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METHODS OF BACTERIOLOGICAL INVESTIGATION. 63 :

mind that for many bacteria it is absolutely necessary that the
character of the artificial culture medium be very similar to that
of the substratum in which they are naturally found.
There are many kinds of bacteria which have not yet been
cultivated, and which are therefore styled strictly obligatory par-
asitic, which I doubt not would grow well enough if we were
able to prepare a medium for them identical with their natural
nutriment.
Except for especial purposes, I would not advise any one whose
time is so limited as that of a dentist or medical practitioner to
attempt to make his own gelatine or agar-agar, as he will save
much time in purchasing it already made.

Having obtained a pure culture of any given micro-organism,
the further study of it is to be directed to the following points

1. Its morphology, development, etc., as reveale powers of the microscope.
2. The characteristics of its growth on dilferent nutrient
media.
3. Its behavior in regard to atmospheric oxygen.
4. Its physiological action (fermentation, putrefaction).
5. Does it possess diastatic, hydrolitic (inverting), or pepton-
izing action ?
6. Does it form coloring-matter ?
7. How is it aifected l)y various antiseptics?
8. Does it possess pathogenic properties, and if so. what are
they ?
The first question is answered by direct microscopical obser-
vation of stained and unstained specimens, with the help of
good, homogeneous immersion lenses, particular attention being
paid to the production or non-production of spores, since this is
a matter not alone of botanical, but also of great hygienic inter-
est, inasmuch as the resistance of a bacterium to devitalizing
acrents is to a o-reat dcfirree determined bv the r)resenee or absence
of spores. A knowledge of the morphology and development
of a bacterium is not, however, essential to the study of questions
2 to 8. Pare cultures may be ol)tained and all desirable experi-