66 VETERINARY BACTERIOLOGY 



Some of the nitrogenous products of bacterial decomposition 

 are worthy of note, inasmuch as they are used in the laboratory in 

 the differentiation of certain species. The most important of 

 these are indol and skatol. They are organic compounds having 



the following formulas: 



/CH 3 



/V^.H.,X / \*> x\ 



C fl H 4 CH C 6 H 4 < CH. 



Indol. Skatol. 



Indol is produced by certain bacteria when growing in a solution 

 of peptone. It is identified by the addition of nitrous acid, with 

 which it combines to form nitroso indol, a bright red compound. 

 In making the test in the laboratory it is customary to add a 



t 



Fig. 31. Some decay-producing and putrefactive bacteria. 



few drops of concentrated sulphuric acid, followed by a dilute solu- 

 tion of nitrite. The sulphuric acid breaks up the nitrite, with the 

 formation of free nitrous acid, which then unites with the indol. 

 Indol and skatol are also formed in the intestines by the activity 

 of certain of the bacteria found there and are of considerable 

 physiological significance. 



Reduction Processes in Inorganic Compounds. Changes similar 

 to those just discussed are sometimes brought about by bacteria 

 in inorganic compounds. When nitrates are in solution together 

 with organic substances and under anaerobic conditions, the 

 bacteria present in many cases will reduce the nitrates to nitrites 

 and the nitrites to free nitrogen, apparently in order to utilize 

 the oxygen. This process is usually called denitrification because 

 the medium loses nitrogen, but is more correctly a reduction or 

 deoxidation. Sulphates are reduced to sulphites and even to 

 sulphids under similar conditions. For example, the sewage from 



