Mechanisms of Resistance to Colicins 327 



different colicins. For example, all colicins of group E attach 

 to the same receptor of the susceptible bacteria; a mutant 

 resistant to one will be resistant to all colicins of the group. 

 Yet they differ in activity spectra, thermoresistance, sus- 

 ceptibility to proteolytic enzymes and so on (Fredericq, 

 1950). 



Colicin receptors behave exactly like bacteriophage recep- 

 tors, but are generally independent and separate from them. 

 For example, a mutant resistant to a given colicin can retain 

 all phage receptors as well as receptors for other colicins which 

 characterize the mother strain. Similarly, a mutant resistant 

 to a given bacteriophage remains susceptible to all colicins as 

 well as to other phages which are active on the strain from 

 which it is derived. 



Systematic investigations revealed, however, that a specific 

 reciprocal cross-resistance occurs constantly in the case of 

 certain colicins and definite bacteriophages. Thus, all mutants 

 selected by colicin K or by phage T 6 are in every case resistant 

 to both agents simultaneously (Fredericq, 19496). The same 

 holds true for colicin E and phage BF 23 (Fredericq, 1949a), 

 and for colicin C and phages T 1-T 5 (Fredericq, 1951). 



Each of these phages adsorbs on a receptor common to 

 the phage and to the corresponding colicin, by means of a 

 protein located at the tip of their tail. This protein must be a 

 substance related to colicins because it has the same specificity 

 and, like a colicin, it kills susceptible bacteria even under 

 conditions where the phage particles are not reproduced 

 (Fredericq, 1952a and c). X-ray-inactivation curves of 

 colicin K and of the lethal protein of phage T 6 revealed that 

 both agents are of the same molecular size (Latarjet and 

 Fredericq, 1955). 



The presence of independent colicin receptors and of their 

 genetic determinants was also demonstrated in recombina- 

 tion experiments. Mutants resistant to colicins, derived from 

 the fertile Esch. coli K 12 strain, are easily obtainable, and 

 crosses between susceptible and resistant parents were studied 

 using Lederberg's (1947) technique. 



