Page 88 - My FlipBook

62 the micro-oroanisms of the human mouth.
Preparation of Nutrient Gelatine, or Agar-Agar.
Having prepared the solution which we wish to make the
base of our nutrient gelatine, we add to 100 grams of the solu-
tion 5 to 10, or even 20 grams, of the finest gelatine cut into
small [>ieces, and allow it to stand one-half hour to one hour,
wdien the gelatine should be completely dissolved by gently
warming the solution, and carbonate of sodium carefully added
until the reaction becomes neutral, or very slightly alkaline.
It is then boiled for about an hour, in order to effect a com-
plete precipitation of the salts and coagulable substances (other-
wise the medium when done will appear cloudy), and while
hot filtered directly into test tubes, or into flasks from which
it may subsequently be transferred to the tubes, each tube re-
ceiving about one and one-half to two inches. The tubes are
then exposed to steam at a temperature of 100° C. in the steam
sterilizer for one-quarter to one-half hour. Twelve hours later
they are again exposed for the same length of time, which will
usually suffice to effect a complete sterilization of the contents.
A solution which I have used very frequently consisted of
water 100.0, peptone 2.0, sugar 1.0, beef-extract 1.5, to which five
to twenty grams of gelatine is added, according to the strength
required. In summer 20 per cent, will often be found neces-
sary to prevent melting of the cultures at room temperature.
The beef-water-peptone-gelatine now so extensively used is made
in the following manner : " One-half kilogram of good, finely
minced meat, is covered wdth 1 liter of distilled water, Avell mixed
and left for twenty-four hours in the refrigerator. It is then
pressed through gauze, and enough distilled water added to bring
the quantity up to 1 liter. To this water we add 10 grams of
dry peptone, 5 grams of table-salt, and 50 to 100 grams of gela-
tine" (Hiippc). As a universal culture material for oral bacteria
this is improved by the addition of 10 grams of sugar. The
further preparation is conducted in the manner described above.
Nutrient agar-agar is prepared in the same way as nutrient
gelatine, except that only 1 to 2 per cent, of agar-agar is added,
instead of 5 to 20 per cent.
Naturally, every intelligent experimenter will be able to vary
the culture medium according to the circumstances, bearing in