Genetic Aspects of Drug Resistance 57 



When the problem of bacterial resistance first arose, the back- 

 ground of information about genetic mechanisms in micro- 

 organisms, and particularly in bacteria, was still very meagre; 

 consequently the genetical analysis of such resistance pro- 

 ceeded slowly and encountered a number of difficulties that 

 would not be met with at the present stage of our knowledge. 



The accumulated evidence is ample to show that drug 

 resistance follows a pattern to be expected of characters de- 

 termined by genes. This evidence is derived from statistical 

 analyses of the mode of origin of bacterial resistance; from 

 genetical studies of recombination between different grades of 

 resistance, and also between resistance and other genetic 

 characters ; from studies of the transfer of genetic characters, 

 including resistance, from one bacterium to another by trans- 

 formation and transduction techniques; and from studies of 

 spontaneous mutability and of mutability induced by various 

 mutagens and by a mutator factor. 



None of this evidence excludes the possibility that extra- 

 genic mechanisms, in addition to genie mechanisms, may 

 operate in the development of drug resistance. However, if 

 we can assume that the findings made regarding numerous 

 other bacterial properties hold true for resistance as well, it 

 can reasonably be inferred that the role of extragenic factors 

 is a minor one. 



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