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® 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Unpublished work of Ayers and Rupp of this laboratory 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 by 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,” Harden and Walpole,?° and Thomp- 
* Loe 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. 
1” A. Harden and G. 8S, Walpole, Chemical Action of Bacillus Lactis Aero- 
genes (Escherich) on Glucose and Mannitol; Production of 2: 3-Butylene- 
glycol and Acetylmethylcarbinol in Proc, Roy. Soc. (London, 1906), Ser. 
3, 77, pp. 399-405. ’ 
