CHAPTER VIII 

 ADAPTIVE AND CONSTITUTIVE ENZYMES 



IN the last chapter the " training " of Eherthella typJiosa 

 to assimilate ammonium salts instead of amino -acids 



and of C. diphtherice and M. tuberculosis to grow on 

 synthetic media were cited as examples to show that 

 organisms can be made to grow on an initially unfavour- 

 able medium. Another example is the training of certain 

 yeasts to ferment galactose (a power which the majority 

 of yeasts do not possess), whilst the production of lactose 

 fermenting variants of Esch. coli, of rhamnose fermenting 

 strains of Eherth. typhosa and of sucrose fermenting 

 variants of Shigella dysenterice probabh^ occur by a similar 

 mechanism. As we have said, it has been suggested by 

 some workers that this might not be due to a true 

 " training " or induction of variants but to a selection 

 of the appropriate strain from a mixed inoculum con- 

 taining only very small numbers of the " non-exacting " 

 strain, in the case of variation of nutrient requirements, 

 or of those organisms fermenting galactose, etc., in the 

 other cases. 



There is evidence, however, that this is not always 

 the correct explanation, although it may be true in some 

 instances. Often there is a considerable lag before the 

 mutant ai)pears, much more than would be expected if 

 organisms of the " trained " type Ave re already present, 

 even in very small numbers. Moreover, in the majority 

 of cases the bacteria concerned were isolated as single 

 colony cultures and repeatedly subcultured and would 

 thus be expected tQ be pure strains, although it must 



69 



