Violet Red Bile Agar with MUG is often abbreviated as VRBA w/ MUG. This medium is used for the fluorogenic detection of Escherichia coli in food, milk, water, and other sanitary materials. Like Violet Red Bile Agar, it is a selective medium which detects the growth of lactose fermenting coliforms. Coliform colonies lower the pH of the medium subsequently causing their colonies to look red (neutral red dye). Crystal Violet and bile salts inhibit the growth of Gram-positive microorganisms while the addition of 4-methylumbelliferyl-β-D-glucuronide (MUG) allows for the rapid detection of E. coli. MUG is used as a substrate to detect glucuronidase activity which results in fluorescence under UV light.