COLICTNS AND OTHER BACTERIOCINS 385 



the somatic or O antigen of the producing bacteria (Goebel, 

 Barry, Jesaitis, and Miller, 1955). 



2. Action of Colicins on Sensitive Bacteria 



a. Specificity 



The action of colicins is very specific. First of all, it is re- 

 stricted to members of the Enterobacteriaceae family. But inside 

 this family, there is a certain degree of group specificity (colicins 

 produced by shigellas, for instance, are mainly active on shigel- 

 las), and even of species specificity, among strains of Shigella 

 sonnei for example (Fredericq, 1948). Finally, like the phages, 

 each particular colicin exhibits a definite host-range: some of 

 them act on only a limited number of strains, whereas others have 

 a much wider range of action. This strain specificity depends on 

 the presence of specific receptors on the surface of the susceptible 

 bacteria, as shown by P. Bordet (1948) and by Bordet and Beu- 

 mer (1948). Specific antibacterial sera protect sensitive bac- 

 teria against the action of colicins. Materials contained in 

 bacterial extracts as well as intact sensitive bacteria adsorb coli- 

 cins and neutralize their activity. 



The ability of a bacterial strain to adsorb a colicin and conse- 

 quent sensitivity to its action may be lost by mutation. In the 

 zones where growth of the sensitive indicator is inhibited rare 

 colonies may be observed (Gratia, 1925). These colonies, like 

 those surviving the action of phage, are formed by resistant mu- 

 tants which arise spontaneously during bacterial growth (Fred- 

 ericq, 1948). The resistant mutants appear to lack the specific 

 receptor for colicin fixation. 



In a bacterial population sensitive to various colicins, muta- 

 tions to resistance against each of the several kinds usually occur 

 independently of each other. For example E. coli (f), used by 

 Gratia and by Fredericq for their investigations on colicins, is 

 susceptible to a wide variety of these agents. A mutant resist- 

 ant to colicin V, called 0/V, is still sensitive to other colicins. 

 From (f>/V one may isolate a double mutant, <f)/V,E, resistant to 



