Baird Parker Agar Base

Casein pancreatic digest, beef extract and yeast extract provide nitrogen, vitamins, minerals and amino acids essential for growth. Lithium chloride, trypsin inhibitor and potassium tellurite inhibit the accompanying flora, and glycine and sodium pyruvate facilitate the staphylococci growth. Bacteriological agar is the solidifying agent.

The ISO 6888-1 Standard recommends adding Egg Yolk Tellurite Emulsion Supplement (LC-5129) to Baird Parker Agar Base to make the complete medium Baird Parker Agar, used for the general count of coagulase-positive staphylococci in products intended for human or animal feed, through the counting of colonies obtained in a solid medium. The identity of Staphylococcus aureus isolated on Baird Parker Agar must be confirmed with a coagulase reaction.

The ISO 6888-2 Standard recommends the addition of Rabbit Plasma Fibrinogen (RPF) Supplement (LC-6024) to Baird-Parker Agar Base, to make Rabbit Plasma Fibrinogen (RPF) Agar used for food (such as cheese made from raw milk and certain raw meat products) that may be contaminated with Staphylococcus forming non-characteristic colonies in Baird-Parker Agar or with base flora that can mask the colonies sought. This medium should be used immediately after preparation.

In Baird-Parker Agar the characteristic colonies are black or grey, shiny and convex and surrounded by a clear area. After 24 h of incubation an opalescent ring may appear in the clear area. Non-characteristic colonies may be bright black or gray, with or without a narrow white border, without a clear area or opalescent ring.

In Rabbit Plasma Fibrinogen Agar, staphylococcus colonies are small, black or gray, even white, surrounded by a halo of precipitation indicating the coagulase activity. At the start of incubation, Proteus colonies can present similar physical characteristics to coagulase-positive Staphylococcus aureus colonies. However, after 24 or 48 hours of incubation, they can acquire a brown color that expands and invades the plate, and which allows them to be distinguished from staphylococcus.

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

Preparation

Suspend 63 grams of the medium in 1 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. Aseptically add 5 mL of Tellurite Egg Yolk Emulsion (LC-5129) per 100 mL of base medium, or one vial of the RPF Supplement ISO (LC-6024) to 90 mL of base medium. Homogenize gently and dispense into Petri dishes.

Instructions for use

Enumeration of coagulase-positive staphylococci by the technique using Baird-Parker Agar:

1. Inoculate 0.1 mL of the liquid sample or 0.1 mL of the initial suspension. Inoculate also the first decimal dilution.
2. Spread the inoculum over the plate surface as quickly as possible.
3. Incubate the plates at a temperature of 35-37 °C for 24 ± 2 hours, then reincubate for a further 24 ± 2 hours at the same temperature.
4. Examine the characteristic colonies at 24 hours and 48 hours.
5. Select a defined number of characteristic colonies to confirm in tubes of Brain Heart Infusion and incubate at 35-37 °C for 24 ± 12 h.
6. Add 0.1 mL of the culture obtained to 0.3 mL of the rabbit plasma and incubate at 35-37 °C.
7. Observe the coagulation of the plasma at 4-6 hours. If the assay is negative, continue incubating for 24 hours.
8. Coagulation of half of the volume of the plasma is considered a positive result.

Enumeration of coagulase-positive staphylococci by the technique using Rabbit Plasma Fibrinogen Agar:

1. Inoculate 1 mL of the liquid sample or 1 mL of the initial suspension by the pour plate method. Inoculate also the first decimal dilution.
2. Incubate the plates at a temperature of 35-37 °C for 18-24 hours. If needed, reincubate for a further 18-24 hours at the same temperature.
3. Count the characteristic colonies.

• Baird-Parker. J. App. Bact. 25:12. 1962.
• Baird-Parker. J. Ann. Microbiol. 30:409, 1963.
• Sharp, Neave and Reider. J. App. Bact. 28:390. 1962.
• Baird-Parker and Devenport J. App. Bact. 28:390. 1965.
• Tardio and Bact. J. AOAC. 54:728, 1971.