206 BACTERIOLOGY 



acid on the other organic matters in the culture-medium, 

 so that its presence may in this way escape detection. In 

 view of this, Petri recommends the use of dilute sulphuric 

 acid. He states that when indol is present the characteristic 

 rose color appears a little more slowly with the dilute acid, 

 but it is more permanent, and there is never any likelihood of 

 its presence being masked by other color-reactions. 



Muir and Ritchie recommend the use of ordinary fuming 

 or yellow nitric acid for this test. In this method two or 

 three drops of the acid are added to the culture under con- 

 sideration. If indol be present, the red color appears as a 

 result of the reducing action of the nitrous acid upon it. 

 The defect in this method is that it reveals only the presence 

 of indol, and fails to indicate whether or not reducing-bodies 

 were coincidently 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 conspicuously manifested in connection 

 with substances containing 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 



