Agar Medium F (Crystal Violet, Neutral Red, Bile Agar with Glucose)

Principle And Interpretation

This medium is a selective medium and cited as Agar Medium F (Crystal violet, neutral red, bile agar with glucose). It is recommended for detection of Enterobacteriaceae species by European Pharmacopoeia from food, pharmaceutical and dietary supplement preparations (1). Mossel et al (2,3,4) added glucose to the medium observing improved detection of coliforms. Incubation can be carried out at different temperatures and incubation time depending upon the group of Enterobacteriaceae to be recovered (5).

Pancreatic digest of gelatin and yeast extract provide nitrogenous compounds and other nutrients essential for bacterial metabolism. This media is selective due to presence of the inhibitors; bile salts and crystal violet. Crystal violet inhibits gram-positive organisms especially Staphylococci. Neutral red indicator helps to detect lactose monohydrate and glucose monohydrate fermentation. Lactose and glucose fermenting strains grow as red or pink and may be surrounded by a zone of acid precipitated bile. Sodium chloride maintains the osmotic equilibrium in the medium. T he red colour is due to absorption of neutral red and a subsequent colour change of the dye when the pH of medium falls below 6.8.