PUTREFACTION OF PROTEINS 95 



or results from the spontaneous oxidation of phosphine when 

 this is formed under anaerobic 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 inter- 

 mediate products in some cases. The strong odor of 

 ammonia in stables and about manure piles is the every-day 

 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 patho- 

 genic 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. 



The three physiological activities already discussed explain 

 how bacteria break down the chief complex, energy-rich 

 substances — carbohydrates, fats and proteins which consti- 

 tute the bulk of the organic material in the bodies of 

 plants and animals, as well as the waste products of the 

 latter, into energy-free compounds like carbon dioxide, water, 

 ammonia, nitric, sulphuric and phosphoric acids — ^mineralize 

 them, as is frequently said. By so doing the bacteria act 

 as the great scavengers of nature removing the dead animal 



