Baird-Parker Agar was developed by Baird Parker in 1962 and is a modification of the Tellurite-glycine formulation of Zebovitz et al. It is a moderately selective medium for the isolation and differentiation of coagulase-positive staphylococci, especially Staphylococcus aureus. It is primarily used in the processing of food, cosmetics, and environmental samples rather than clinical samples.
Baird-Parker agar medium is formulated on the principle that staphylococci are able to reduce tellurite to tellurium and show lecithinase reaction in the presence of egg yolk.
Components like casein enzymic hydrolysate, meat extract, and yeast extract provide nitrogen, carbon, sulfur, and vitamins. Pyruvate and sodium pyruvate not only protect injured cells and helps recovery but also stimulate the growth of Staphylococcus aureus.
Lithium chloride and potassium tellurite act as inhibitor agents for contaminating microflora. The tellurite additive is toxic to egg yolk-clearing strains other than S. aureus and imparts a black color to the colonies.
Composition of the media
|Casein peptone||10 g|
|Meat extract||5 g|
|Yeast extract||1 g|
|Lithium chloride||5 g|
|Sodium pyruvate||10 g|
|Final pH (at 25 °C) 6.8±0.2|
Egg yolk tellurite enrichment is added as a supplement.
Preparation of the media
- Suspend desired quantity (as per the manufacturer’s instruction) of the medium in 950 ml purified water.
- Heat with frequent agitation and boil for one minute to completely dissolve the medium.
- Autoclave at 121°C for 15 minutes.
- After cooling to 45- 50°C, add 50 mL of egg yolk tellurite supplement and 3 ml sterile 3.5% potassium tellurite solution or 50 ml egg yolk tellurite emulsion.
- Mix thoroughly before dispensing.
Result and Interpretation
On 18-24 hrs incubation, colonies of Staphylococcus aureus appear black and shiny, with a fine white rim, surrounded by a clear zone. Upon further incubation, for 48 hrs, an opaque zone is developed around colonies, which can be due to lipolytic activity.
Coagulase test on the colonies with the above characteristics should be additionally done for confirmation.
On testing food samples for the presence of Staphylococcus aureus, quantitative results can be obtained by counting the Staphylococcus aureus-like colonies and reported as a number of Staphylococcus aureus colonies per gram of food.