VOGEL & JOHNSON AGAR
Vogel & Johnson Agar is a selective agar medium used for the detection of Staphylococcus aureus and other coagulase-positive, mannitol-fermenting Staphylococci in clinical specimens and food sources. Tryptein and Yeast Extract provide the essential nutrients for bacterial growth. D-Mannitol is the main carbohydrate source. The presence of Tellurite, Lithium Chloride and high concentration of Glycine make the medium selective. Coagulase-positive Staphylococci ferment D-Mannitol, thus acidifying the medium and causing the color shift of the pH indicator Phenol Red from red to yellow, and reduce Tellurite, thus forming a black precipitate in the center of the colony. Therefore, resistant colonies form black colonies surrounded by a yellow zone on this medium.