HiCrome™ ECO157 : H7 Selective Agar Plate, Modified

Principle And Interpretation

Enterohaemorragic E.coli strains are also termed as verocytotoxin-producing E.coli (VTEC/ EHEC). Although many different serotypes of Escherichia coli are known to produce verocytotoxin (5) those of Escherichia coli and O157:H are so far the common types causing human infections. O157 VTEC strains have several unusual biochemical characters that are exploited in methods for their laboratory identification. They belong to the minority of E.coli that are β-glucuronidase negative and do not ferment sorbitol or rhamnose within 24 hours. These can be isolated from faecal specimens by plating on media containing D-sorbitol instead of lactose.

HiCrome™ EC O157:H7 Agar is based on the formulation described by Rappaport and Henigh (5). The medium contains sorbitol as fermentable carbohydrate and chromogenic mixture instead of lactose and indicator dyes respectively.

The chromogenic substrate is specifically and selectively cleaved by Escherichia coli O157: H7 resulting in a dark purple to magenta coloured moiety. E.coli forms bluish green coloured colonies.

Tryptone and yeast extract provides carbonaceous and nitrogenous compounds, long chain amino acids, vitamins and growth nutrients. Sodium chloride maintains osmotic equilibrium. Addition of HiCromeTM EC O157:H7 Selective Supplement (FD187) makes the medium selective (6). Potassium tellurite selectively inhibits Aeromonas and Providencia species. Novobiocin inhibits gram-positive bacteria. Sodium lauryl sulphate helps to inhibit the accompanying gram-positive flora.