Rainbow Medium O157

For the detection, isolation, and presumptive identification of verotoxin-producing strains of Escherichia coli, particularly serotype O157:H7. The medium contains chromogenic substrates that are specific for two E. coli-associated enzymes: β-galactosidase (a blue-black chromogenic substrate) and β-glucuronidase (a red chromogenic substrate). The distinctive black or gray coloration of E. coli O157:H7 colonies is easily viewed by laying the Petri plate against a white background. When O157 is surrounded by pink or magenta non-toxigenic colonies, it may have a bluish hue. The addition of selective agents improves performance. E. coli O157:H7 colony coloration will be slightly bluer with these selective agents added. Tellurite is highly selective for E. coli O157:H7 and can reduce background flora considerably. Novobiocin inhibits Proteus swarming and the growth of tellurite-reducing bacteria. Rare strains of O157:H7 are tellurite sensitive.