In the breaking down of the complex protein molecule even by a single kind of bacterium there is not a perfect descending scale of complexity as might be supposed from the statement that there result proteoses, peptones, polypeptids, amino-acids. These substances do result, but at the time of their formation simpler ones are formed also, even CO2, NH3 and H2S. It appears that the entire molecule is shattered in such a way that less complex proteins are formed from the major part, while a minor portion breaks up completely to the simplest combinations possible. A more complete knowledge of these decompositions will aid in the further unravelling of the structure of proteins. The presence or absence of free oxygen makes a difference in the end-products, as has been indicated. There are bacteria which oxidize the ammonia to nitric acid and the H2S to sulphuric acid. (See [Oxidation], Chapter XI.) Bacteria which directly oxidize phosphorus compounds to phosphoric acid have not been described. It does not seem that such are necessary since this is either split off from nucleic acid or results from the spontaneous oxidation of phosphine when this is formed under anaërobic
conditions.
Not only are proteins decomposed as above outlined, but also their waste products, that is, the form in which their nitrogen leaves the animal body. This is largely urea in mammals, with much hippuric acid in herbivorous animals and uric acid in birds and reptiles. These substances yield NH3, CO2 and H2O with a variety of organic acids as intermediate products in some cases. The strong odor of ammonia in stables and about manure piles is the everyday evidence of this decomposition.
Where the putrefaction of proteins occurs in the soil with moderate amounts of moisture and free access of air a large part of the products is retained in the soil. Thus the ammonia and carbon dioxide in the presence of water form ammonium carbonate; the nitric, sulphuric and phosphoric acids unite with some of the metals which are always present to form salts. Some of the gases do escape and most where the oxygen supply is least, since they are not oxidized.
The protein-splitting reactions afford valuable tests in aiding in the recognition of bacteria. In the study of pathogenic bacteria the coagulation and digestion of milk, the digestion or liquefaction of blood serum, the liquefaction of gelatin and the production of indol and H2S are those usually tested for. In dairy bacteriology the coagulation of milk and the digestion of the casein are common phenomena. Most bacteria which liquefy gelatin also digest blood serum and coagulate and digest milk, though there are exceptions. In soil bacteriology the whole range of protein changes is of the greatest importance.
Fig. 68.—Diagram to illustrate the circulation of phosphorus through the agency of bacteria.
Fig. 69.—Diagram to illustrate the circulation of carbon through the agency of bacteria.
