CHAPTER XVII. 
THE CLASSIFICATION OF BACTERIA. 
THE PERMANENCE OF VARIETIES. 
Bacteria have been classified in many different 
ways by many different observers. Asarule, 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- 
ties 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 
