32 SELMAN A. WAKSMAN AND JACOB S. JOFFE 



medium the lower the final hydrogen-ion concentration attained. 

 Ayers (1916) has shown that streptococci reach a more or less 

 . definite hydrogen-ion concentration. 



Itano (1916) found that the minimum exponent (pH) for B. 

 subtilis was between 4 and 5 (4.82) and the maximum between 

 9 and 10 (9.43). The pH of the media was altered in such a way 

 that the different exponents were brought toward the optimum 

 pH (7.5-8.5). Boas and Leberle (1918) recently reviewed the 

 acid production by molds and yeasts and stated that acidity 

 depends upon the splitting of the nitrogenous substances, when 

 the action hereby produced represses the production of acid from 

 carbohydrates; that the acidity is due chiefly to the decompo- 

 sition of the carbohydrates, when the nitrogen source does not 

 leave an injurious acid as in the case of ammonium salts of strong 

 acids ; thus the acidity may vary with both carbon and nitrogen 

 sources in the medium. Malfitano (1900) has pointed out that 

 with the age of cultures of molds proteolysis takes place resulting 

 in a decrease in acidity. Boas and Leberle (1918) found that the 

 final H-ion concentration obtained lies within wide limits, for A. 

 fumigatus between pH 1.56 and 5.7 and for yeasts between pH 

 2.94 and 3.80, tending in each case towards a maximum. When 

 a maximum is reached, a slow decrease in actual acidity takes 

 place, particularly during a long period of incubation, due partly 

 to utilization of the acid and partly to its destruction. The lar- 

 gest quantities of acid are produced by molds and yeasts from 

 the carbon sources; this acid production is repressed by the acid 

 radical of ammonium salts of strong acids used as sources of nitro- 

 gen. The carbohydrate induces sooner or later an enzymatic 

 splitting of protein reacting substances from the nitrogen source, 

 chiefly ammonia, which modifies the reaction. 



Davis (1918) and others observed that B. diphtheriae grown in a 

 nutrient bouillon produces an initial increase in the hydrogen ion 

 concentration followed by a steady decrease until apparently a 

 limited alkaline reaction is attained; this may finally change to 

 a secondary acidity, the latter never reaching the high point of 

 the first acidity. 



