REDUCING POWER OF BACTERIA. 201 



is that it reveals only the presence of indol, and fails to 

 indicate whether or not reducing-bodies were coinci- 

 dently formed with the indol. As a test for indol alone 

 it is convenient and entirely trustworthy. 



REDUCING POWER OF BACTERIA. The power to 

 reduce chemical compounds from a higher to a lower 

 state may be said to be common to all bacteria. In 

 some bacteria, perhaps the majority, it is most conspicu- 

 ously manifested in connection with substances contain- 

 ing sulphur, hydrogen sulphide being formed. In other 

 bacteria it is best seen in connection with 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 me- 

 tabolic activities manifested by the bacteria in question. 



Test for Hydrogen Sulphide. The reduction of sul- 

 phur compounds may be determined by growing the 

 bacteria in peptone solution containing ferric tartrate, 

 when the presence of hydrogen sulphide will be indi- 

 cated by the brownish-black or jet-black color of the 

 precipitated iron-sulphide. 



The complete reduction of nitrates is brought about 

 by many bacteria. Other bacteria are capable of carry- 

 ing the reducing action as far as the formation of ammo- 



