CLONAL PHENOMENA 



to grow on similar plates with the same content of antibiotic. 

 There are also theoretical objections to the mutation hypo- 

 thesis which appear less serious. Dean and Hinshelwood, 

 for instance, consider that 'Adaptive responses to so many 

 G and N sources and to so many concentrations of so many 

 drugs occur that a very broad and almost continuous spectrum 

 of mutant types would have to be postulated in all normal 

 cultures'. 



Probably the most satisfactory way of covering the facts 

 is to adopt the compromise supported by Ravin (1953). He 

 suggests that the essence of the matter is that there is a geno- 

 typic limitation of adaptability . If we have three alternative 

 genotypes A, B, C, of a bacterial strain each can be expected 

 to have a certain limited capacity for non-mutational adapta- 

 tion, which will differ according to the genotype. If they 

 are represented as superscripts x^y, etc., we might have the 

 series of mutational changes : 



In this case an ability to adapt to condition PF would demand 

 mutation to B. 



With perhaps some slight reservations, it seems by far the 

 most effective way of handling the reported data to accept 

 an immense range of potential mutability in bacteria and to 

 confine physiological adaptation to those conditions in which 

 the occurrence of mutation can be satisfactorily excluded. 



2. Mutation 



When mutations are specifically looked for in bacteria they 

 are found to be ubiquitous and multiform. In general, 

 mutation will occur at rates of the order of one per million 

 dividing bacteria, and to isolate a mutant present in such an 

 excess of the original form requires that some environment 

 be provided in which the mutant will survive or multiply 

 while the parent form is inhibited or eliminated. There are 

 two standard ways of providing this. 



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