Reagent proposed by KOVÁCS (1928) for detecting microbial indole in the identification of indole-positive and indole-negative microorganisms.
Mode of Action
Some microorganisms can cleave tryptophan which is especially abundant in tryptically digested peptone to give pyruvic acid, ammonia and indole. Indole then reacts with 4-dimethylaminobenzaldehyde to form a dark red dye. As tryptophan also gives a color reaction with 4-dimethylaminobenzaldehyde, it must be separated from the indole. This is achieved by selectively extracting indole with butanol.
Experimental Procedure and Evaluation
The strain purity of the organism to be tested must first be established; it is then inoculated into an appropriate culture medium (e.g. Standard I Nutrient Broth
(Cat. No. 1.07882.), DEV Tryptophan Broth (Cat. No. 1.10694.), SIM Medium (Cat. No. 1.05470.), etc.) and incubated for 18-24 hours at the optimal incubation temperature. The medium is then covered with a layer of KOVÁCS' indole reagent of about 0.5 cm. If indole is present the reagent layer turns cherry red in color after a few minutes.
The reagent solution must be stored in the dark in the refrigerator, otherwise it may turn brown and cannot be used.