For selective isolation and easy detection of Escherichia coli O157:H7 from food samples. Chromogenic EC O157:H7 Selective Agar Base is based on the formulation described by Rappaport and Henigh. The chromogenic substrate is specifically and selectively cleaved by Escherichia coli O157: H7 resulting in a dark purple to magenta colour moiety. E. coli forms light pink to mauve colour colonies. Enterohaemorragic E. coli strains are also termed as verocytotoxin-producing E. coli (VTEC). Escherichia coli O157:H7, a member of the enterohemorrhagic E. coli (EHEC) group, gained recognition as the causative agent for an outbreak of severe bloody diarrhea traced to consumption of improperly cooked hamburgers. Although many different serotypes of Escherichia coli are known to produce the toxin, those of Escherichia coli O157:H7 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 ß-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.
Casein enzymic hydrolysate provides carbonaceous, nitrogenous and growth nutrients. The medium contains sorbitol and chromogenic mixture instead of lactose and indicator dyes respectively. Sodium chloride maintains osmotic equilibrium. Sodium lauryl sulphate helps to inhibit the accompanying gram-positive flora. Addition of Chromogenic EC O157:H7 Selective Supplement (TS 248) makes the medium selective. Potassium tellurite selectively inhibits Aeromonas and Providencia species. Novobiocin inhibits gram-positive bacteria.