ADAPTIVE ENZYME FORMATION 



of Bacillus cereus, where the inducer seems to be bound to the 

 bacterial structure but not incorporated in the genome. 



Nevertheless, it must be stressed that the capacity to 

 produce an adaptive enzyme of a given quality is directly 

 dependent on genetic character. In view of the frequent 

 comparison that has been made between the processes of 

 antibody production and of adaptive enzyme formation, it 

 is important to stress that only genetically competent strains 

 will produce y^-galactosidase when stimulated by an appro- 

 priate inducer. It is not a matter of just presenting any 

 a- or y^-hexoside to any Gram-negative bacillus and expecting 

 it to produce the enzyme needed to split the compound. 

 Another feature of this reaction which I shall refer to later 

 and which may be relevant to the nature of antibody 

 formation is the recent demonstration by Monod's school of 

 a two-stage process by which the inducer first initiates an 

 increased permeability of the cell wall for itself, and then 

 stimulates the pre-existent genetically controlled synthetic 

 mechanisms to produce and liberate the specific enzyme. 



There are many other types of adaptive change about 

 which there is no agreement. Examples are the 'training' 

 of a bacterium to use a new source of carbon or to grow in 

 just sublethal concentrations of bactericidal agents. Most 

 workers prefer to regard such changes as due to minor muta- 

 tions and differential selection of the mutants. Hinshelwood 

 and his school (Hinshelwood, 1946; Dean and Hinshelwood, 

 1953), however, have collected much evidence and argued 

 firmly for the view that there are variations due to physio- 

 logical alterations, for example, in the composition of the 

 enzyme population within the bacterium, which are not 

 genetic and which can be recognized by the readiness with 

 which they are lost on transfer to a non-selective medium. 

 They find, for instance, that when a strain of bacteria is 

 isolated from the resistant growth appearing on a plate with 

 a low concentration of antibiotic and grown in medium 

 without antibiotic, the descendant bacteria will usually fail 



