382 BACTERIOPHAGES 



Salmonella, Shigella, and Proteus species. They are all called 

 colicins, because a substance produced by any member of the 

 group may be active on strains belonging to any other species of 

 the family, including E. coli. These colicins can be distinguished 

 from one another by such properties as specificity of action, the 

 aspect of the zone of inhibition they produce on a given indicator, 

 and other physicochemical properties (Fredericq, 1948). 



Similar antibiotics are produced by certain strains of Pseudo- 

 monas pyocyanea, but their range of action seems to be restricted to 

 the Pseudomonadaceae family. They have therefore been called 

 pyocins (Jacob, 1954b). The general term of bacteriocins has 

 been proposed to include such antibiotics of protein nature whose 

 production is lethal and whose action, restricted to a narrow 

 range of related species, is conditioned by the presence of spe- 

 cific receptors (Jacob, Lwoff, Siminovitch, and Wollman, 1953). 

 More recently substances possessing most of these properties 

 have been described in gram-positive bacteria and called mega- 

 cins (Ivanovics and Alfoldi, 1954). 



In many of their properties, particularly in their specificity of 

 action, bacteriocins resemble bacteriophages (Fredericq, 1948, 

 1953), and this explains why a chapter on bacteriocins has been 

 included in this book. The following discussion will be mainly 

 concerned with the properties of colicins, which have been the 

 object of most studies. Other bacteriocins will be described 

 briefly at the end of the chapter. 



b. Detection and Titration 



The methods used to reveal the presence of a bacteriocin in a 

 culture medium, or its production by a given bacterial strain, 

 are identical with those used for the detection of bacteriophage. 

 Spot tests of the culture medium may be made on the surface of 

 an agar layer seeded with sensitive indicator bacteria, or bacterio- 

 cinogenic and indicator bacteria may be cross-streaked on the sur- 

 face of an agar plate. The second method is made more sensi- 



