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136                PATHOGENIC BACTERIA.

Petruschky's Whey.—In order to differentiate be-
tween acid and alkaline producers among the bacteria,
Petruschky has recommended a neutral whey colored
with litmus. It is made as follows:

To a liter of fresh skimmed milk i liter of water is
added. The mixture is violently shaken. About 10
are now taken out as a sample to determine how much
hydrochloric acid must be added to produce coagulation
of the milk, and, having determined the least quantity
required for the whole bulk, it is added. After coagulation
the whey is filtered off, exactly neutralized and boiled.
After boiling it is generally found clouded and acid in
reaction. It is therefore filtered again, and again neu-
tralized. Litmus is finally added to the neutral liquid, so
that it has a violet color, which can readily be changed to
blue or red by alkalies or acids.

The medium is a very useful aid in differentiating;
the typhoid and colon bacilli, showing well the alkaline
formation of the typhoid bacillus.

Peptone Solution, or Dunham's solution, is very use-
ful for the detection of certain faint colors. It is a per-
fectly clear, colorless solution, made as follows:

Sodium chlorid,            0.5*) Boil until the ingredients

Witte's dried peptone,   i.   V    dissolve; then filter, fill
Water,                       100.   j     into tubes, and sterilize.

It is one of the best media for the detection of iudol.
In it the bacillus pyocyaneus produces its blue color. A
very important fact in regard to peptone has been pointed
out by Garini,1 who found that many of the peptones
upon the market were impure, and on this account failed
to show the indol reaction for bacteria known to produce
indol. He recommends the use of the biuret reaction
for testing the peptone to be employed. The reagent
jised is Fehling's copper solution, with which pure pep-
tone strikes a violet color not destroyed upon boiling-,

1 Centralbl.f. Bakt. u. Parasitenk., 1893, xiii., p. 790.