The Vogel-Johnson agar is a solid, selective and differential culture medium, specially formulated for the isolation of Staphylococcus aureus. This medium was created by Vogel and Johnson in 1960 from the modification of the tellurite glycine agar formulated in 1955 by Zebovitz, Evans and Niven..
The modification consisted in increasing the concentration of mannitol present in the medium and in the incorporation of a pH indicator. The current formula is composed of triptein, yeast extract, mannitol, dipotassium phosphate, lithium chloride, glycine, phenol red, agar, 1% potassium tellurite solution and water..
It should be noted that there are other media that, like Vogel-Johnson agar, are selective for the isolation of S. aureus, such as salty mannitol agar and Baird Parker agar. In this sense, it could be said that the foundation of Vogel-Johnson agar is a mixture between salty mannitol agar and Baird Parker agar..
In the first the colonies of S. aureus They are distinguished by fermenting the mannitol and turning the pH indicator yellow. On the other hand, in the second the S. aureus it is characterized by reducing tellurite to tellurium and forming gray to black colonies. Both properties are observed in Vogel-Johnson agar..
This medium, like its counterparts, is useful for the detection of Staphylococcus aureus in food samples, sanitary controls of industrial products and in clinical samples.
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Vogel-Johnson medium contains triptein and yeast extract; Both substances provide long-chain amino acids that serve as sources of carbon and nitrogen necessary for bacterial growth. Bacteria capable of growing in this medium will take the nutrients from these substances..
Vogel-Johnson agar is capable of inhibiting the growth of Gram negative bacteria and even some Gram positive bacteria, favoring the development of coagulase positive staphylococci. The inhibitory substances are potassium tellurite, lithium chloride and glycine..
The substances that make this differential medium are mannitol and potassium tellurite. Mannitol is a carbohydrate, and when it is fermented, acids are produced that make the medium turn from red to yellow, which happens thanks to the presence of the red phenol pH indicator..
While, the colorless tellurite when it is reduced to free metallic tellurium, takes a dark gray to black color..
The Staphylococcus aureus ferments mannitol and reduces tellurite to tellurium. That is why the typical colonies of S. aureus in this medium they are gray or black surrounded by a yellow medium.
Bacteria that grow in this medium and do not reduce tellurite or ferment mannitol, will form transparent colonies surrounded by a red colored medium, even more intense than the original color, due to the alkalization of the medium due to the use of peptones..
On the other hand, bacteria that reduce tellurite but do not ferment mannitol will grow as gray or black colonies surrounded by a deep red medium..
If the medium were prepared without the addition of potassium tellurite, colonies of S. aureus would develop as yellow colonies, surrounded by a yellow medium, as in salty mannitol agar.
Dipotassium phosphate maintains the osmotic balance of the medium and adjusts the pH to neutrality 7.2. While the agar gives the solid consistency to the culture medium.
This solution is not included in the dehydrated medium, since it cannot be sterilized in an autoclave. For this reason, it is prepared separately and added to the already sterilized medium..
Some commercial houses sell the ready-to-use 1% potassium tellurite solution. If you want to prepare in the laboratory, proceed as follows:
Weigh out 1.0 g of potassium tellurite and measure 100 ml of distilled water. Dissolve the potassium tellurite in a part of water and then complete the amount of water until reaching 100 ml. Sterilize the solution by filtration method.
Weigh out 60 gr of the dehydrated medium, and dissolve in 1 liter of distilled water. The mixture is heated to a boil to aid in complete dissolution. During the dissolution process the medium is stirred frequently.
Sterilize in autoclave at 15 pounds pressure and 121 ° C for 15 minutes. Remove from the autoclave and let it rest until the medium reaches a temperature of approximately 45 to 50 ° C. Add 20 ml of the previously prepared 1% potassium tellurite solution.
Mix and pour into sterile Petri dishes. Allow to solidify and order inverted on plate holders to later store in the refrigerator until use.
The final pH of the prepared medium must be 7.2 ± 0.2.
Before sowing a sample, wait for the plate to reach room temperature..
The color of the prepared medium is red.
Although it can be used for the isolation of S. aureus in any type of samples, it is mainly used for the microbiological analysis of pharmaceutical products, cosmetics and food.
It is recommended that the inoculum be dense. Sowing can be done by striation with a platinum handle or by surface with a Drigalski spatula.
Plates are incubated at 35-37 ° C for 24 to 48 hours in aerobiosis.
The following control strains can be used to perform quality control on the Vogel-Johnson medium:
Staphylococcus aureus ATCC 25923, Staphylococcus aureus ATCC 6538, Staphylococcus epidermidis ATCC 12228, Escherichia coli ATCC 25922 or Proteus mirabilis ATCC 43071.
The expected result is as follows: for strains of S. aureus Satisfactory growth with black colonies surrounded by yellow medium. In order to S. epidermidis regular growth with translucent or black colonies surrounded by red medium.
Likewise, for E. coli total inhibition is expected, and for Proteus mirabilis partial or total inhibition; if it grows, it will do so sparingly and the colonies will be black surrounded by red half.
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