Mechanisms of Resistance to Colicins 333 



of the same family. They are protein substances with a high 

 molecular weight and kill susceptible cells after fixation on 

 specific receptors. 



Susceptibility patterns are determined by a number of 

 receptors, specific for each of the colicins to which the cells are 

 susceptible. The presence of these receptors is governed by 

 genetic factors, as shown by mutation and recombination 

 experiments. Resistant mutants are characterized by the 

 independent loss of only one receptor and by resistance to the 

 corresponding colicin. Resistances to different colicins behave 

 as independent units in recombination experiments and show 

 well defined linkages with other markers of the parents. 

 Colicin receptors behave exactly like phage receptors and in 

 some cases the same receptor may be common to a given 

 phage and colicin. 



The hereditary property of producing colicin may be 

 transduced to non-colicinogenic strains. The transduced 

 strains retain all their other original markers, except that 

 they become resistant to the particular colicin they now 

 produce. This specifically acquired resistance is not due to 

 the loss of the corresponding receptor but is dependent on the 

 presence of the colicinogenic factor. This immunity is quite 

 comparable to the immunity of lysogenic bacteria towards the 

 phage they carry. 



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