Mechanisms of Resistance to Colicins 325 



transduced strains always produce a colicin identical with that 

 of the transducing strain. A strain already transduced for a 

 given colicin may still be transduced by a strain producing 

 another colicin and so yield two distinct colicins. A strain 

 already transduced may in turn transfer the newly acquired 

 colicinogenic property to another strain (Fredericq, 1954c). 



Transfer of a colicinogenic property is extremely rapid, at 

 least during the initial stage, and can be obtained by mixing 

 the two cultures for only a few minutes. So far, it has proved 

 impossible to extract the transducing agent or even to 

 separate it from the living cells. 



Transduction of a colicinogenic property seems to require 

 contact and probably conjugation of two cells. Transduction 

 in series, investigated in Esch. coli K 12, indeed revealed that 

 the transducing activity is linked w4th F+ sexual polarity and 

 that an F~ colicinogenic strain is unable to transduce, unless 

 it is first transformed into F+ (Fredericq, 1954c). 



Despite the intervention of the F+ sexual factor, trans- 

 duction of the colicinogenic properties appears to be quite 

 independent of recombination of the other genetic characters. 

 In crosses experiments, colicinogenic factors are not linked 

 with any of the numerous other markers studied and colicin 

 production is not allelic with absence of colicin production. 

 When the F~ parent is colicinogenic, all recombinants, 

 without exception, are likewise colicinogenic, whatever 

 markers may be involved in the selection; but some recom- 

 binants, up to 70 per cent, still receive the colicinogenic 

 properties when they are borne by the F+ parent. Colicino- 

 genic factors are therefore completely independent of the 

 normal genetic structure of the bacteria (Fredericq and 

 Betz-Bareau, 1953a and h, 1956). 



Resistance by loss of specific receptors 



Patterns of susceptibility towards colicins differ extremely 

 from one strain to another. Some strains, like Esch. coli B 

 or K 12, are susceptible to all colicins; others may be resistant 



