330 Pierre Fredericq 



and Wollman, 1951). It was later found, however, that 

 this ML strain is both cohcinogenic and lysogenic and that 

 lysis must rather be attributed to development of its prophage 

 (Fredericq, 1954^^; Kellenberger and Kellenberger, 1956). 

 Nevertheless, colicin induction may also be obtained in strains 

 obviously non-lysogenic but without lysis (Fredericq, 1955; 

 Hamon and Lewe, 1955; Panijel and Huppert, 1956). Colicins, 

 indeed, are bactericidal but not bacteriolytic agents. 



Cohcinogenic factors are therefore potential lethal agents 

 whose pathogenicity is only disclosed by the achievement of 

 their potentiality. As long as they remain in the latent condi- 

 tion they induce immunity to the corresponding colicin. 



Strains which were originally non-colicinogenic and which 

 have been made cohcinogenic by transduction behave 

 exactly like spontaneously cohcinogenic ones. The introduc- 

 tion of a cohcinogenic factor into transduced cells gives 

 immunity to the colicin whose synthesis it controls, but 

 results in the killing of the cells where that synthesis succeeds 

 (Fredericq, 1954a). 



Immunity of cohcinogenic cultures to the colicin they 

 produce is quite different and independent from resistance 

 through loss of colicin receptors. Cells made cohcinogenic 

 by transduction keep the receptor of the colicin they now 

 produce, if they already had it before transduction : 



(1) When a strain possessing the receptor for colicins of 

 group E is transduced by a strain producing a colicin of this 

 group, it is no longer receptive to that colicin. However, it 

 retains the receptor, because it remains susceptible to the 

 other colicins of the group, as well as to phage BF 23 which 

 attaches on the same receptor. On the contrary, when non- 

 receptivity is achieved by mutation through loss of the 

 receptor, the resistant mutants always resist all colicins of 

 the group as well as phage BF 23. Immunity is often more 

 specific than resistance and may serve to distinguish colicins 

 which adsorb on the same receptor (Fredericq, 1956). 



(2) Immunity and resistance behave as non-allelic markers 

 in recombination experiments. Crosses of two parents which 



