12 BULLETIN" 782, U. S. DEPARTMENT OF AGRICULTURE, 



FERMENTATION OF CARBOHYDRATES. 



DEXTROSE. 



In some previous work with a small number of cultures of alkali- 

 forming bacteria, it was found by Ayers and Johnson (2) that while 

 they apparently did not ferment sugars, an alkaline reaction was 

 usually produced. In recent work, however, with a larger number 

 of cultures it has been found that certain carbohydrates were 

 fermented but that special means have to be taken to determine 

 this action. 



Extract broth is not always a suitable medium in which to deter- 

 mine the fermentation of dextrose, as is shown by the results in Table 

 4. The broth had the following composition: Dextrose 1.0 per cent, 

 beef extract 0.4 per cent, peptone 1.0 per cent, and distilled water. 

 The reaction was brought to the neutral point, the solution filtered, 

 the dextrose added, and the medium sterilized. The final reaction 

 was about plus 0.2 (Fuller's scale). The cultures were incubated 

 at 30° C. for 7 days and a duplicate set for 21 days, and titrated 

 at the end of each incubation period. The figures in the table repre- 

 sent the change in titration from the control, 10 cubic centimeters 

 of medium being titrated with tenth-normal sodium hydrate. The 

 fermentation was considered positive when 1 per cent or more normal 

 acid was formed, since this is the usual amount considered to indicate 

 fermentation. An examination of the results shows that after 7 

 days' incubation 15 of the cultures showed an acid fermentation in 

 dextrose and 10 showed such a reaction after 21 days. With 13 of 

 the cultures a higher acidity was found after 7 days than after 21 

 days, while 7 showed more acid after the longer incubation. After 

 7 days' incubation 3 of the cultures were acid but after 21 days were 

 alkaline, while 3 other cultures were alkaline after 7 days and became 

 acid after 21 days. It will be noted in Table 4 that 3 cultures 

 produced gas from dextrose. These results show that the fermenta- 

 tion of dextrose by the alkali-forming group of organisms was very 

 indefinite when determined in the ordinary manner by titration in 

 the regular broth medium. Some cultures showed a distinct and 

 others only a slight fermentation, while many produced only an 

 alkaline reaction. 



As previously noted, it was found that these bacteria produced 

 alkaline reaction when grown in beef-extract broth without sugar. 

 The broth contained 0.2 per cent beef extract and 0.5 per cent 

 sodium-dibasic phosphate. While this is not exactly the same 

 composition as that of ' the dextrose-extract broth, the results of 

 titrations help to explain some of the peculiar results obtained in 

 the dextrose fermentation, and they are incorporated in Table 4. 

 It may be seen that all the cultures of this group gave an alkaline 

 change in titration after 14 days' incubation in beef-extract broth; 



