Sensitive reagents for detection of indole production by bacteria

A number of substituted 4-aminobenzaldehydes were compared with more commonly employed reagents for the detection of indole-producing organisms among the Enterobacteriaceae. Factors such as rate of colour intensity and stability were compared in tests with reference organisms and clinical isolates, and with appropriate concentrations of pure indolic standards. 2-Methoxy 4-dimethylaminobenzaldehyde and 4-N pyrrolidylbenzaldehyde produced immediate and intense red to purple colourations when used in the indole test both in liquid media and on multipoint inoculated plates. Both reagents were considered to be superior to the traditional Ehrlich's reagent and also to 4-dimethylaminocinnamaldehyde which is often used as an alternative.