ELEMENTARY BACTERIOLOGY LABORATORY EXERCISES 27 
tested and touch to the surface of the reagent. A blue color indicates the pres- 
ence of nitrites. 
Preparation of Trommsdorf’s Solution 
(1) Add slowly, with constant stirring, a boiling solution of 20 grams of 
zinc chloride in 100 c.c. of distilled water to a mixture of 4 grams of starch in 
water. Continue heating until the starch is dissolved as much as possible, and 
the solution is nearly clear. 
(2) Dilute with water and add 2 grams of zinc iodide (potassium iodide 
will do). 
(3) Dilute to 1 liter and filter. 
(4) Store in well-stoppered bottles in the dark. 
EXERCISE 42 
INDOL PRODUCTION 
Inoculate a tube of Dunham’s peptone solution with Bacterium coli. Incu- 
bate at 37° C. until the next period and then test for the presence of indol. 
Indol is formed from tryptophane by the action of some bacteria. Trypto- 
phane is an amino acid and hence occurs in peptone which is a digestion pro- 
duct of proteins. It is of course essential for this test that proteins which contain 
tryptophane be used in the making of the peptone. The bacteriological peptones 
now on the market contain tryptophane. 
Dunham’s Peptone Solution 
1.0% peptone 
0.5% sodium chloride 
Distilled water 
Test for Indol (Erlich Method) 
The reagent is a 2 per cent solution of paradimethylaminobenzaldehyde in 
95 per cent alcohol. One cubic centimeter of this reagent is added to the culture, 
then drop by drop concentrated hydrochloric acid is added until a red zone 
appears between the alcohol and peptone solution. Not more than 0.5 cc. of the 
acid is required. On standing the zone deepens and widens. The red color is 
soluble in chloroform and the test may be confirmed by shaking the culture with 
chloroform to see if the pigment is extracted. If it proves soluble, the test is 
considered positive. 
