CULTIVATION OF BACTERIA.	137
while impure peptone gives a red or reddish-yellow pre-
cipitate. Both the peptone and copper solution should
be in a dilute form to make successful tests. The
addition of 4 c.cm. of the following solution—
Rosalie acid,	0.5,
80 per cent, alcohol,	100.
makes it become an excellent reagent for the detection
of acids and alkalies. The solution is pale rose in color.
If the bacterium produces acids, the color fades; if alka-
lies, it intensifies. As the color of rosalic acid is destroyed
by glucose, it cannot be used in culture-media contain-
ing it.
Theobald Smith calls attention to the fact that Dun-
ham's solution is unsuited to the growth of many bac-
teria, some failing altogether to grow in it, and recom-
mends that, instead, bouillon free of dextrose shall be used.
All bacteria grow well in it, and the indol-reaction is
pronounced in sixteen-hour-old cultures. His method of
preparation1 is as follows: beef-infusion, prepared either
by extracting in the cold or at 60° C, is inoculated in
the evening with a rich fluid culture of some acid-pro-
ducing bacterium (Bacillus coli), and placed in the ther-
mostat. Early next morning the infusion, covered with
a thin layer of froth, is boiled, filtered, peptone and salt
added and the neutralization and sterilization carried on
as usual,
To test for the presence of indol, the bacterium is
planted in the culture-medium, allowed to grow for
upward of twelve hours, and then subjected to the com-
bined action of a nitrite and chemically pure sulphuric
acid. In making the test, Smith adds to each tube i c. cm.
of a o.oi per cent, solution of KNO2, freshly prepared,
and 10 drops of chemically pure H2SO4. The presence
of indol is characterized by the production of a red
color.
1 Journal of Exp. Medicine, vi., Sept, 5, 1897, p. 546.