''age of culture effects 79 



with age is characteristic of enzymes concerned mainly with 

 catabolic or protective mechanisms. There is also the 

 possibility that some, at any rate, of these variations are 

 artefacts produced by the permeability of the cell-wall to 

 substrates varying with age of culture. That this is not always 

 the case, however, has been demonstrated with extracellular 

 enzymes such as proteases, whose formation in the external 

 environment follows just such a curve as that shown for 

 Type II enzymes in Fig. 6. In the case of certain amino-acid 

 decarboxylases, showing a Type II variation, it has been 

 possible to estimate the amount of enzyme formed in the cells 

 by breaking these down with acetone and ether, when the 

 amount of enzyme formed within the cell is found to vary with 

 the age of the culture in the same way as the Type II variation 

 found with the intact cells. If the Type II variation is 

 characteristic of catabolic systems, it may be that Type I 

 variation is characteristic of anabolic systems which the cell 

 must possess for growth to take place. Our knowledge is 

 not yet sufficiently extensive for any such generalisations to 

 be made. In Streptococci we find enzymes showing both 

 types of variation : tyrosine decarboxylase showing Type II ; 

 arginine dihydrolase (see p. 171) and the enzymes involved 

 in glucose fermentation showing Type I variation. 



Where the formation of enzymes within the cell is also 

 conditioned by ^H, the Type II variation may be modified if 

 growth occurs in the presence of fermentable carbohydrate. 

 Thus an enzyme whose formation is optimal only when the 

 growth pH approximates to the optimum activity ^H {e.g. 

 hydrogenase) may give a Type II variation with age, but 

 the activity may decrease again before the end of growth 

 owing to the pH of the medium becoming considerably acid. 

 These variations are very important in the experimental 

 study of bacterial metabolism. For example, CI. aceto- 

 hutylicum possesses hydrogenase and acetoacetic acid 

 decarboxylase (acetoacetic acid — ^acetone), both of which show 

 a Type II variation with age of culture. If we studv the 

 formation of these enzymes in suspensions of organisms 



