CH. V] VARIATIONS IN FERMENTING POWER 61 



to the absence or inhibition of either the " inverting " ferment 

 or the " acid-forming " ferment. 



That the several steps in the process of fermentation 

 result from the activity of different enzymes is suggested by 

 the following considerations. Typhoid and dysentery bacilli 

 never, of themselves, produce gas and cannot be made to do 

 so by training or selection ; both however produce acidity in 

 dextrose and mannite and can be "trained" to do so in 

 lactose. 



B. coli is known to produce formic acid from glucose and 

 then to split up the formic acid into gases ; Penfold has 

 shown that growth on monochlor-acetic-acid-agar may inter- 

 fere with the "formic acid-forming'' property of this organism 

 without affecting its " formic acid-splitting " power, that is to 

 say its power to form gas from formic acid. 



4. This observer has gone still further and has shown that, 

 during the stage of add production, more than one kind of 

 acid may be formed by an organism but that the formation 

 of each acid is the work of a special enzyme. The inhibition 

 of the " formic acid-forming '' enzyme in the case of B. coli 

 did not interfere with the production of acidity in dextrose. 



5. Penfold likewise showed that the splitting up of each 

 acid, with the formation of gases, was the work of a special 

 enzyme and that an enzyme which could produce gas from one 

 acid could not do so from another. The strain of B. coli which 

 was deprived of its power to form formic acid was, on this 

 account, deprived of its power to produce gas, for, although the 

 organism could produce other acids (as shown by the reaction 

 of the medium) it could not split these up. If however sodium 

 formate was added to the medium the organism at once yielded 

 gas ; or, again, if the organism were grown in dextrose 

 with B. typhosus (which possesses the power of producing 

 formic acid from dextrose but cannot split the acid up) it 

 once more yielded gas. It is obvious, then, that in the case of 

 B. coli its " acid-splitting " enzyme is only capable of splitting 

 up formic acid and cannot form gas from other acids. 



6. The development by a strain of bacteria in contact 

 with a certain sugar, of the power to ferment that sugar is an 



