CHAPTER X.
THE RECOGNITION OF BACTERIA.

THE most difficult thing in bacteriology is to be able
to recognize the bacteria which come under observation.

A certain few micro-organisms are so characteristic in
shape and grouping as to be separated by a microscopic
examination. Some, as the tubercle bacillus, are charac-
teristic in their reaction to the anilin dyes, and can be
differentiated at once by this peculiarity. Some, as the
Bacillus mycoides, are so characteristic in their agar-agar
growth as to eliminate others. The red color of Bacillus
prodigiosus and the blue of Bacillus janthinus will speak
almost positively for them. The potato culture of the
Bacillus mesentericus fuscus and its close relative the vul-
gatus is quite sufficient to enable us to pronounce upon
them. Unfortunately, however, there are several hun-
dreds of described species which lack any one distinct
character that may be used for differential purposes, and
require that for their diagnosis we shall wellnigh ex-
haust the bacteriological technique in an almost fruitless
•effort to recognize them.

A series of useful tables has been compiled by Eisen-
berg, and is now almost indispensable to the worker.
Unfortunately, in tabulating bacteria we constantly meet
species described so insufficiently as to make them worse
than useless on account of the confusion caused.

The only way to recognize a species is to study it
thoroughly and compare it, step by step, with the descrip-
tions and tables of known species compiled by Eisenberg
and others.

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