The cornmeal agar is a solid culture medium, with low nutritional power, useful for the sub-cultivation of certain fungi and for the demonstration of chlamydospores in strains of the complex Candida albicans. In English it is known as Corn Meal Agar.
The conventional cornmeal medium has a very simple composition, it contains cornmeal, agar-agar and water. Due to its low nutritional level, it is ideal for use in the maintenance of fungal strains for moderate periods of time, especially black fungi..
Sporulation of the complex Candida albicans it is favored in this medium, if 1% of Tween 80 is added during the preparation of the agar. The formation of chlamydospores is characteristic of this species and practically the only one that affects humans..
There are other species that form chlamydospores, but they are unlikely to affect humans, such as Candida australis, present in penguin droppings, or C. clausenii, which is a rarely found saprophyte. Likewise, exceptionally the species C. stellatoidea Y C. tropicalis could form them.
On the other hand, the addition of glucose to the cornmeal medium favors the formation of pigments in strains of Trichophytom rubrum.
It is important to note that there are fungi that do not form hyphae or pseudohyphae in cornmeal agar, such as for example Cryptococcus neoformans, differentiating from other genres.
Cornmeal agar can be homemade in the laboratory or commercial media can also be used..
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Corn flour is the substrate, agar is the solidifying agent, and water is the solvent..
Cornmeal agar can be supplemented with tween 80 (sorbitan monooleate or polysorbate polyester 80). This compound reduces the surface tension of the medium due to its emulsifying power.
It also creates a hostile environment that inhibits exaggerated cell multiplication and stimulates the growth of hyphae, also favoring the production of chlamydospores; the latter considered structures of resistance. This structure helps in the identification of the species of Candida albicans.
For its part, glucose in this medium increases the pigment-forming capacity of some fungi..
It should be noted that the corn flour medium with glucose does not serve to demonstrate chlamydospores in cCandida albicans complex.
Weigh out 47 g of yellow corn flour and dissolve in 500 ml of distilled water. Heat to 60 ºC, while stirring the preparation for a period of approximately 1 hour. Then filter through a piece of gauze and cotton, optionally it can be filtered again by passing the preparation through a Whatman No. 2 filter paper.
Make up the volume to 1000 ml with distilled water. Add 17 g of agar-agar, heat until dissolved. Autoclave for 15 minutes at 121 ºC.
Serve in sterile Petri dishes. Keep in fridge.
The color of the prepared medium is whitish with a lumpy appearance.
If you want to prepare corn flour with glucose to the preparation described above, add 10 g of glucose.
Weigh 17 g of the dehydrated medium and dissolve in 1 liter of distilled water. The mixture can be heated, shaking gently to dissolve completely. Sterilize in an autoclave at 121 ºC, at 15 lb, for 15 minutes.
Pour into sterile Petri dishes. Let solidify. Invert and store in the refrigerator until use. Temper before use.
The pH should be 6.0 ± 0.2 at 25 ºC.
To comply with ISO 18416, cornmeal agar must be prepared as follows:
Weigh 65 grams per liter and add 10 ml of Tween 80. Heat and boil for a few minutes until dissolved, taking care not to overheat too much. Sterilize at 121 ºC for 15 minutes.
To enhance the chromogenic power of colonies of Trichophyton rubrum and differentiate them from T. mentagrophytes, 0.2% glucose can be added to the original formula. You do not need to have Tween 80, as glucose inhibits the formation of chlamydospores.
Mainly, the use of corn flour agar is intended for the study of Candida strains, helping their identification through the characteristic observation of chlamydospores in the albicans species. That is, the use of this agar serves as an auxiliary method of identification of these yeasts..
Both saprophytic and pathogenic species can develop on this agar, but each forms characteristic mycelial structures. For example, species of the genus Torulopsis do not produce mycelium and reproduce only by blastoconidia..
Likewise, the Trichosporon and Geotrichum species produce arthroconidia on corn flour agar and it is sometimes difficult to distinguish between one and the other..
Arthroconidia of the genus Geotrichum produce an extension of the hyphae resembling a hockey stick..
Also the production of pigments using corn flour agar supplemented with glucose is useful in the identification of Trichophytom rubrum.
Suspicious Candida colonies obtained in the primary culture medium - Sabouraud agar - from clinical samples, cosmetics, soils, among others, are subcultured on corn flour agar. The medium is seeded and incubated at 22ºC for 24 to 48 hours. Incubation time can be lengthened if necessary.
For this purpose, the corn flour agar with Tween 80 must be inoculated using the Dalmau technique. This method consists of taking a portion of the suspected colony with the platinum handle and making three parallel cuts in the middle, keeping the handle at 45º. The cuts must be separated by a distance of 1 cm from each other.
Subsequently, a previously flamed cover-object is placed on the streaks sown, in such a way that half is covered and the other is uncovered..
Incubate the seeded plates at 30 ° C for 48-72 h and then examine microscopically through the cover slip..
For the maintenance of strains, the seeded and grown plates are stored in a refrigerator (4 to 8 ºC). In this way, they can last several weeks and be used for teaching or research purposes..
For the sterility control, a plate is incubated without inoculation at room temperature, it is expected that there will be no growth or color change..
For quality control, known strains such as: Staphylococcus aureus, ATCC 6538, Escherichia coli ATCC 25922, Aspergillus niger ATCC 16404, Candida albicans ATCC 1023, Saccharomyces cerevisiae ATCC 9763.
The expected results are partial inhibition for S. aureus Y E. coli. While satisfactory growth is expected in the rest of the strains.
Aspergillus niger grows with black and sporulated colonies in an approximate time of 5 days of incubation.
Candida albicans yeast colonies with chlamydospore production.
Saccharomyces cerevisiae produce large yeast cells.
A yellow precipitate forms at the bottom of the plate that should not be confused with colonies..
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