COMPARATIVE STUDY OF THE COCCI 6l 



bacterium is most active, it uses up oxygen and re- 

 duces the litmus, which is accordingly decolorized; 

 and, conversely, when activity grows less, oxygen dif- 

 fuses down from the surface, making the litmus pink 

 again. 



Coagulation usually depends" upon the amount of acid 

 produced, and is more ^accurately and easily studied in 

 sugar broth cultures. 



Action upon carbohydrates. The characteristics usu- 

 ally observed in sugar broth are turbidity and sediment, 

 relation to oxygen, gas production, and acid production. 

 We have given reasons, in discussing nutrient broth, for 

 considering turbidity and sediment unimportant; and 

 the relation to oxygen is most sharply defined by surface 

 growth in the agar tube. None of the cocci that we 

 studied produced gas, and therefore acid production only 

 was recorded. For this purpose ordinary straight tubes 

 were used; and the sugars tested were dextrose and 

 lactose. A preliminary test indicated that saccharose is 

 less commonly fermented by the cocci than are dextrose 

 and lactose. 



The media were made up in the usual manner, with 

 2 per cent of the sugar to be tested. The reaction 

 was made neutral, and after tubing and sterilization 

 it was between 0.5 and 1.0 per cent. After standing 

 for two weeks sterile blanks showed a slight further 

 rise in acidity; so control tubes were always kept with 

 each batch inoculated and titrated at the end of the 



