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Baird Parker agar is a selective media for the enumeration Staphylococcus Aureus in food. This was first reported by Baird Parker. It is highly recommended by both national and international organizations. You can achieve selectivity with potassium tellurite or lithium chloride. If Proteus species are present in the sample, sulfamezathine should be added. The critical component sodium pyruvate, which is essential for both the recovery of damaged Staphylococcus Aureus cells as well as their subsequent growth, is vital. This medium is used to form Staphylococcus aureus colonies that are grey-black or brown-grey due to tellurite reduction. Staphylococcus aureus colonies that have egg yolk factor are enclosed by an opaque zone. Often, this zone is surrounded with an outer clear area.
Composition of Baird Parker Agar
|HM Peptone B#
Final pH (at 25°C) 7.0±0.2
Principle of Baird Parker Agar
The coagulase test has shown a high correlation with the presence of a zone of lipolysis within this medium. This is due to the Lecithinase, a Staphylococci enzyme that breaks down the egg yolk. Nearly 100% of Staphylococci that are coagulase-positive can reduce tellurite, which causes black colonies. Other Staphylococci cannot.
Sources of nitrogen, carbon and sulphur include yeast extract, HM peptone A, and tryptone. Sodium pyruvate is a non-toxic, effective treatment that protects and promotes cell recovery. It also stimulates Staphylococcus Aureus growth by not destroying selectivity. Other than Staphylococcus aureus, most of the contaminating microflora can be inhibited by potassium tellurite and lithium chloride.
- Sodium pyruvate: Stimulates S. aureus growth without destroying selectivity
- The egg yolk: Staphylococci that contain lecithinase, not only enrichment but also clear areas around colonies
- Tellurite additive: Nearly 100% of coagulase-positive Staphylococci can reduce toxic tellurite strains clarifying eggs yolks, which results in black colonies
- Glycine, lithium chloride: Inhibit other organisms than S. aureus.
- After autoclaving, sulfamethazine can be added to ensure that almost all Proteus are inhibited.
Preparation and Method of Use of Baird Parker Agar
- Take 63.0 grams and mix it with 950 ml of distilled water.
- To dissolve the medium completely, heat to boiling
- For 15 minutes, sterilize by using an autoclave at 15 lbs pressure (121 degC).
- Cool to 50°C, then add 50 ml concentrated egg yolk emulsion and 3ml sterile 3.5% Potassium Tellurite solution (3FD047) or 50ml Egg Yolk Tellurite Emulsion (5FD046).
- If desired, add 1 vial of BP Sulpha Supplement to increase selectivity
- Alternately, one vial Fibrinogen plasma trypsin inhibitor supplement may be substituted for Egg yolk Tellurite emulsion to identify coagulase and positive Staphylococci.
- Mix everything together and pour onto sterilized Petri dishes.
- Before using, dry the surface of the agar plates for a short time.
- Use a glass spatula to spread 0.1ml of sample dilutions in Buffered Peptone Water onto the agar until it dries. For larger dishes, you can use up to 0.5ml.
- Place the inverted dishes in a warm place at 35°C.
- After 24 hours, examine the area and search for Staphylococcus aureus colonies.
- For another 24 hours, you can re-incubate any negative cultures.
Result Interpretation of Baird Parker Agar
Presumptive positive tests for S. aureus include the presence of convex, shiny, black colonies measuring 1-1.5mm in size. Colonies that do not form black pigmentation should be interpreted as negative.
|Grey-black shiny. The positive, opaque zone around the colony
|Brown-black, Negative for lecithinase
|Shades of brown-black (very small), Negative for lecithinase
|Black, Negative for lecithinase
|Dark brown matt, Negative for lecithinase
|Large brown-black, Negative for lecithinase
|Irregular and may produce clearing. Wide opaque zones may be produced in 24hrs
|White, Negative for lecithinase
Uses of Baird Parker Agar
- Baird Parker Agar can be used to detect coagulase activation by adding fibrinogen plasma.
- The ISO committee also recommends this medium for Staphylococci isolation and enumeration.
- It is used to isolate and count coagulase positive staphylococci in food and clinical samples.
- Officially, AOAC International adopted Baird Parker Agar. It is now recommended by the USP to be used in Microbial Limit Tests.
Limitations of Baird Parker Agar
- Some contaminating organisms may live in close proximity to coagulase-positive colonies and could partially digest the coagulase halos reaction.
- For confirmation, a biochemical test must be done.
- A coagulase reaction must be used to confirm the identity of Staphylococcus Aureus isolate on Baird Parker Agar
- The medium is not recommended for the detection of coagulase-positive Staphylococcus Aureus, but other bacteria could grow in it.
Warnings and precautions
In Vitro diagnostic use. Before opening the container, read the label. Protective gloves/protective clothing/eye protection/face safety are recommended. Good microbiological laboratory practices should be followed when handling culture and specimens. When handling clinical specimens, it is important to follow established precautions. Individual safety data sheets may contain safety guidelines.
- Baird-parker agar, Janet E.L. Corry, G.D.W. Curtis, Rosamund M. Baird, https://doi.org/10.1016/S0079-6352(03)80030-9.