CHAPTER XI. 

 THE DIFFERENTIATION OF BACTERIA. 



THE most difficult thing in bacteriology is the determina- 

 tion of the species of bacteria that come under observation. 



A few micro-organisms are characteristic in morphology 

 and in their chemic and other products, and present no diffi- 

 culty. Thus, the tubercle bacillus is characteristic in its 

 reaction to the anilin dyes, and can usually be recognized by 

 this peculiarity. Some, as Bacillus mycoides, have charac- 

 teristic agar-agar growths. The red color of Bacillus pro- 

 digiosus and the blue of Bacillus janthinus speak almost 

 positively for them. The potato cultures of Bacillus mesen- 

 tericus fuscus and vulgatus are usually sufficient to enable 

 us to recognize them. Unfortunately, however, there are 

 several hundreds of described species that lack any one dis- 

 tinct characteristic that may be used for differential pur- 

 poses, and require that for their recognition we shall well- 

 nigh exhaust the bacteriologic technic in order to determine 

 them. 



Useful tables have been compiled by Kisenberg, Migula, 

 Chester, and others, and are indispensable to the worker. 

 The most useful is probably the "Manual of Determinative 

 Bacteriology," by F. D. Chester (1901), from which, through 

 the courtesy of the author and publisher, the following 

 synopsis of groups is taken. Unfortunately, in tabulating 

 bacteria we constantly meet species described so insuffi- 

 ciently as to make it impossible to properly classify and 

 tabulate them. 



The only way to determine a species is to study it thor- 

 oughly, step by step, and compare it with the descriptions 

 and tables. In this regard the differentiation of bacteria 

 resembles the determination of the higher plants with the 

 aid of a botanic key, or the qualitative analysis for the de- 

 tection of unknown chemic compounds. Such a key for 

 specific bacterial differentiation is really indispensable even 

 though it be imperfect, and every student engaged in research 



