CHAPTER III. 



GENERAL BIOLOGY, INCLUDING THE 



CHEMICAL CHANGES WROUGHT 



BY BACTERIA. 



Bacteria. The bacteria with which the physician is chiefly 

 concerned as disease-producing are but a very small number 

 when compared with the multitude of species in nature. 

 The lay mind is apt to consider any germ as noxious, but 

 instead of this it can be said that without the activity of many 

 saprophytes, life on the earth would soon be extinct. Animals 

 require organic material from plants for their nourishment, 

 but their cells do not possess the power to put together 

 (synthesize) elementary substances necessary for their own 

 complex cell composition. Bacteria have the power both of 

 breaking down and building up; that is, they may reduce 

 some compounds to their elements or build up elements into 

 more complex substances. 



Perhaps the most striking examples of this property are 

 to be found among the earth organisms, some of which break 

 down organic matter into ammonia and liberate nitrogen, 

 others then taking up this gas from the atmosphere and 

 combining it with other elements in a form that plants can 

 assimilate. 



The products of their breaking down and building up are 

 utilized by plants and are presented to animals as food in 

 such a form that the animals can use them for their cell 

 needs. It is not the purpose of this book to dwell upon this 

 abstract matter of general biology, but the principles of the 

 activities of non-pathogenic bacteria can well be seen in 

 those inhabiting the intestines. 

 (32) 



