Page 92 - My FlipBook

66 THE MICRO-ORGANISMS OF THE HUMAN MOUTH.

The study of bacteria in an unstained condition is, however,
always attended with considerable difliculty, and their detection
and diiterentiation in tissues, or when mixed with various organic
matters, is sometimes impossible. Tliese difficulties are in a great
measure, and in fact usually, entirely overcome by first properly
staining the micro-organisms with some one of the basic aniline
stains above mentioned.
For examining bacteria in liquids we employ the

Cover- Glass Preparations.
A minute (piantity of the material to be examined (pure cul-
ture, or any soft substance supposed to contain bacteria) is spread
over the surface of a cover-glass in a small drop of water and
allowed to dry in the air (or the material is crushed in water
between t^vo cover-glasses, wliich are then pulled apart so that
each receives a coating). The glass is then slowly passed three
times through the flame of a spirit lamp or Bunsen burner, to
fix the material, and two to three drops of the stain (aqueous
solution) applied by means of a glass rod. The staining requires
from thirty seconds to ten minutes ; by tilting the glass to one side
it is easy to see when the preparation has taken on the coloring-
matter. The excess of staining matter is then removed with a
gentle stream of water, or by floating the cover-glass in a vessel
of pure water, when the preparation may be placed upon a slide
and examined under the microscope. For permanent prepara-
tions the cover-glass, after washing, is dried and mounted in
Canada balsam.
In order to stain the spores, float the cover-glass from ten min-
utes to one hour on a hot concentrated solution of fuchsine in ani-
line water, wash ott' the excess of coloring-matter, and treat the
preparation for a few seconds or minutes with absolute alcohol
c'ontaining a trace of fuchsine. Then stain in the usual manner
witli methylene blue (Hueppe). The bacilli appear blue, the
spores red. Instead of fuchsine and methylene blue, methyl
violet and vesuvine mav be used.