Chapter 14 



METABOLISM OF MICROORGANISMS 



Interrelations of microorganisms, animals and plants 



Microorganisms form an integral and indispensable part of a living 

 world. Some consideration of their chemical activities is not only de- 

 sirable but essential if an over-all view of biochemistry is to be obtained. 

 Chlorophyll-containing plants are the ''factories" in which organic matter 

 is made and energy is stored. Many of the organic compounds produced 

 in nature contain nitrogen, and in the fixation of atmospheric nitrogen, 

 bacteria, such as rhizobia, are undoubtedly the most important agents. 

 The building of organic matter is balanced by its destruction; here again 

 microorganisms seem to play the leading role. Dead plant and animal 

 materials are converted by microorganisms into simple compounds such 

 as carbon dioxide, ammonia and nitrates, which are used again by 

 plants. Although animals contribute to the breakdown of organic matter, 

 in the over-all effect microorganisms undoubtedly are the principal agents. 

 The balance between construction and destruction of organic matter is 

 often spoken of in connection with single elements and designated as 

 the carbon cycle and the nitrogen cycle in nature. 



The intimate association of microorganisms and animals is of course 

 obvious from the fact that animals act as hosts to vast numbers of bac- 

 teria in the intestinal tract. The importance of bacteria to the host is 

 conspicuous in the case of ruminants, where they probably are indis- 

 pensable. They break down cellulose and other resistant plant materials 

 to compounds that can be utilized by the animal, synthesizing all the 

 B vitamins needed by the ruminant. Even nonruminants appear to 

 derive a large part of their supply of certain vitamins, e.g., biotin, from 

 the synthetic action of bacteria. 



On the debit side of the association account is the production of dis- 

 ease by infectious microorganisms in animals and plants. The practical 

 problem then is to promote the development of useful microorganisms 

 and to retard the growth of harmful types. 



From a scientific viewpoint, the study of the metabolism of micro- 

 organisms has been a most fruitful effort. A first insight into inter- 

 mediary metabolism came from a study of yeast. This has been extended 

 to animals and bacteria, and from these studies a diversified but also a 

 unified pattern of metabolism is emerging. 



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