VOGEL-JOHNSON AGAR

Code: CM0641

A selective medium for the isolation of Staphylococcus aureus from clinical specimens and food.

Typical Formula*	gm/litre
Tryptone	10.0
Yeast extract	5.0
Mannitol	10.0
Dipotassium phosphate	5.0
Lithium chloride	5.0
Glycine	10.0
Phenol red	0.025
Agar	16.0
pH 7.2± 0.2 @ 25°C

* Adjusted as required to meet performance standards 

Directions
Suspend 61g in 1 litre of distilled water and bring gently to the boil to dissolve completely. Sterilise by autoclaving at 121°C for 15 minutes. Cool to 50°C and add 5.7ml of sterile 3.5% potassium tellurite solution SR0030 (equivalent to 20ml of 1% potassium tellurite).

Description
Vogel-Johnson Agar, by selecting and identifying coagulase positive and mannitol-fermenting strains, permits the early detection of Staphylococcus aureus from heavily contaminated foods and clinical specimens. It corresponds to the specification of the United States Pharmacopoeia¹ in terms of its formula.

Vogel and Johnson² modified the Tellurite Glycine Agar formula of Zebovitz et al.³ by doubling the mannitol concentration to 1% (w/v) and adding Phenol Red as a pH indicator. The enhanced fermentation reaction which occurs as a result of the increase in mannitol content is clearly indicated by the development of yellow zones surrounding the colonies.

Staphylococcus aureus is able to reduce tellurite to metallic tellurium resulting in growth as black colonies.

During the first 24 hours of incubation contaminating organisms are almost completely inhibited by tellurite, lithium chloride and the high glycine concentration. Virtually all the organisms that grow in this time are coagulase positive.

Organisms that grow as black colonies surrounded by a yellow zone after incubation at 35-37°C for 24 hours may be presumed to be Staphylococcus aureus.

Prolonged incubation may result in the growth of black coagulase-negative colonies and if these organisms also ferment mannitol they may be falsely identified from their appearance as Staphylococcus aureus. In these circumstances further tests of identity should be carried out before concluding that the organism is Staphylococcusaureus.

Techniques
Food samples
1. Dry the surface of the plates.
2. With a stirile glass spatula spread from 0.1 to 1.0ml of diluted food (macerated in 0.1% Peptone Water) over the surface of each well dried plate.
3. Incubate at 35-37°C and examine after 24 and 48 hours.

Clinical specimens
1. Dry the surface of the prepared plates.
2. Inoculate directly with the specimen.
3. Incubate at 35-37°C and examine after 24 and 48 hours.

Colonial appearance
Staphylococcus aureus appear as black, convex shiny colonies surrounded by a yellow zone.

Storage conditions and Shelf life
Store the dehydrated medium at 10-30°C and use before the expiry date on the label.
Store the prepared medium at 2-8°C.

Appearance
Dehydrated medium: Straw-pink coloured, free-flowing powder
Prepared medium: Orange coloured gel

Quality control

Positive control:	Expected results
Staphylococcus aureus ATCC® 6538*	Good growth; black colonies with yellow zones
Negative control:
Escherichia coli ATCC® 8739 *	No growth