Mechanisms of Resistance to Colicins 329 



resistance by action of both loci has not yet been investigated 

 (Fredericq, unpubHshed). 



The occurrence of receptors common to a phage and a given 

 coHcin is also confirmed by recombination experiments. In 

 all crosses studied, there is indeed a perfect correlation 

 between resistance/susceptibility to phage T 6 and to colicin 

 K, and between resistance/susceptibility to phage BF 23 

 and to colicin E. In the cross just mentioned between Esch. 

 coli B and K 12, both loci of resistance to colicin E induce 

 resistance also to phage BF 23 and, on recombination, yield 

 cells susceptible to both agents. 



In conclusion, sensitivity is controlled by a number of 

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

 tible. Resistance may be achieved by mutation through loss 

 of the corresponding receptor and behaves as an allelic marker 

 to sensitivity in recombination experiments. 



Immunity by transfer of colicinogenic factors 



Colicinogenic strains are not susceptible to the particular 

 colicin they produce but may, of course, be susceptible to 

 other colicins. This immunity is a direct consequence of the 

 colicinogenic property, and non-colicinogenic mutants are as a 

 rule susceptible to the colicin produced by the mother strain 

 (Fredericq, 1948cZ). 



Colicin is not produced by every cell of a colicinogenic 

 culture. Only cells which do not yield colicin are immune, 

 the others are killed by the colicin they synthesize. They are, 

 however, too few to be seen and the culture as a whole appears 

 normal. But colicin synthesis may be induced in nearly all 

 cells of a colicinogenic culture by many mutagenic or carcino- 

 genic agents, more particularly ultraviolet irradiation, 

 peroxide, ethyleneimine or halogenoalkylamine (Jacob, 

 Siminovitch and Wollman, 1952, 1953 ; Lwoff and Jacob, 

 1952). 



Colicin induction was first observed in the ML strain of Esch. 

 coli and led to massive lysis of the cells (Jacob, Siminovitch 



