458 FERMENTATION OF UREA 



done under natural conditions, though much more slowly, by bacteria. 

 Some of the latter, principally strict or facultative anaerobics, break 

 down or decompose the proteid material into still comparatively 

 complex molecules with the production of a fetid smell. This process 

 is called putrefaction. Other bacteria, mostly aerobics, produce a 

 more complete decomposition of the proteids into very simple com- 

 pounds, like carbon dioxide, water, and ammonia. This process is 

 known as decay. There is no generic difference between the processes 

 of putrefaction and decay, but only one of degree. Some of the most 

 common bacteria of putrefaction and decay have already been 

 described, such as the Bacillus proteus, the Bacillus subtilis, the 

 Bacillus mesentericus vulgatus, etc. The ammonia derived from 

 decomposing urea and proteids may be formed in such a manner as 

 to remain in the soil, or it may escape into the air. from which it is 

 subsequently washed down into the soil with the rain. Plants, as 

 stated, can utilize ammonia salts only to a slight extent for providing 

 for their nitrogen requirements, and, hence, nitrogen in this form is 

 of little value to higher vegetables which require nitrogen in the form 

 of nitrates. Some bacteria have the power to ferment urea and 

 change it into salts of ammonia; other bacteria possess the property 

 of oxidizing ammonia salts into nitrates; this process is called nitri- 

 fication. The oxidation of ammonia salts into nitrates, however, does 

 not occur immediately, but through the intermediary process of the 

 formation of nitrites, that is, salts of nitrous acid, which represents a 

 lower stage of oxidation than nitric acid. 



FERMENTATION OF UREA. 



Quite a number of bacteria possess the property of fermenting 

 urea and decomposing it into ammonia, from which salts are formed 

 when the opportunity offers for the base to unite with an acid. It is 

 probable that this converting power depends upon an enzyme known 

 as urase. Some of the principal urea-fermenting bacteria are the 

 following : 



Micrococcus Urese. This is a globular bacterium from 0.8 to 1 

 micron in diameter, frequently found in diplococcus or tetrad form. 

 According to Leube it forms on gelatin plates after twenty-four 

 hours, white cultures of the size of a millet seed, w^hich have a mother- 

 of-pearl luster, a sharp margin, and a smooth surface. After ten days 

 the colonies are quite large and resemble somewhat a drop of stearin 

 which has fallen upon and solidified on a surface. The growth does 

 not liquefy gelatin. 



Micrococcus Ureae Liquefaciens. This is a larger organism than the 

 preceding. The cocci have a diameter from 1.25 to 2 micra. They 

 appear singly or in chains of three to ten individual cocci. On gelatin 

 plates the organism, after two days, forms in the depth small, white 



