Violet Red Bile Agar (VRB) is a selective medium used to detect and enumerate lactose-fermenting coliform microorganisms. The coliform group consists of several genera of bacteria belonging to the Enterobacteriaceae family. The historical definition of this group was based on the method used for detection, i.e. lactose fermentation. This group is defined as all facultative aerobic and anaerobic, gram-negative, non-spore-forming and rod-shaped bacteria that ferment lactose with gas and acid formation within 48 hours at 35°C. This medium is recommended for the microbiological analysis of milk and other dairy products as well as for the examination of water.

The medium contains bile salts and crystal violet which serve as inhibiting agents against certain gram-positive microorganisms, especially staphylococci. The neutral red is used as a pH indicator.

The degradation of lactose to acid is indicated by the neutral red pH indicator, which changes its color to red, and by the precipitation of bile acids. Lactose-fermenting microorganisms produce pink to red colonies which are usually surrounded by a reddish area of precipitated bile. Microorganisms that do not ferment lactose produce colourless colonies.