Baird Parker Agar- Composition, Principle, Preparation, Results, Uses

Baird Parker Agar is a selective and differential medium for the isolation and enumeration of coagulase-positive staphylococci from food and clinical samples. It contains the following ingredients and their quantities per liter of distilled water:

• Tryptone: 10 g
• HM peptone B: 10 g
• Yeast extract: 5 g
• Sodium pyruvate: 10 g
• Lithium chloride: 5 g
• Glycine: 12 g
• Agar: 20 g

The medium is supplemented with egg yolk emulsion and potassium tellurite solution after autoclaving. The egg yolk emulsion provides enrichment and also serves as an indicator of lecithinase and lipase activity. The potassium tellurite solution inhibits most of the contaminating microflora except Staphylococcus aureus and also imparts a black color to the colonies that can reduce tellurite. The final pH of the medium is 7.0 ± 0.2 at 25°C.

Baird Parker Agar is based on the principle that coagulase-positive staphylococci, especially Staphylococcus aureus, can reduce tellurite to tellurium and produce lecithinase that breaks down the egg yolk in the medium. These two reactions allow the selective and differential identification of S. aureus from other bacteria.

The medium contains various ingredients that serve different purposes:

• Tryptone, meat extract and yeast extract are sources of nitrogen, carbon, sulfur, vitamins and trace elements that support the growth of bacteria.
• Sodium pyruvate not only protects injured cells and helps recovery but also stimulates the growth of S. aureus without destroying the selectivity of the medium.
• Lithium chloride and potassium tellurite act as inhibitory agents for most of the contaminating microflora except S. aureus.
• Glycine enhances the growth of staphylococci.
• Egg yolk emulsion provides enrichment and aids in the identification process by demonstrating lecithinase activity (egg yolk reaction).

The principle of Baird Parker Agar is correlated with the coagulase test, which is a confirmatory test for S. aureus. Coagulase is an enzyme that causes plasma to clot by converting fibrinogen to fibrin. Coagulase-positive staphylococci are usually pathogenic, while coagulase-negative staphylococci are usually commensal or opportunistic pathogens.

Baird Parker Agar is a selective and differential medium for the isolation and enumeration of coagulase-positive staphylococci from food and clinical samples. To prepare and use this medium, follow these steps:

1. Suspend 63.0 grams of Baird Parker Agar powder in 950 ml of distilled water. The powder contains tryptone, HM peptone B, yeast extract, sodium pyruvate, lithium chloride, glycine, agar and other ingredients that provide nutrients and selectivity for the growth of staphylococci.
2. Heat the mixture to boiling to dissolve the medium completely. Stir well to avoid clumping.
3. Sterilize the medium by autoclaving at 15 lbs pressure (121°C) for 15 minutes. This will kill any unwanted microorganisms that may contaminate the medium.
4. Cool the medium to 50°C and aseptically add 50 ml of concentrated Egg Yolk Emulsion and 3 ml of sterile 3.5% Potassium Tellurite solution. The egg yolk provides enrichment and lecithinase detection, while the potassium tellurite inhibits most of the competing flora and imparts a black color to coagulase-positive staphylococci.
5. For additional selectivity, if desired, add rehydrated contents of 1 vial of BP Sulpha Supplement per liter of medium. This supplement contains sodium metabisulphite and sodium azide, which inhibit gram-negative bacteria and some gram-positive bacteria other than staphylococci.
6. Alternatively, 1 vial of Fibrinogen Plasma Trypsin Inhibitor Supplement may be used per 90 ml of medium in place of Egg Yolk Tellurite Emulsion for identification of coagulase-positive staphylococci. This supplement contains fibrinogen plasma and trypsin inhibitor, which allow the detection of coagulase activity by forming fibrin clots around the colonies.
7. Mix well and pour the medium into sterile Petri plates. The plates should have a depth of about 4 mm and a diameter of about 90 mm. The medium should be yellow and opaque after adding the egg yolk or fibrinogen plasma.
8. Dry the surface of agar plates for a minimal period of time prior to use. Excessive drying may affect the performance of the medium.
9. With a sterile glass spatula or loop, spread 0.1 ml aliquots of sample dilutions made up in Buffered Peptone Water on the agar surface until it is dry. Up to 0.5 ml may be used on larger dishes. The sample dilutions should be prepared according to the standard methods for food or clinical samples.
10. Incubate the inverted dishes at 35°C for 24 to 48 hours. Do not stack more than three plates during incubation to ensure proper aeration.
11. Examine the plates after 24 hours and look for typical colonies of Staphylococcus aureus. Re-incubate negative cultures for a further 24 hours if necessary.

Baird Parker Agar allows the selective and differential isolation of coagulase-positive staphylococci, especially Staphylococcus aureus, from food and clinical samples. The medium relies on the following characteristics of S. aureus:

• The ability to reduce potassium tellurite to metallic tellurium, which produces black colonies on the medium.
• The production of coagulase, an enzyme that clots plasma and causes a clear zone around the colonies.
• The production of lecithinase or lipase, enzymes that hydrolyze egg yolk and cause an opaque zone within the clear zone.

The presence of black, convex, shiny colonies 1-1.5 mm in diameter with a clear zone and an opaque zone is a presumptive positive test for the presence of S. aureus. Colonies that do not form black pigmentation or clear zones should be interpreted as negative.

However, some strains of S. aureus may not produce lecithinase or lipase, resulting in only a clear zone without an opaque zone. Conversely, some strains of coagulase-negative staphylococci may produce black colonies with or without clear zones. Therefore, the identity of S. aureus isolated on Baird Parker Agar must be confirmed with a coagulase reaction. A 4+ coagulase reaction is necessary for unquestioned identification of S. aureus. Other tests, such as anaerobic glucose fermentation, lysostaphin sensitivity, and thermonuclease production, may also be used for confirmation.

Baird Parker Agar is a useful medium for detecting and enumerating coagulase-positive staphylococci from food and clinical samples. However, it is not sufficient to incriminate a food as the cause of food poisoning or to identify S. aureus without further tests.

Baird Parker Agar is a selective and differential medium that is widely used for the isolation and enumeration of coagulase-positive staphylococci from food and clinical samples. Coagulase-positive staphylococci, especially Staphylococcus aureus, are important pathogens that can cause food poisoning, skin infections, wound infections, endocarditis, toxic shock syndrome, and other diseases. Therefore, it is essential to detect and identify them in food and clinical samples to ensure food safety and public health.

Baird Parker Agar has been officially adopted by AOAC International (Association of Official Analytical Chemists) and is recommended in the USP (United States Pharmacopeia) for use in the performance of Microbial Limit Tests. Microbial Limit Tests are performed to determine the bioburden or the number of viable microorganisms in a sample. Baird Parker Agar can be used to test various types of samples, such as raw materials, finished products, water, cosmetics, pharmaceuticals, etc.

Baird Parker Agar is also recommended by the ISO (International Organization for Standardization) committee for the isolation and enumeration of staphylococci in food products. ISO is an international standard-setting body that provides specifications and guidelines for various fields and industries. Baird Parker Agar can be used to test various types of food products, such as meat, dairy, eggs, seafood, etc.

Baird Parker Agar can also be used to detect coagulase activity by adding fibrinogen plasma. Fibrinogen plasma is a supplement that contains fibrinogen, a protein that is involved in blood clotting. Coagulase-positive staphylococci can produce coagulase, an enzyme that converts fibrinogen into fibrin. Fibrin forms a clot around the bacterial cells, protecting them from phagocytosis and other host defenses. By adding fibrinogen plasma to Baird Parker Agar, coagulase-positive staphylococci can be distinguished from coagulase-negative staphylococci by the formation of a white halo around the colonies.

Baird Parker Agar is a useful medium for the detection and identification of coagulase-positive staphylococci in various samples. It has several advantages over other media, such as high selectivity, high sensitivity, high correlation with the coagulase test, and easy interpretation of results. However, it also has some limitations that need to be considered, such as potential false positives or negatives, need for further confirmation tests, and possible interference from other microorganisms. Therefore, Baird Parker Agar should be used in conjunction with other methods and criteria for the accurate diagnosis of staphylococcal infections.

Baird Parker Agar is a selective and differential medium for the isolation and enumeration of coagulase-positive staphylococci from food and clinical samples. However, it has some limitations that need to be considered:

• The identity of Staphylococcus aureus isolated on Baird Parker Agar must be confirmed with a coagulase reaction. Coagulase-negative staphylococci may also grow on the medium and produce black colonies, but they are usually smaller and less shiny than those of S. aureus.
• Though the medium is recommended for detection of coagulase-positive S. aureus, other bacteria may grow on it as well. For example, some strains of Bacillus cereus, Enterococcus faecalis, Micrococcus luteus, and Listeria monocytogenes may produce black colonies or zones of lipolysis on Baird Parker Agar. Therefore, further biochemical tests have to be performed for confirmation.
• Colonies of some contaminating organisms growing in close proximity to the coagulase-positive colonies may partially digest the coagulase halo reaction. This may result in false-negative results or reduced clarity of the zones of lipolysis. Therefore, care should be taken to avoid overcrowding the plates and to examine them within the recommended incubation time.
• Baird Parker Agar may not detect all strains of S. aureus that are relevant for food safety. For example, some strains of methicillin-resistant S. aureus (MRSA) may not grow well on the medium or may not produce typical black colonies. Therefore, alternative methods may be needed to detect these strains.

In conclusion, Baird Parker Agar is a useful medium for the isolation and enumeration of coagulase-positive staphylococci from food and clinical samples, but it has some limitations that require further confirmation tests and careful interpretation of the results.

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