Principles and uses
Violet Red Bile Agar with Glucose (VRBG) is a selective medium, containing Bile and Violet Red dye, for the isolation and enumeration of enterobacteria.
It is based on MacConkey Medium  for the detection and enumeration of bile-tolerant Gram-negative Enterobacteriaceae in dairy products and foods. In this medium, the lactose is replaced by glucose as the carbohydrate. VRBG agar is becoming the preferred medium for use in investigations into raw materials, processed foods and plant hygiene. The Enterobacteriaceae group includes lactose-fermenting coliforms bacteria and non-lactose fermenting species like Salmonella and Shigella.
Pancreatic digest of gelatin provides nitrogen, vitamins, minerals and amino acids essential for growth. Yeast extract is the source of vitamins, particularly of the B-group. Glucose is the fermentable carbohydrate providing carbon and energy. Glucose fermenters form red colonies in the presence of the pH indicator neutral red. Bile salts and crystal violet inhibit Gram-positive bacteria. Sodium chloride supplies essential electrolytes for transport and osmotic balance. Bacteriological agar is the solidifying agent. The European Pharmacopoeia, USP in paragraph 2.6.13: “Microbiological examination of non-sterile products: test for specified microorganisms” recommends this medium for the testing of bile-tolerant Gram-negative bacteria in products.
ISO 21528 proposes VRBG Agar for the detection and enumeration of Enterobacteriaceae.
The pour plate method suppresses the growth of Gram-negative non-fermenting bacteria due to its semi-anaerobic conditions. The fermentation of glucose is likewise stimulated and results in the formation of purple-red colonies, clearly visible, surrounded by a zone of the same color. Note that coliforms will ferment the glucose and produce acid with or without gas. Klebsiella and Citrobacter, which are more heat-resident than coliforms, also grow in this medium and can indicate a production process defect (insufficient heating).