The DMACA Indole Discs are used for Indole test to determine the ability of an organism to split indole from the tryptophan molecule, and thus to aid differentiation between Escherichia coli from Klebsiella.In the presence of oxygen, some bacteria are able to split tryptophan into indole and alpha-aminopropionic acid. The presence of indole can be detected by the addition of DMACA (p-Dimethylaminocinnamaldehyde) reagent indicated by formation of bluish-purple colour. Escherichia coli ATCC 25922 : Positive reaction ,blue- purple colour ; Klebsiella pneumoniae ATCC 13883 : Negative reaction