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 . 1 gram 



Distilled water . . . 20 . c.c. 



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



(6) Sulfanilio 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 



