CHAPTER XX 

 THE SAPROGENIC AND SAPROPHILIC BACTERIA 



Introduction. Definitions. In the previous chapter it 

 was pointed out that enzymes produced by bacteria are re- 

 sponsible for all the changes occurring in the media in which 

 the organisms are growing and not the bacterial cell in toto. 



It is a well-known fact that plants as a whole secrete only 

 a small amount of carbon dioxide after the natural absorption 

 of carbon and oxygen in this form (CO 2 ), and never liberate 

 and very rarely absorb any nitrogen from the air. Nitro- 

 gen which has been taken up from the nitrates of the soil is 

 returned thereto only when the plant dies and its protein 

 compounds are decomposed. The same can be said of the 

 greater amount of the nitrogen contained in animal cells. It 

 is liberated on the decomposition of the animal. There is, 

 however, in the animal a small amount of nitrogen secreted 

 in the milk and excreted in the urine (urea, uric acid, hippuric 

 acid, etc.). These compounds are eventually broken down 

 into ammonia and free nitrogen by the action of bacterial 

 enyzmes. 



Those bacteria which bring about the profound decompo- 

 sition or putrefaction of nitrogenous compounds are called 



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