This bacterium is often endemic to cattle and spread through contaminated meat, and the Food Safety Inspection Service (USDA-FSIS) has declared it to be an “official adulterant” and requires meat producers to exclude it from their products.
Rainbow Agar O157 has both selective and chromogenic properties that make it particularly useful for isolating pathogenic E. coli strains. 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).
- Strain O157:H7 is typically glucuronidase negative so it forms unique and distinctive black or gray colonies.
- Many other non-O157 toxigenic strains overproduce β-galactosidase relative to β-glucuronidase on this medium and consequently they are typically colored purple, violet or blue.
- Most non-pathogenic E. coli strains are glucuronidase positive, and range from pink to magenta with occasional purple or blue strains.
- Nearly all other bacterial species are either inhibited on this medium or they grow as white or cream-colored colonies.
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