Vogel-Johnson Agar Medium

 Principle

• Vogel-Johnson agar medium (VJ medium) is composed of tryptone, yeast extract, mannitol, dipotassium phosphate, lithium chloride, glycine, phenol red, and agar.
• Tryptone and yeast extract provide nitrogenous and carbonaceous compounds, long-chain amino acids, vitamin B complex, and other growth nutrients. Mannitol is the source of carbon. Dipotassium phosphate acts as a buffering agent.
• Lithium chloride and glycine inhibit most microorganisms except staphylococci. Phenol red is a pH indicator. Agar is the solidifying agent.
• The addition of tellurite 1% provides more selectivity to the medium for the isolation of staphylococci. After 24 hours of incubation, most of contaminating organisms are almost inhibited by tellurite, lithium chloride, and high glycine content.
• Staphylococcus aureus also be inhibited by these inhibitors but gets compensated by Mannitol and glycine. VJ Agar selects and differentiates the coagulase-positive staphylococci which ferment mannitol and reduce tellurite.

 Use: Recommended for selective isolation of mannitol fermenting Staphylococcus aureus from heavily contaminated food and clinical specimens (after the addition of potassium tellurite).