326 F. JACOB AXD E. L, WOLLINIAN 



However, whereas in some strains this spontaneous rate of production cannot 

 be increased by artificial means, in other strains the production of phage 

 can be induced at will in the whole population. 



B. Induction of Phage Production in Lysogenic Bacteria 



In 1950, it was found by Lwoff and associates that when cultures of 

 certain lysogenic strains of B. megatherium are exposed to suitable doses of 

 ultraviolet Hght, the whole population lyses and releases phage particles. 

 After exposure to UV light, bacterial growth first proceeds without divisions, 

 during a time period corresponding to one or two generations. No phage is 

 released during this period. At the end of this latent period, lysis begins and 

 the first phage particles appear in the medium. If the proper dose of UV 

 Hght has been used, lysis is rapidly completed and an average of about one 

 hundred phage particles per lysed ceU is released. During the latent period 

 lysogenic bacteria can be prematurely disrupted in the same way as infected 

 bacteria (with KCN + phage, lysozyme, chloroform, etc.). During the first 

 two-thirds of the latent period, no infectious particles are found, but at the 

 end of this time mature particles begin to appear and their number increases 

 linearly with time until the full yield is completed. 



Induction allows a careful analysis of phage production by lysogenic 

 bacteria. Especially it makes it possible to compare phage development in 

 the same phage-bacterium system, whether after infection of sensitive bac- 

 teria, or after induction of lysogenic bacteria (Figs. 3 and 4). It has thus been 

 observed that the characteristics of phage development, such as latent 

 period, average burst size, are very similar in both systems. Such an analogy 

 indicates that the processes concerned with the actual multiplication of 

 phage material, i.e. vegetative phase and maturation, are probably identical 

 in induced lysogenic and in infected sensitive bacteria. For phage to be pro- 

 duced by lysogenic bacteria, there must be a shift from the stable prophage 

 state to the transitory vegetative state. Once the vegetative state is reached, 

 phage multiplication proceeds in the same fashion after induction of lyso- 

 genic bacteria as after infection of sensitive cells with homologous phage 

 particles. 



C. Factors Controlling IndvMion 



In the induced production of phage by lysogenic bacteria, it is convenient 

 to distinguish three main factors: a genetic factor responsible for the sus- 

 ceptibility of lysogenic strains to the action of inducing agents; the inducing 

 agent itself; and the physiological conditions which will aUow the lysogenic 

 cells to respond to the action of the inducing agent by the production of 

 phage. These three factors will now be examined in turn. 



