Vogel Johnson Agar Medium 22. (In accordance with IP 2007)

Principle And Interpretation

Vogel-Johnson Agar Medium is prepared according to the formula of Vogel and Johnson (1). Originally it was developed by Zebovitz (2) as a Tellurite Glycine Agar, a selective medium for the detection of coagulase positive Staphylococci. This medium is used to detect Staphylococcus in pharmaceutical and cosmetics products (4). Staphylococcus is a prevalent pathogen in food poisoning due to its enterotoxin production. It is a commensal found on skin and scalp of human body.

Vogel-Johnson modified the medium by adding phenol red as a pH indicator and by increasing the mannitol quantity (3). Pancreatic digest of casein and yeast extract provide nitrogenous compounds, vitamin B complex and other growth nutrients. Dibasic potassium phosphate gives buffering capacity to the medium. During first 24 hours of incubation, contaminating organisms are almost inhibited by tellurite, lithium chloride and high glycine content. Staphylococcus aureus may be inhibited by these inhibitors but get compensated by mannitol and glycine.

Coagulase-positive Staphylococci reduce potassium tellurite to metallic free tellurium and thus produce black colonies surrounded by yellow zones. This yellow colour is due to phenol red indicator, which turns yellow in acidic condition by the fermentation of mannitol. Prolonged incubation may result in the growth of black coagulase negative colonies.