In addition to expressing β-D-glucuronidase, E. coli are coliform bacteria that are able to produce indole from tryptophan at (44+-0.5) c within (21+-3) h. Therefore, in case of any doubt of E. coli colonies on primary agar medium, indole test may be used as an additional confirmation. E. coli also give positive results in the methyl red test and can decarboxylate l-glutamic acid but are not able to produce acetyl methyl carbinol, utilize citrate as the sole source of carbon, or grow in KCN broth.