CHAPTER XVII. 



THE CLASSIFICATION OF BACTERIA. 



THE PERMANENCE OF VARIETIES. 



BACTERIA have been classified in many different 

 ways by many different observers. As a rule, the genera 

 are based upon morphological characters and the species 

 upon biochemical, physiological, or pathogenic prop- 

 erties. While the form, size, and method of division 

 are the most permanent characteristics of bacteria, and 

 so are naturally utilized for classification, nevertheless, 

 in this basis of division there are decided difficulties. 

 Thus while the form and size of bacteria are fairly con- 

 stant under the same conditions, they are in many quite 

 different under diverse conditions. Another serious 

 drawback is that these morphological characteristics 

 give no indication whatever of the relations of the bac- 

 teria to disease and fermentation the very characteris- 

 tics for which as physicians we study them. Other 

 properties of bacteria which are fairly constant under 

 uniform conditions are those of spore and capsule for- 

 mation, motility, reaction to staining reagents, relation 

 to temperature, to oxygen and other food material, and, 

 finally, their relation to fermentation and disease. 



Taking any one of these properties of bacteria as a 

 basis, we can classify them; but even here there will 

 be groups which under certain conditions would be 



17 



