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 (synthe- 

 size) the elementary constituents necessary for their 

 complex cell composition. Bacteria have the power 

 both of breaking dow r n 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 



