244 NUTRITION AND METABOLISM 



have recently been found to hydrolyze urea. All urea bacteria can 

 live without urea, feeding on organic matter like other bacteria, but 

 most of them require an alkaline medium. 



Hippuric acid is split by certain bacteria to benzoic acid and 

 amino-acetic acid which can be oxidized completely. Uric acid can be 

 changed in several ways. In some of these changes, urea is found as 

 an intermediary product. 



PRODUCTS FROM MINERAL COMPOUNDS 



Minerals are used freely by microorganisms for cell construction, 

 consequently, they do not leave the living cell like fermentation 

 products. But a few organisms can actually decompose mineral 

 matter and when this takes place mineral products are secreted. Two 

 main processes can be distinguished, oxidation and reduction. 



OXIDATIONS are the result of the organisms seeking a supply of 

 energy. Several oxidations of minerals have been indicated previously, 

 as the oxidation of ammonia to nitrites, of nitrites to nitrates, of hypo- 

 sulphites to sulphates, of hydrogen sulphide to sulphur and of sul- 

 phur to sulphuric acid, of ferrous salts to ferric salts. All these 

 microbial changes are simple processes and can be followed by chem- 

 ical analysis much more easily than organic fermentations. The 

 organisms which cause these changes, do not, as a rule, thrive in 

 organic substances and for this reason pure cultures can be obtained 

 only with difficulty. Their activity is of great importance in soil 

 fertility. 



REDUCTIONS of minerals, too, are of great significance. As a typical 

 example, nitrates may be reduced to nitrites, to ammonia, to nitrogen 

 gas, and, rarely, to nitrogen oxides. The reduction may be performed 

 either by the direct removal of oxygen, or by the formation of free 

 oxygen. The reduction of nitrates to nitrites can be written in the 

 following three ways: 



KNO 3 - O = KNO 2 

 KNO 3 = KNO 2 + O 



KN0 3 + 2H = KN0 2 + H 2 0. 



The result in all three cases is the same. Many bacteria can reduce 

 nitrates to nitrites or to ammonia. A few can reduce them to nitrogen. 



