326 Pierre Fredericq 



to most of them. Sensitivity is controlled by a number of 

 receptors, specific for each colicin to which a strain is suscep- 

 tible. A resistant mutant, derived from a strain susceptible 

 to many different colicins, does not lose all at once the general 

 susceptibility of that strain. It only resists the particular 

 colicin which selected it but remains sensitive to the other 

 colicins acting on the mother strain (Fredericq, 19466, 

 1948(^). 



These colicin receptors are fixation receptors. Colicins 

 must first attach to the susceptible bacteria before killing 

 them. Survival curves are exponential and express the 

 adsorption phenomenon underlying colicin action (Fredericq, 

 19526; Jacob, Siminovitch and Wollman, 1952; Fredericq and 

 Delcour, 1953). Colicin receptors may be extracted from 

 susceptible bacteria and these extracts specifically neutralize 

 the corresponding colicins by fixing them. Similar extracts 

 prepared from resistant bacteria are inactive (Bordet and 

 Beumer, 1948). Antibacterial sera have no direct anticolicin 

 action but they protect sensitive bacteria against the later 

 action of colicins, probably by masking the receptors (Bordet, 

 1948). 



These colicin receptors are quite stable hereditary proper- 

 ties which characterize each susceptible strain, but may get 

 lost by mutation. Resistant mutants arise spontaneously and 

 are merely selected by the corresponding colicins (Fredericq, 

 1948a). Mutations affecting colicin receptors are independent 

 of the other properties of the strain; and, conversely, muta- 

 tions affecting other characters have no effect upon the 

 receptors (Fredericq, 19486). Mutations affecting these 

 receptors are also specific and independent of one another, 

 but in some cases cross-resistance may be observed (Fredericq, 

 1948(^). A strain originally susceptible to many colicins can 

 be transformed into a completely resistant one if all its 

 receptors are taken away step by step in a series of successive 

 mutations (Fredericq, 1948c). 



Colicins may be grouped according to the receptor on which 

 they adsorb, but the resulting groups include chemically quite 



