90 BACTERIOLOGICAL CHEMISTRY 



be remembered that there may be a continuous production 

 of variants. 



An alternative and more likely explanation of these 

 variations is that the organisms produce a new enzyme 

 or series of enzymes, under the stimulus of the changed 

 medium, which enables them to deal with the new 

 nutrient substances provided. Some of these enzymes 

 may be merely capable of breaking down a new sugar, 

 as is the case with, galactose trained yeasts or the 

 Esch. coll mutants ; or they may be responsible for 

 synthetic reactions which were lacking in the parent 

 organism, as occurs when Eherth. typhosa is trained to 

 grow on ammonium salts in the absence of tryptophane. 



Karstrom, as a result of growing certain organisms 

 on a variety of media, showed that the bacterial enzymes 

 may be divided into two groups. To the first group 

 belong the 



1. Constitutive Enzymes which are always produced 



by a given organism, whatever the medium on 

 which it grows. These enzymes appear to be 

 essential members of the " battery " of enzymes 

 carried by the cell. 



The second group comprises the 



2. Adaptive Enzymes which appear in a given 



organism as the result of growth on a medium 

 containing the corresponding substrate. These 

 enzymes only appear when their specific sub- 

 strate is present, and on that account seem not 

 to be essential enzymes. 



The constitutive enzymes can be further divided into 

 two sorts : {a) those which always appear in approxi- 

 mately the same amount even on a medium from which 

 their particular substrates may be lacking, and (b) those 

 which, although always produced to some extent, occur 

 in increased amount when the organism is grown on the 

 specific substrate, 



