LYSOGENY 371 



not multiply, and is randomly distributed to one of the daughter 

 cells at each division. 



Although the variability in the bacterial response to infection 

 with a given temperate phage depends on environmental factors, 

 the capacity of a phage to lysogenize is genetically controlled. 

 Mutants can be isolated in which the ability to lysogenize is 

 altered (c mutants). Valuable information about the process of 

 lysogenization has been gained through genetic analysis of the 

 c mutants (Levine, 1957; Kaiser, 1957). The power of a 

 phage to lysogenize appears to be controlled by a number of 

 closely linked factors in the genetic material of the phage. 

 Moreover, mixed infection with certain pairs of c mutants, each 

 of which is unable to lysogenize alone, may result in lysogeniza- 

 tion. The mixed infection simulates a heterozygous diploid 

 in the trans configuration and each of the mutant phages appears 

 to perform a reaction that the other one cannot. Evidently the 

 temperate parental phage carries out both reactions and by 

 such tests, the existence of two or three different reactions in- 

 voh^ed in the process of lysogenization has been demonstrated. 



5. Production of Phage by Lysogenic Bacteria 



a. Spontaneous Production 



Large cultures of lysogenic bacteria contain free phage par- 

 ticles. During the exponential phase of bacterial growth, the 

 number of phage particles increases proportionally to the num- 

 ber of bacteria. Using single bacteria isolated with a micro- 

 manipulator, Lwoff and Gutmann (1950) demonstrated that 

 phages are not secreted by living and multiplying bacteria, but 

 are released by lysis of a small fraction of the bacteria. The con- 

 stant ratio between free phage particles and bacteria observed 

 in growing cultures expresses the fact that, at each generation, 

 a given fraction of the population lyses and releases a burst of 

 phage. The ratio observed depends on the burst size and the 

 frequency of lysis. Under given conditions of culture the rate 

 of production, expressed as the probability per bacterium per 



