Chromogenic substrate member for identifying Escherichia coli bacteria. It comprises a growing medium for said bacteria and container means (1) for the growing medium (4) and a liquid sample to be tested. The container means also contains a chromogenic reagent compound consisting in 8-hydroxyquinoline-β-D glucuronide and an activator compound consisting in X-glucuronide where X is selected from the group consisting in alkyl, aryl or indoxyl radical, the halogeno substitutes and the nitro substitutes thereof.