CHAPTER XI 

 THE IDENTIFICATION OF SPECIES 



THE most difficult thing in bacteriology is the identification 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 difficulty. 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 characteristic agar-agar growths. The red 

 color of Bacillus prodigiosus 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 hun- 

 dreds of described species that lack any one distinct characteristic 

 that may be used for differential purposes, and require that for their 

 recognition we shall well-nigh exhaust the bacteriologic technic. 



Tables for the purpose have been compiled by Eisenberg, Migula, 

 Lehman and Neumann, Chester, and others, and are indispensable 

 to the worker. The most useful are probably the "Atlas and Grun- 

 driss der Bakteriologie und Lehrbuch der speziellen bakteriologischen 

 Diagnostik," by Lehmann and Neumann,* and 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 insufficiently as to make it 

 impossible to properly classify and tabulate them. 



The only way to determine a species is to study it thoroughly, 

 step by step, and compare it with the description and tables. In 

 this regard the differentiation of bacteria resembles the determina- 

 tion of the higher plants with the aid of a botanic key, or the qualita- 

 tive analysis for the detection of unknown chemic compounds. 

 Such a key for specific bacterial differentiation is really indispensa- 

 ble, even though it be imperfect, and every student engaged in re- 

 search work should have one. As Chester says: "probably nine- 

 tenths of the forms of bacteria already described might as well be 

 forgotten or given a respectful burial. This will then leave com- 

 paratively few well-defined species to form the nuclei of groups in 

 one or another of which we shall be able to place all new and suffi- 

 ciently described forms." "That typical forms or species of bac- 

 teria do exist, no one can deny. These typical forms furthermore 



* J. F. Lehmann, Miinchen, 1907. 

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