84 UNIVERSITY OF WISCONSIN STUDIES 



lation of hydrogen ion concentration to growth has been 

 worked out in considerable detail for this group by Clark 

 and Lubs 8 and was made the basis of the "methyl red test." 

 The exact point at which growth stops is dependent in some 

 measure on the buffer effect of the medium and, therefore, is 

 constant only under fixed conditions. If the buffer and 

 fermentable constituents of the medium are properly pro- 

 portioned, the limiting hydrogen ion concentration is reached 

 and the fermentation stops. If, on the other hand, the 

 sugar content is low and the buffer action high, the sugar may 

 be entirely fermented without raising the hydrogen ion con- 

 centration to the point at which fermentation stops. In this 

 ease the reaction "reverts" to the alkaline side. This action 

 is very well illustrated by chart I of Clark and Lubs' paper. 

 It is pointed out, in this paper, that this "reversion" cannot 

 be due to the production of ammonia as has been commonly 

 assumed. 



Unpublished work of Ayers and Rupp of this laboratory 

 has shown that this phenomenon is brought about b} r two simul- 

 taneous, but independent, fermentations. While the sugar 

 is being fermented, a secondary fermentation is converting 

 the salts of the acids, which are the end products of the first 

 fermentation, to gases, carbonates and other products. 



The reaction at any given time is dependent on the relative 

 rates at which these two fermentations are progressing. 



The change from an acid reaction toward the alkaline side 

 is not due, as has been assumed, to a neutralization by ammonia 

 from a protein decomposition but to an actual decrease in 

 the acids through a secondary fermentation and in some 

 measure to a neutralization by _ carbonates which appear as 

 end products of the decomposition of the acid salts. 



The work of Harden, 9 Harden and Walpole, 10 and Thomp- 



Loc cit. 



"Arthur Harden, The Chemical Action of Bacillus Coli Communis and 

 Similar Organisms on Carbohydrates and Allied Compounds in Jour. Chem. 

 Soc. (London, 1901), 79, pt. 1, pp. 610-628. 



10 A. Harden and G. S. Walpole, Chemical Action of Bacillus Lactis Aero- 

 genes (Escherich) on Glucose and Mattnitol; Production of 2: 3-Butylene- 

 aJycol and Acetylmethylcarbinol in Proc. Roy. Soc. (London, 1906), Ser. 

 B, 77, pp. 399-405. 



