CHAPTER XI. 

 THE DETERMINATION 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 Eisenberg, Migula, 

 Lehman and Neumann, Chester, and others, and are indis- 

 pensable 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. Unfor- 

 tunately, in tabulating bacteria we constantly meet species 

 described so insufficiently 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 



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