Mechanisms of Resistance to Colicins 331 



are both non-reccptive to the same cohcin, but for diflerent 

 reasons (the one because it produces that cohcin and the 

 other because it has lost the corresponding receptor) may 

 yield cells which recombine presence of the receptor received 

 from the immune parent and absence of immunity received 

 from the resistant one, and are therefore fully susceptible 

 (Fredericq, unpublished). 



Immunity is more or less pronounced according to the type 

 of colicin produced. Strains made colicinogenic for a colicin 

 of group E are apparently resistant to that colicin even if 

 they keep the corresponding receptor. They tend, however, 

 to lose that receptor in the course of subcultures. It is prob- 

 able that spontaneous mutants are selected because cells that 

 keep the receptor are somewhat inhibited. Strains transduced 

 for colicin I are resistant to the level of colicin they release but 

 may be inhibited by the same colicin in higher concentration. 

 Immunity to colicin V is still less pronounced and strains 

 rendered V-colicinogenic are partially susceptible to their own 

 colicin. Their broth cultures are less turbid than those of the 

 same strains before transduction ; agar streaks develop poorly 

 and isolated colonies are small and irregular (Fredericq, 

 1956). 



In conclusion, transduction of colicinogenic factors intro- 

 duces into transduced cells a potential lethal agent which 

 controls colicin synthesis and induces immunity to that 

 particular colicin as long as it remains in a latent condition. 



Discussion 



Resistance to colicins may be induced by two very different 

 mechanisms : by mutation through loss of the corresponding 

 receptors and by transfer of colicinogenic factors. These two 

 mechanisms exactly parallel what happens with bacterio- 

 phages. 



Resistance by loss of a specific receptor is readily under- 

 standable in both cases. Colicins, like bacteriophages, must 

 first attach to the susceptible cells before completing their 



