ACETYLMETHYLCAEBINOL FORMATION 137 



(6) By decarboxylation and condensation of two molecules 

 of pyruvic acid to form acetylmethylcarbinol, 



2CH3 . CO . COOH > CH3 . CO . CHOH . CH3 + 2C0o .... (9) 



Which of the reactions predominates depends upon the pK of 

 the growth medium. Acetylmethylcarbinol is formed only 

 when growth occurs at an acid ^H and the pH of optimum 

 activity of the enzyme involved lies between 4-0 and 5-5. 

 The enzyme has been obtained in a cell-free state, and is 

 developed within the cell only when growth occurs at an acid 

 pB.; if the ^H is maintained at an alkaline value by the 

 addition of alkali throughout growth, so that the fermentation 

 acidity is neutralised, then the acetylmethylcarbinol enzyme 

 is not formed and the fermentation of the organism is con- 

 sequently essentially similar to that of Esch. coli. It is not 

 possible to convert Aerobacter aerogenes into Esch. coli by 

 continued growth in alkaline media, as immediately growth 

 is resumed in acid conditions acetylmethylcarbinol formation 

 again takes place. 



Reaction 9 thus occurs under acid environmental conditions. 

 Since it involves the conversion of two molecules of acid into 

 one molecule of a neutral substance, it acts as a neutralisation 

 mechanism (see p. 71), coming into action when the growth 

 environment becomes acid. This fact is further utiHsed as a 

 method of differentiating between Aerobacter and Escherichia. 

 If the two organisms are cultivated in a medium containing 

 a small amount of glucose, then both will ferment the glucose 

 with the formation of acid. However, the acetylmethyl- 

 carbinol formation by Aerobacter will result in the neutralisa- 

 tion of some of this acid and, as long as the glucose is not 

 present in excess, the final pH will be lower in the Escherichia 

 culture than in the Aerobacter culture. Consequently, if we put 

 up our cultures in peptone containing 0-2 per cent, glucose and 

 methyl red as indicator, we find that Esch. coli will turn the 

 indicator red in the course of its growth, while Aerobacter 

 will not; the Esch. coli is therefore said to be " methyl red 

 positive." 



