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Full text of "Pathogenic Bacteria"

CUL 77 VA TION OF BACTERIA.              129

micro-organism accidentally enter, the whole quantity
will be spoiled ; if, on the other hand, it is kept in tubes,
several of them may be lost without much inconvenience.
Under proper precautions of sterilization and protection
it should all keep well.

Agar-agar.—Agar-agar is the commercial name of a
Japanese sea-weed which dissolves in boiling water with
resulting thick jelly when cold. The jelly, which solidi-
fies between 30° and 40° C., cannot again be melted ex-
cept by the elevation of its temperature to the boiling-
point, so that this culture-medium, which is nearly trans-
parent, is almost as useful as gelatin. In addition to its
readiness to liquefy and solidify, it is sufficiently firm
to allow of the incubation-temperature—i. e. 37° C.—at
which gelatin is always liquid, and no better than bouillon.

The preparation of this medium is generally described
in the text-books as one u requiring considerable patience
and much waste of filter-paper.'' In reality, it is not dif-
ficult if a good heavy filter-paper be obtained and no
attempt be made to filter the solution until the agar-agar
is perfectly dissolved. It is prepared as follows : To 1000
c.cm. of bouillon made as described above, preferably of
meat instead of beef-extract, 10 grams of agar-agar are
added. The mixture is boiled for an hour, or, if possible,
two. At the end of the first hour it is cooled to about
60° C., and after neutralization, which may not be neces-
sary if the bouillon was neutral, an egg beaten up in
water is added, and the liquid is boiled again until the
egg is entirely coagulated. The reaction of the agar-agar
should be neutral rather than alkaline, as, for an un-
known reason, alkalinity seems to interfere slightly with
filtration.

After the boiling, which should be brisk, has caused'
the thorough solution of the agar-agar, it is filtered, just
as the gelatin was, through a carefully-folded pharmaceu-
tical filter wet with boiling water. It may expedite mat-
ters to pour in about one-half of the solution, keep the
remainder hot, and subsequently add it when necessary.