CHAPTEE III. 



THE CLASSIFICATION OF BACTERIA PERMANENCE OF SPECIES 

 CHEMICAL COMPOSITION AND NUTRITION. 



Genera and Species. Bacteria have been classified in many different 

 ways by different observers. As a rule the genera are based upon mor- 

 phological characters and the species upon biochemical, physiological, 

 or pathogenic properties. 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 

 constant under the same conditions, we have already noticed that they 

 may be quite different under diverse conditions. Another serious draw- 

 back for our purposes is that these morphological characteristics give 

 no indication whatever of the relations of the bacteria to disease and 

 fermentation the very characteristics for which as physicians we study 

 them. Other properties of bacteria which are fairly constant under 

 uniform conditions are those of spore and capsule formation, 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 placed in one class and under others in another. 



Thus, the power to produce spores may be totally lost or held in 

 abeyance for a time. 



The relations to oxygen may be gradually altered, so that an anaerobic 

 species grows in the presence of oxygen. Parasitic bacteria may be so 

 cultivated as to become saprophytic varieties, and those which have 

 no power to grow in the living body may acquire pathogenic proper- 

 ties. 



The possibility of making any thoroughly satisfactory classification 

 is rendered still more difficult by the fact that many necessarily imper- 

 fect attempts have already been made, so tha,t there is a great deal 

 of confusion, which is steadily increased as new varieties are found 

 or old ones reinvestigated and classified differently in the different 

 systems. 



As one of the more successful attempts to classify bacteria, the system 

 devised by Migula is here given, simply as an example. The morphology 

 of bacteria is used as the basis of the division: 



