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TYPHOID FEVER.                        371

The center of the superficial colonies is the only portion
which shows the yellowish-brown color. The margins
of the colony appear somewhat reticulated. The gelatin
is not liquefied.

Unfortunately, the appearances of the colonies of the
Bacillus typhi and the Bacillus coli communis are iden-
tical, and make it next to impossible to select a single
colony of either with any certainty. The only solution
of the problem is to transfer a large number of colonies
to some culture-medium in which a characteristic of one
or the other species is manifested, and then study the
growth; or to grow the colonies upon some special
medium in which differences, such as rapidity of growth
or acid-production, etc. cause the colonies of the differ-
ent species to assume characteristic appearances.

A method recently suggested by Eisner1 has materially
aided the separation .of these allied bacteria by using a
culture-medium upon which the two bacilli develop dif-

The Eisner medium can be made by allowing i kgm.
of grated potatoes (the small red German potato is best)
to macerate in i liter of water over night. The juice is
carefully pressed out, and filtered cold to get rid of as
much starch as possible. The filtrate is now boiled and
filtered again. The next step is a neutralization, in
which Eisner used litmus as an indicator, and added 2.5-
3 of a ^ normal solution of sodium hydrate to each
10 of the juice. Abbott prefers to use phenol-
phthalein as an indicator. The final reaction should be
slightly acid. Ten per cent, of gelatin (no peptone or
sodium chlorid) is now dissolved in the solution, which
is boiled for the purpose, and must then be again neu-
tralized to the same point as before. After filtration, the
medium receives the addition of i per cent, of potassium
iodid. It is filled into tubes and sterilized.

When water or feces suspected of containing the ty-
phoid bacillus are mixed in this medium and poured

1 Zdtschrift fur Hygiene, xxii., Heft i, 1895 ; Dec. 6, 1896.