The Baird Parker agar It is a solid, selective and differential culture medium. It was created in 1962 for the detection and counting of coagulase positive staphylococci (Staphylococcus aureus).
It is composed of pancreatic hydrolyzate of casein, meat extract, yeast extract, lithium chloride, glycine, sodium pyruvate, potassium tellurite, agar and egg yolk emulsion..
Baird Parker Agar is based on the ability of the S. aureus to reduce tellurite and produce lecithinase. Both properties generate a colony with specific characteristics for this species. Therefore, it is highly effective in detecting this microorganism..
The typical colonies of S. aureus They are black or dark gray, with a colorless border and a light halo that surrounds them, differentiating them from other microorganisms. This pathogen can be found in clinical samples, waters, cosmetics, and raw or cooked foods..
Its diagnosis or detection is of utmost importance, due to the variety of pathologies it produces, such as food poisoning, scalded skin syndrome, toxic shock syndrome, abscesses, meningitis, septicemia, endocarditis, among others..
Article index
Pancreatic casein hydrolyzate, meat extract and yeast extract are the sources of nutrients, vitamins and minerals necessary for general microbial development, while pyruvate and glycine are compounds that promote the specific growth of Staphylococcus aureus.
Baird Parker Agar is selective because it contains substances that inhibit the growth of the accompanying flora, while favoring the development of S. aureus. The inhibitory compounds are lithium chloride and potassium tellurite..
This means allows to differentiate the S. aureus of the rest of the coagulase negative Staphylococci. S. aureus has the ability to reduce tellurite to free metallic black tellurium, forming black or dark gray colonies.
Likewise, the egg yolk provides the substrates to demonstrate the presence of the enzyme lecithinase and lipase.. S. aureus is lecithinase positive and therefore a clear halo will be observed around the colony, indicating that the lecithin was hydrolyzed.
In this sense, the appearance on this agar of glossy black or dark gray colonies with a light halo around them indicates the presence of S. aureus.
If a precipitation zone forms it is indicative of lipase activity. Some strains of S. aureus they are lipase positive and other negative.
In the event that the S. aureus If lipase is positive, an opaque area will be observed around the black or dark gray colony, and then the light halo due to the action of lecithinase.
Colonies of bacteria other than S. aureus capable of growing in this medium will develop colorless or brown colonies, without a halo around.
Atypical black colonies can also be seen with or without a colorless border, but without a light halo. These colonies should not be taken into account, they do not correspond to S. aureus.
Take a fresh chicken egg, wash it well and place it in 70% alcohol for 2 to 3 hours. The egg is then opened aseptically and the white is carefully separated from the yolk. Subsequently, 50 ml of the yolk are taken and mixed with 50 ml of sterile physiological solution..
Some commercial houses sell 1% potassium tellurite ready to use. It is added to the medium before the medium solidifies.
To prepare this solution in the laboratory, 1.0 g of potassium tellurite is weighed and dissolved in one part of water. Subsequently, the amount of water is completed until reaching 100 ml. The solution must be sterilized by the filtration method.
Weigh out 60 g of the dehydrated medium and dissolve in 940 ml of distilled water. Let the mixture rest for approximately 5 to 10 minutes.
Apply heat by stirring the medium frequently to improve the dissolution process. Bring to a boil for a minute. Sterilize in the autoclave at 121 ° C for 15 minutes.
Let stand until it reaches a temperature of 45 ° C and add 50 ml of the egg yolk emulsion and 10 ml of 1% tellurite. Mix well and pour 15-20 ml onto sterile Petri dishes..
Allow to solidify, arrange inverted in plackets and store in the refrigerator until use.
The final pH of the prepared medium must be 6.8 ± 0.2.
Before seeding a sample, wait for the plate to reach room temperature. Sow plates by streak or surface seeding with Drigalski spatula.
The color of the dehydrated medium is light tan and the color of the prepared medium is light amber.
The clinical samples are sown directly by discharging part of the material at one end of the plate, and from there it is streaked by exhaustion. Incubate for 24 to 48 hours at 35-37 ° C.
Weigh 10 g of the food sample and homogenize in 90 ml of 0.1% peptone water, from there dilutions are prepared if necessary. Inoculate the plates in triplicate with 0.3 ml of the prepared solutions, and seed on the surface with a Drigalski spatula. Incubate for 24 to 48 hours at 35-37 ° C.
This methodology allows to count the typical colonies obtained and is ideal when the presence of S. aureus above 10 CFU per g / ml of sample.
If the amount of S. aureus It is small or there is a lot of accompanying flora, it is suggested to enrich the sample in tryptic soy broth with 10% NaCl and 1% sodium pyruvate. This will favor the growth of S. aureus and will inhibit the development of the accompanying flora. Turbid tubes are seeded on Baird Parker agar..
In a sterilized vacuum filtration system, 100 ml of study water is filtered, and subsequently the 0.4 micron microporous membrane is removed with a sterile forceps and placed on a Baird Parker plate. Incubate for 24 to 48 hours at 35-37 ° C. This technique allows for the counting of typical colonies of S. aureus.
Known strains such as Staphylococcus aureus ATCC 25923, Staphylococcus aureus ATCC 6538, Staphylococcus epidermidis ATCC 12228, Escherichia coli ATCC 25922 or Proteus mirabilis ATCC 43071.
In the case of strains of S. aureus ATCC is known to reduce tellurite, and they are lipase and lecithinase positive. Therefore, there must be a satisfactory development and grow convex colonies with a black center and a colorless border, with an opaque halo and a light outermost halo..
For its part, S. epidermidis poor development is expected on this medium, with brownish-gray to black colonies, without light halo.
In order to E. coli Y P. mirabilis it is expected to be totally or partially inhibited. In case of growth, brown colonies will develop without opaque area, or light halo.
-The medium should not be heated after adding the tellurite and the egg yolk.
-The preparation of the egg yolk emulsion and its addition in the middle is a very vulnerable step for contamination. Care must be taken.
-If there are typical colonies of S. aureus must be corroborated by mounting a coagulase test on said strain.
-If there are doubtful results with coagulase, other confirmatory tests should be mounted.
-Take care not to confuse the presence of typical colonies of S. aureus with atypical black colonies.
Yet No Comments