Violet Red Bile Glucose Agar is used for detecting and enumerating Enterobacteriaceae in food and dairy products.
SUMMARY AND EXPLANATION
The Enterobacteriaceae group includes lactose-fermenting coliform bacteria, lactose-nonfermenting strains of E. coli, and lactose-nonfermenting species, such as Salmonella and Shigella. When examining some foods, it is desirable to detect Enterobacteriaceae rather than the coliform bacteria.
Enterobacteriaceae are glucose-fermenting bacteria. Mossel et al. modified lactose-containing Violet Red Bile Agar by adding glucose to improve the recovery of Enterobacteriaceae. Later work by Mossel et al. demonstrated that lactose could be omitted, resulting in the formulation known as Violet Red Bile Glucose Agar (VRBGA).
Violet Red Bile Glucose Agar is recommended for the detection and enumeration of Enterobacteriaceae in food and dairy products. Violet Red Bile Glucose Agar is also listed in the USP as the recommended solid medium for use in the isolation of bile-tolerant gram-negative bacteria from nonsterile pharmaceutical products.
Violet Red Bile Glucose Agar contains pancreatic digest of gelatin as a source of carbon, nitrogen, vitamins and minerals. Yeast extract supplies B-complex vitamins which stimulate bacterial growth. Glucose is a carbohydrate. Bile salts and crystal violet inhibit gram-positive bacteria. Glucose fermenters produce red colonies with red-purple halos (bile precipitation) in the presence of neutral red, a pH indicator. Sodium chloride maintains the osmotic balance. Agar is the solidifying agent.