224 BACTERIOLOGY 



the alterations produced in certain pigments, as litmus, 

 methylene-blue, indigo, etc., the normal color disappearing 

 in part or entirely according to the nature and activity of 

 the process. Other bacteria have the property of reducing 

 certain salts, as in the reduction of nitrates to nitrites, or 

 even to ammonia by the denitrifying bacteria. In some 

 instances these reductions result from the fact that the 

 bacteria liberate hydrogen from the compounds, in others 

 it results from the fact that the bacteria abstract oxygen 

 from such compounds, while in still other instances the 

 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 20.0c.c. 



Acetic acid (25 per cent, solution) . . . 150 . c.c. 



(6) Sulfanilic acid 0.5 gram 



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



