146 PHYSIOLOGY OF BACTERIA 



No studies on the endpoints of other fermentations are known to 

 the author. It is probable that in the urea fermentation, the accumu- 

 lation of hydroxyl ions is the primary limiting factor, but there is no 

 real experimental evidence for this. Nothing at all is known about 

 the endpoints of the fermentation of proteins. 



(6) CONSTANCY OF THE ENDPOINT 



Bacteriologists assume in their every-day technique 

 that the final concentration of fermentation products 

 of a given culture is fairly uniform under uniform 

 conditions. This is shown by the fact that we use the 

 amounts of acid or of gas formed by microorganisms as a 

 means of classification. It is generally admitted, how- 

 ever, that this endpoint varies with the cultural condi- 

 tions, especially with the temperature, and also more or 

 less with the medium. 



Different species and varieties of yeast are characterized by the 

 quantities of alcohol they produce. The streptococci were divided 

 by Ayers, Johnson and Davis (1918) into high acid producers (pH less 

 than 5) which are mostly non-pathogenic, and into low acid producers 

 (pH = 5.4 to 6) which were largely pathogenic. In the colon 

 group, the methyl red test is a measure of final acidity, and the 

 customary determination of the amount of gas formed is also proof 

 of the assumption that the endpoint of fermentation is something 

 quite definite. This also pertains to the distinction of streptococci 

 by the amounts of CO 2 and volatile acids formed (Hucker, 1928a 

 and b). 



(c) INFLUENCE OF CONCENTRATION OF CELLS 



Coming back once more to the relation of fermentation 

 to enzyme action, we will have to consider the question : 

 Does the endpoint depend upon the amount of enzyme 

 present? According to all the data with zymase, the 

 percentage of fermented sugar increases with the amount 

 of zymase. But whether or not this is true also with 



