566 



J. HOWARD BROWN 



large amount of acid he would do well to select a medium with 

 a high buffer index and high reserve alkalinity. This is illus- 

 trated in the experiment recorded in table 4, in which Bact. coli 

 was grown in glucose bouillons of low and high buffer indices. 

 The degree of acidity rose much more quickly and the final hydro- 

 gen ion concentration was higher in the bouillon of low buffer 

 index but the amount of acid produced was much greater in the 

 bouillon of high buffer index. The experiments recorded in 

 tables 4 and 5 also illustrate the statement of Clark (1915a) 

 that ''unless the media employed by different laboratories are 

 identical, at least in their buffer effect, the titratable acidity 



TABLE 5 

 Acid production by a streptococcus 



The titration is expressed in terms of per cent normal acid or cubic centimeters 

 of N /20 NaOH required to reduce 5 cc. of culture to pH 8.0. 



produced by the same organism may be found to be very dif- 

 ferent. " They also confirm his observation that "the greater 

 the buffer effect of the medium, the lower the final hydrogen 

 ion concentration attained." It may be added, therefore, 

 that if it is desired to compare the final hydrogen ion con- 

 centrations or the titratable acidities of similar cultures in 

 different media, at least the buffer indices of the media should 

 be known. It is not claimed that the titratable acidity is always 

 a measure of the amount of acid produced, nor that the buffer 

 content is the only factor which determines the amount of acid 

 which may be produced or the amount of growth which a medium 

 can support. There may be simultaneous production of acid 

 and alkali by some organisms. It has been shown above that 

 the buffer content may be altered by the growth of the culture. 



