332 Pierre Fredericq 



action. Indeed, some bacteriophages have a tail constituent 

 which is a substance related to colicins and by means of which 

 they adsorb on receptors common to given colicins. 



Immunity of colicinogenic strains is quite comparable to 

 that of lysogenic strains. In both cases, it results from the 

 presence of a potential lethal agent but persists only as long 

 as this agent remains in a latent condition. Induction of 

 colicin synthesis, like that of phage synthesis, kills the bac- 

 teria in which it succeeds. 



It is perhaps easier to conceive of the immunity of lyso- 

 genic than of colicinogenic bacteria. In lysogenic bacteria, 

 the presence of a DNA structure, the prophage, prevents the 

 development of related DNA structures introduced into the 

 cells. A single mechanism is probably responsible for hinder- 

 ing the development of the carried prophage as well as of the 

 related infecting particles. In colicinogenic bacteria, on the 

 other hand, a genetic structure, the colicinogenic factor, must 

 not prevent the development of similar structures but the 

 action of a quite different agent, the colicin, whose synthesis 

 it potentially controls. This is an unprecedented example of 

 cellular immunity to a well defined chemical substance, but 

 nothing is yet known about its mechanism. 



Colicinogenic factors, like prophages, control a potential 

 lethal synthesis and are therefore pathogenic. In spite of the 

 fact that they cannot mature into infectious particles set free 

 by lysis of the cells, and require for their transfer the mech- 

 anism of conjugation, they could however be considered as 

 bacterial viruses distinct from bacteriophages; the more so 

 as they are independent of the normal genetic structure of 

 the bacteria. Their relations with bacteriophages point 

 perhaps to a common parental ancestry. 



Summary 



Colicins are a group of highly specific antibiotic substances 

 produced by some strains of the family Enter obacteriaceae, 

 mostly Escherichia and Shigella^ and acting upon other strains 



