HiCrome™ Chromogenic Coliform HiCynth™ Agar

Principle And Interpretation

HiCrome™ Chromogenic Coliform Agar is a selective medium recommended for the simultaneous detection of Escherichia coli and total coliforms in water samples (1). The medium contains three chromogenic substrates. The enzyme B-D-galactosidase produced by coliforms cleaves 6-chloro-3-indoxyl-ẞ- D-galactopyranoside to form pink to red coloured colonies (3). The enzyme ß-D-glucuronidase produced by E.coli, cleaves 5-bromo-4chloro-3-indoxyl-ẞ- D-glucuronic acid (2). Colonies of E.coli give dark blue to violet coloured colonies due to cleavage of both the chromogens. The presence of the third chromogen IPTG enhances the colour reaction. Addition of L-Tryptophan improves the indole reaction thereby increasing the detection reliability. HiCrome™ Chromogenic Coliform HiCynth™ Agar is a modification prepared by replacing animal peptones with chemically defined peptones to avoid BSE/TSE risks associated with animal peptones.

HiCynth™ Peptone No. 2 and HiCynth™ Peptone No. 5 provides nitrogeneous and carbanaceous compounds, long chain amino acids, vitamins and nutrients for the organisms. Sodium pyruvate and sorbitol provide growth nutrients and source of fermentable carbohydrate and other essential growth nutrients for the organisms. Phosphates buffer the medium. The media formulation helps even sublethally injured coliforms to recover and grow rapidly. Tergitol-7 inhibits gram-positive as well as some gram-negative bacteria other than coliforms (3).

The medium is inoculated either by pour plate technique or by spreading the sample on the surface of plated medium. Membrane filter technique can also be used. To confirm E.coli, add a drop of Kovacs reagent on the dark blue to violet colony. Formation of cherry red colour indicates a positive reaction.