84 LABORATORY BACTERIOLOGY 
Test the culture in sugar-free bouillon for the presence of 
indol if it is at least 72 hours old. Read the chapter on this 
organism in the text-books. 
Inoculate groups A and D of media with B. suipestifer 
from a culture furnished. 
127. The indol (cholera-red) test. Add 1 cc. of 0.01% solu- 
tion (fresh) of potassium nitrite and a few drops of concen- 
trated sulphuric acid to the culture in sugar-free bouillon. 
A pinkish color indicates the presence of indol. In an old 
(3- to 5-day) culture the reaction is usually stronger than in 
one more recently made. 
If sugar-free bouillon is not at hand, a tube of Dunham's 
solution can be used instead, with quite good results. 
Ring method, When there is a small quantity of indol it 
can be detected more readily by the “ring method.” Add 1 
or 2 cc. of a 25% solution of H,SO,, allowing it to run down 
on the inside of the tube containing the culture. Add 1 cc. 
of potassium nitrite. If indol is present, a pinkish ring will be 
observed between the layer of acid in the bottom of the tube 
and the culture above it. 
128. Dunham’s peptone solution. This is simply a solution 
of peptone and sodium chloride in distilled water. The for- 
mula is as follows : 
Dried peptone tee Se BBL I gram 
Sodium chloride. . . . . : 0.5 gram 
Distilled water. . . . 100 ce, 
Dissolve the peptone and salt in the water, distribute it 
in the tubes (5 cc. each), and sterilize the same as bouillon. 
1JIn place of this, &. exteritidis or some other member of the inter- 
mediate group may be substituted. 
