Pathogenic Bacteria

CUL 77VA TION OF BACTERIA. 125

enough water added to bring the total amount up to 1000
c.cm. This liquid is called the meat-infusion. To it 10
grams of Witte's or Fail-child's dried beef-peptone and 5
grams of sodium chlorid are added, and the whole boiled
until the albumins coagulate. The reaction is then care-
fully tested, in order that whatever sarcolactic acid may
have been present in the meat may be neutralized by the
addition of a few drops of a saturated aqueous solution of
sodium carbonate. The solution is added drop by drop,
and the reaction frequently tested with litmus-paper.
When a neutral reaction, or, better, a faint alkaline re-
action, is attained, the mixture is well stirred, boiled
again for about half an hour to precipitate the alkaline
albumins formed, and-filtered. The use of phenolphtha-
lein to determine the reaction of the culture-media is much
more reliable than litmus, and in many laboratories has
replaced it. The method of using it suggested by Timpe
is to continue the addition of the carbonate of sodium
solution until a drop of it produces a red spot upon plie-
nolphthalein-paper. Such a paper can easily be made by
using a solution of 5 grains of plienolphthalein to i liter
of 50 per cent, alcohol. The bibulous paper is cut into
strips, moistened with the solution, and then hung up to
dry. It keeps quite well. Acids do not change the
appearance of the paper, but small traces of alkali turn
it red.

If it is necessary to be extremely accurate concerning
the acidity or alkalinity of the culture-medium, the
method of titration with plienolphthalein can be em-
ployed. For this purpose a small quantity of the culture-
fluid—say 10 c.cm.—receives an addition of a drop or
two of a weak alcoholic solution of plienolphthalein
(i : 300), and then drop by drop from a burette a dilute
soda solution is added until a faint rose color occurs, when
a simple calculation will show that if so much is required
to bring about the required color in the 10 c.cm., so much
more will be required for the total amount. The occur-
rence of the rose color marks the change from a neutral