HYDKOGEN IONS, ETC., OF BACTERIOLOGICAL MEDIA 565 



In a certain lot of bouillon containing 1 per cent of glucose 

 Bad. coll produced an alkaline reaction after incubation for 

 four days. This bouillon had a high buffer index and reserve 

 alkalinity. The experiment recorded in table 3 was designed 

 to explain this phenomenon and shows that the reserve alkalinity 

 of a medium may be of diagnostic importance. It is seen that 

 in a bouillon with a reserve alkalinity (pH n - 5) of 3.5 per cent 

 normal, 1 per cent of glucose was hardly sufficient to insure 

 continued acidity. The culture in the same medium contain- 

 ing 0.75 per cent of glucose actually became alkaline to brom 

 cresol purple (pH 7.0) in one hundred forty-four hours. • On 



TABLE 4 

 Acid 'production by Bacterium coli 



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

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



the other hand if the reserve alkalinity was reduced to about 

 2.0, less than 1 per cent of glucose was sufficient to maintain 

 the acidity of the culture. 



Of what value would the buffer indices illustrated in figure 

 1 be to the bacteriologist in selecting his medium? To mention 

 only one or two illustrations; if he were working with a member 

 of the Bacterium coli group and wished to determine in a few 

 hours whether the organism would ferment sucrose, he might 

 select a bouillon with a low buffer index, i.e., one in which the 

 formation of a small amount of acid would be revealed by a 

 rapidly rising hydrogen ion concentration. If, on the s other 

 hand, he desired abundant growth and the production of a 



