CHANGES IN THE REACTION OF MEDIA 207 



reduction is of a more complex nature. Each of these 

 changes, therefore, indicates the nature of some of the 

 metabolic activities manifested by the bacteria in question. 



Test for Hydrogen Sulphide. The reduction of sulphur 

 compounds may be determined by growing the bacteria in 

 peptone solution containing ferric tartrate, when the presence 

 of hydrogen sulphide will be indicated by the brownish- 

 black or jet-black color of the precipitated iron-sulphide. 



Reduction of Nitrates. The complete reduction of nitrates 

 is brought about by many bacteria. Other bacteria are 

 capable of carrying the reducing action as far as the for- 

 mation of ammonia, while still others merely reduce the 

 nitrates to nitrites. These reducing functions are encour- 

 aged and may be demonstrated by cultivating the bacteria 

 in peptone solution containing potassium nitrate. 



Test for Nitrites. The method of Griess, as modified by 

 Ilosvay, is quite satisfactory. These reagents are required: 



(a) Naphthylamine 0.1 gram 



Distilled water 20.0 c.c. 



Acetic acid (25 per cent, solution) . . . 150.0 c.c. 



(6) Sulfanilic acid 0.5 gram 



Acetic acid (25 per cent, solution) . -. . 150.0 c.c. 



In preparing solution a the naphthylamine is dissolved in 

 20 c.c. of boiling water, filtered, allowed to cool, and mixed 

 with the dilute acetic acid. Solutions a and b are then mixed. 

 It is best prepared as needed, though it may be preserved 

 for some time in a glass-stoppered bottle. 



In testing for nitrites the reagent is added in the proportion 

 of one volume of reagent to five volumes of culture. When 

 nitrites have been formed a deep-red color appears in a few 

 seconds. If no nitrites have been formed the culture remains 

 colorless. In testing cultures it is always necessary to control 



