesize respiratory 

 as well as induced enzymes (Siminovich, 1953). It appears to 

 be a general feature of temperate phages and their mutants 

 that their development, both after induction of lysogenic cells 

 and after infection of sensitive cells, does not interfere as strongly 

 with the metabolism of the host as it is the case with the virulent 

 phage T2. 



The process of induction is under the control of at least three 

 different factors. 



1 . Evidence for a genetic factor stems from the fact that only 

 certain lysogenic systems are inducible. In general, inducibility 

 behaves as a property of the prophage, not of the bacterial 

 strain. 



