Indole test

"[citation needed] Indole is generated by reductive deamination from tryptophan via the intermediate molecule indolepyruvic acid.

Pure bacterial culture must be grown in sterile tryptophan or peptone broth for 24–48 hours before performing the test.

Following incubation, five drops of Kovac's reagent (isoamyl alcohol, para-Dimethylaminobenzaldehyde, concentrated hydrochloric acid) are added to the culture broth.

A positive result is shown by the presence of a red or reddish-violet color in the surface alcohol layer of the broth.

Bacteria which give negative results for the indole test include: Actinobacillus spp., Aeromonas salmonicida, Alcaligenes sp., most Bacillus sp., Bordetella sp., Enterobacter sp., most Haemophilus sp., most Klebsiella sp., Neisseria sp., Mannheimia haemolytica, Pasteurella ureae, Proteus mirabilis, P. penneri, Pseudomonas sp., Salmonella sp., Serratia sp., Yersinia sp., and Rhizobium sp.