Vogel & Johnson (VJ) Agar (Difco)
VJ Agar, also known as Vogel and Johnson Agar, is used for the early detection of coagulase-positive, mannitol-fermenting staphylococci.
Principles Peptone is a source of carbon, nitrogen, vitamins and minerals. Yeast extract supplies B-complex vitamins which stimulate bacterial growth. Mannitol is the carbohydrate. Inhibition of nonstaphylococcal organisms is achieved by the potassium tellurite additive, which is inhibitory for some species of both gram-positive and gram-negative bacteria, by lithium chloride and by the high glycine content. Staphylococci may be slightly inhibited by the presence of the three inhibitors; however, this is compensated for by the addition of mannitol and glycine. Phenol red is the pH indicator and agar is the solidifying agent.
Coagulase-positive staphylococci reduce the potassium tellurite to metallic free tellurium, producing colonies that are gray-black. The fermentation of mannitol by coagulase-positive staphylococci is detected by a change in the color of the phenol red indicator from red (alkaline) to yellow (acid).