CHANGES BROUGHT ABOUT BY NON-PATHOGENIC ORGANISMS 65 



be decomposition of organic matter brought about by the aerobic 

 bacteria, the latter by the anaerobic types. This distinction 

 is not always acknowledged and adhered to, however. The 

 substances produced by the decomposition by bacteria depend, of 

 course, quite largely upon the nature of the material to be de- 

 composed. The carbohydrates and fats break down into alcohols. 



, but the proteins are split into a great 



variety of substances. Other agents, as acids and alkalis, will 

 break up the proteins in a similar manner and into many of the 

 same substances as do bacteria. Chemists have in recent years 

 demonstrated that proteins are made up of large numbers of 

 molecules of the a-amino acids linked together. An oc-amino acid 

 is an organic acid that has the NH 2 group in the alpha position, 

 that is, next the carboxyl. For example, the amino acid corre- 

 sponding to C 2 H 5 CH 2 COOH, butyric acid, is C 2 H 5 CHNH 2 COOH. 

 When these constituent links of the protein molecule are forced 

 apart, they usually appear in the form of one of about twenty com- 

 pounds which have been grouped as primary protein derivatives. 

 Some of these normal derivatives are further altered by bacteria. 

 Among them have been found certain compounds called ptomains, 

 some of which are known to be very poisonous. The splitting 

 usually continues until much of the organic matter is reduced 

 to comparatively simple compounds, such as H 2 S, CO 2 , CH 4 , and 

 NH 3 . The process ofjprotein disintegration is called proteolysis. 

 It usually occurs in several distinct stages. The proteins are 

 first broken down into relatively complex substances called prote- 

 oses, these then are broken down into peptones. This is called 

 peptonization, as it is essentially the change that may be brought 

 about by the enzyme pepsin from the stomach. The process con- 

 tinues and the peptones become amino acids and at last ammonia 

 is liberated. From an economic point of view this liberation of 

 ammonia has the greatest significance, for from this transformation 

 comes all the nitrogenous material used by plants and indirectly 

 by animals as food. This is the essential transformation that all 

 organic nitrogenous fertilizers, such as barnyard manure and 

 dried blood, will undergo before they can be of any use to higher 

 plants. By such changes water contaminated by sewage purifies 

 itself. 



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