EFFECT OF pB. DURING GROWTH 71 



of individual enzymes we find that those that can function 

 under aerobic conditions only, are produced only when growth 

 is aerobic, being suppressed when growth is anaerobic, and 

 vice versa. Taking the deaminases of Esch. coli as examples, 

 the formation of the oxidative L-alanine deaminase is 5-6 

 times greater under aerobic conditions than under anaerobic, 

 while the formation of the anaerobic serine dehydrase is 

 2-3 times greater when growth is anaerobic than when aerobic. 



pB. of the growth medium 



Esch. coli can grow in a casein-digest medium adjusted to 

 any pH between the approximate limits 4-2 and 9-5. The 

 formation of an enzyme within the bacterial cell is dependent 

 to a large extent upon the pH of the medium at the time of 

 formation of the cell. The effect of the ^H varies with the 

 type and function of the enzyme concerned, and we can 

 distinguish three types up to the present: 



{a) Neutralisation mechanisms: bacteria are able to 

 grow in media covering a wide range of pH by the production 

 of mechanisms whose action is to neutralise the external 

 acidity or alkalinity and so tend to stabilise the internal 

 environment. Thus growth in an acid medium promotes 

 the formation of enzymes catalysing reactions with alkaline 

 end-products, and inhibits the formation of enzymes having 

 acid-forming actions. When Esch. coli grows in an acid 

 medium containing amino-acids, it attacks certain of the 

 amino-acids by decarboxylation liberating COg with the 

 formation of alkaline amines; when it grows in an alkaline 

 medium the amino-acid decarboxylases are no longer formed, 

 but, instead, enzymes attacking amino-acids by deamination 

 are formed and these liberate NHg with the formation of 

 acid products. Other organisms react to acid growth conditions 

 by the formation of enzymes catalysing the formation of 

 neutral substances from acids — as, for example, the reduction 

 of butyric acid to butyl alcohol by CI. acetohutylicum, and the 

 formation of acetylmethylcarbinol from pyruvic acid by 

 Aerobacter aerogenes (see Chap. VII). In all these cases 



