172 



LAURENCE F. FOSTER 



above cases and yet the total quantities of actual acid, as shown 

 by titration, are widely different. No better illustration of the 

 efficiency of a buffer could be offered. Very obviously the utili- 

 zation of glucose is here closely related to the concentration of 

 buffer present. A further fact, of interest and importance, is 

 that the final hydrogen-ion concentration rather than the total 

 acid produced is the factor which limits the fermentative activi- 

 ties of the organism. 



Experiment IV. The influence of horse serum in glucose broth 

 upon the filial Ph of Streptococcus hemolyticu^ 



Ten cubic centimeter lots of beef infusion broth (sugar-free) 

 containing varying amounts of glucose and horse serum were 

 prepared and inoculated with 0.4 cc. of an eighteen-hour culture. 

 After an incubation of three days Ph determinations were made. 

 The results of (2) in experiment II are inserted in table 4 for 

 purposes of comparison. 



TABLE 4 

 Experiment IV 



As in experiment II it is to be noted that 0.3 per cent glucose 

 is the minimum concentration which will permit the attainment 

 of the characteristic final Ph. The greatest differences in Ph 

 between the media containing horse serum and (2) of experi- 

 ment II are seen in the tubes containing 0.1 per cent and 0.2 

 per cent glucose. It seems possible that in these cases the 

 horse serum prevents the increase in acidity of the medium to a 

 small extent through its action as a buffer. In those tubes con- 

 taining sufficient glucose for the production of the final Ph char- 

 acteristic of the organism no differences in the level of this final 

 value are seen. That we do have a decided difference in the 

 rates of acid production will be shown in a later experiment. 



