LYSOGENY 331 



inducing agents, can tlie prophage escape the control of the nucleus and 

 develop into vegetative particles. 



V. Properties of Lysogenic Bacteria 



In most cases, the lysogenic strains obtained by lysogenization possess the 

 same properties as the nonlysogenic original strains, except for the specific 

 properties wliich express the presence of the prophage. First of all is the 

 ability to produce phage which, for any individual of a lysogenic population, 

 is a potential character since its expression is lethal. Another property which 

 is conferred on lysogenic bacteria by the presence of a prophage is their 

 resistance to an infection by the phage they are able to produce or homo- 

 logous phage. This property, called immunity, is expressed in every cell of a 

 lysogenic culture. 



Capacity to produce phage and immunity against the homologous phage 

 or its mutants are in most systems the only recognizable differences between 

 lysogenic and nonlysogenic derivatives of the same strain. However, in 

 certain cases, other differences may be observed. The presence of a prophage 

 may interfere with the ability of lysogenic bacteria to reproduce unrelated 

 phages. It may even determine in the host some specific modification which, 

 at first sight, would appear to be unrelated to lysogeny. These modifications, 

 which are named lysogenic conversions, may bear on properties as different 

 as the synthesis of a toxin or the formation of a surface antigen. Only by 

 chance has it been possible to relate such alterations in bacterial physio- 

 logy to the presence of a given prophage. These different expressions of the 

 presence of a prophage wiU now be considered in turn. 



A. Infection of Lysogenic Bacteria with Phages Homologous or Closely Related 

 to the Prophage : Immunity and Incompatibility 



Lysogenic bacteria, as already observed by BaU (1925), Bordet (1925), 

 Burnet and Lush (1936), WoUman and WoUman (1936), are not susceptible 

 to the action of the phage they release, although they generally adsorb it. 

 When infected with homologous particles, no apparent alteration of gro\\i;h 

 is observed, and the infecting particles do not multiply. This property of 

 lysogenic bacteria is called immunity. Immunity of lysogenic bacteria is a 

 form of resistance completely different from inability to adsorb, as exempli- 

 fied by B/2 unable to adsorb T2. A lysogenic bacterium is always immune 

 against the homologous phage or mutants of this phage. It may or may not 

 be able to adsorb this phage according to whether or not it possesses the 

 homologous receptor. For instance both E. coli Kr2(A) and K12/A are re- 

 sistant to phage A, the former because it is immune, the latter because it is 

 unable to adsorb it. From the lysogenic derivative K12(A) which adsorbs the 



