Kovac's Indole Reagent is used for determination of indole production.In the presence of oxygen, some bacteria are able to split tryptophan into indole and alpha-aminopropionic acid by hydrolytic activity .The presence of indole can be detected by the addition of Ehrlich's or Kovac's reagent (p-dimethylamino benzaldehyde) indicated by formation of a red coloured ring, soluble in ether, chloroform and alcohol.Enterobacter aerogenes ATCC 13048 negative reaction ,no red ring Escherichia coli ATCC 25922 positive reaction, red ring at the interface of the medium