The principle of all the different RIDA®COUNT test cards is based on including specific chromogens into modified standard media. The nutrient composition of the RIDA®COUNT E. coli/Coliform test consists of a modified VRB (Violet Red Bile) medium. Non-coliforms are inhibited by the bile salts in the medium. A coliform specific enzyme, is converted by any coliforms present into a blue dye which leads to the formation of blue or blue-green colonies.

Any E. coli colonies present will reduce another E.coli specific chromogen to form a red-violet dye which combines with the blue dye and leads to the development of purple or indigo coloured colonies.

This system enables easy detection and differentiation of E. coli (red-violet colonies) and other coliform bacteria (blue or blue-green colonies) after an incubation time of 24 h at 35±1 °C.