Violet Red Bile Agar with MUG (Methylumbelliferyl-b-D-glucuronide) is a medium for the simultaneous enumeration of coliform
organisms and Escherichia coli in food and dairy products. The selectivity of the medium is due to the presence of bile salts and crystal
violet. Lactose fermenters produce red/purple colonies often surrounded by a halo of bile precipitate. Escherichia coli produce red/purple fluorescent colonies due to the fermentation of lactose and production of the enzyme glucuronidase, which hydrolyses MUG to yield the fluorescent compound methylumbelliferone, detectable by long-wave UV light. Non-lactose fermenters produce pale colonies. Standard Methods procedures specify VRBA with MUG for detecting E. coli in food and dairy products by fluorescence.