666 I. J. KLIGLER 



of the metabolism of bacteria is the change they produce in the 

 hydrogen ion concentration of the medium as a result of their 

 activities. The acid titration or the ammonia determination 

 indicates only the action on either the carbohydrates or the 

 peptone present in the medium. The hydrogen ion concentra- 

 tion, on the other hand, gives the resultant of the action on 

 both the carbohydrate and nitrogenous components. By deter- 

 mining the hydrogen ion concentration on different days, one 

 may even trace the progressive increase with the active utiUza- 

 tion of the sugar and subsequent decrease, if any occurs, during 

 the active utihzation of the constituents of the peptone. This 

 test, furthermore, serves as a simple index of (a) the amount of 

 sugar a particular organism may utilize without producing suffi- 

 cient acid to inhibit further growth and (b) the rate at which 

 different organisms can utilize a particular sugar. Experiments 

 are under way which indicate that distinct and constant differ- 

 ences exist in both properties among closely related types. 



The method with some slight modifications is that used by 

 Clark and based on Sorensen's colorimetric method of determin- 

 ing the hydrogen ion concentration. Standard solutions of pri- 

 mary and secondary phosphate and of sodium acetate-acetic- 

 acid and the indicators described by Clark and Lubs were used. 



Media were made as above containing varying amounts of 

 peptone, glucose and phosphate. A series of typical cultures 

 was inoculated into these media, incubated at 30° C. and tests 

 made at regular intervals. In order to eliminate the color, due 

 to the breaking down of the glucose during sterilization it was 

 found necessary to sterilize the peptone-phosphate solution and 

 the sugar solution separately, and then add the sugar to the 

 peptone by means of sterile pipettes. This somewhat increased 

 the difficulty in making the medium but assured a water-clear 

 solution, which greatly facihtated the color readings. For test- 

 ing the hydrogen ion concentration 1 cc. of the culture was 

 mixed with 5 cc. of freshly distilled water^ and three to four 

 drops of the indicator added. 



' Clark has suggested a dilution of 1 : 10 but a few preliminary tests showed 

 that it was more desirable to use a 1 : 5 dilution as excessive dilution changes the 

 hydrogen ion concentration. 



