Vogel-Johnson Agar

S. aureus reduces the potassium tellurite to the metal tellurium, which results in the growth of black colonies. The fermentation of mannitol is indicated by the yellow zones around the black colonies and changes the red color of the medium to yellow. Tryptone provides nitrogen, vitamins, minerals and amino acids essential for growth. Yeast extract is a source of vitamins, particularly of the B-group. Mannitol is the fermentable carbohydrate providing carbon and energy. Potassium tellurite, lithium chloride and the high glycine concentration inhibit most microorganisms other than staphylococci. Phenol red is the pH indicator. Dipotassium phosphate is a buffer. Bacteriological agar is the solidifying agent.

Vogel-Johnson Agar plates can be streaked heavily with a swab and incubated at 35 ± 2 °C for 24-48 hours. Look for black colonies surrounded by a yellow zone. During the first 24 hours the majority of microorganisms, except for coagulase-positive staphylococci, are totally or markedly inhibited. At 48 hours many coagulase-negative staphylococci, mannitol-positive and mannitol-negative, begin to appear. Staphylococcus epidermidis, almost always inhibited early, forms small grayish-black colonies without yellow zones. Coagulase-positive staphylococci form black colonies on the red medium. If they ferment mannitol, the colonies are surrounded by a yellow zone. Mannitol-negative organisms do not change the red color of the medium.

Typical formula in g/L, adjusted and/or supplemented as required to meet performance criteria.

Preparation

Suspend 60 grams of the medium in one liter of distilled water. Mix well and dissolve by heating with frequent agitation. Boil for one minute until complete dissolution. Sterilize in autoclave at 121 °C for 15 minutes. Cool to 45-50 °C and aseptically add 6 mL of 3.5% Potassium Tellurite (LC-5208). Homogenize gently and dispense into Petri dishes. To prepare a less selective medium, only add 3 mL of 3.5% Potassium Tellurite.

Instructions for use

Inoculate and incubate at a temperature of 35 ± 2 °C for 24-48 hours.

• United States Pharmacopoeia XXI (1985) Microbial limit tests. Rockville Md.
• Vogel R.A. and Johnson M.J. (1961) Pub. Hlth. Lab, 18, 131.
• Zebovitz E., Evans J.B. and Niven C.P. (1955) J. Bact. 70. 687.