Violet Red Bile Agar contains peptone to provide vitamins and minerals, as well as a carbon and nitrogen source. Yeast extract promotes bacterial growth by supplying B-complex vitamins. Lactose provides a carbohydrate source. Bile salts and crystal violet aid in the inhibition of Gram-positive bacteria. Neutral red is a pH indicator in which glucose-fermenters will produce red colonies and halos (due to bile precipitation). Sodium chloride preserves osmotic balance. Agar is a solidifying agent.
VRBA may also be formulated with 4-methylumbelliferyl-β-glucuronidase (MUG) for the detection glucuronidase activity. E. coli hydrolyzes MUG with the use of the enzyme glucuronidase. The hydrolyzation of MUG produces a fluorescent compound that may be detected with UV light at 366 nm.