CORN FLOUR AGAR: BACKGROUND, PREPARATION AND USE - BIOLOGY - 2025

The corn meal agar is a solid culture medium, low nutritional power, useful for subculturing of certain fungi and for demonstrating the chlamydospores in strains of Candida albicans complex. 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.

A. Graphic representation of the Candida albicans Complex on corn flour agar. B. Chlamydospores of the Candida albicans Complex seen under the microscope, formed on cornmeal agar. Source: A. GrahamColm / B. By: CDC / Dr. William Kaplan, Courtesy: Public Health Image Library.

The sporulation of the Candida albicans complex 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 encountered saprophyte. Likewise, exceptionally the species C. stellatoidea and C. tropicalis could form them.

On the other hand, the addition of glucose to the cornmeal medium favors the formation of pigments in Trichophytom rubrum strains.

It is important to highlight that there are fungi that do not form hyphae or pseudohyphae in cornmeal agar, such as Cryptococcus neoformans, differing from other genera.

Cornmeal agar can be made at home in the laboratory or commercial media can also be used.

Basis

Cornmeal 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 Candida albicans species.

For its part, glucose in this medium increases the pigment-forming capacity of some fungi.

It should be noted that the cornmeal medium with glucose is not useful for the demonstration of chlamydospores in Candida albicans complex.

Preparation

Homemade Cornmeal Agar Preparation

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. Store in a refrigerator.

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.

Commercial corn flour agar

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.

Corn flour agar with Tween 80

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.

Corn flour agar with glucose

To improve the chromogenic power of Trichophyton rubrum colonies 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.

Use

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, Trichosporon and Geotrichum species produce arthroconidia on cornmeal 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.

Sown

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 22 ° C for 24 to 48 hours. The incubation time can be lengthened if necessary.

Chlamydospore demonstration

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 should be separated by a distance of 1 cm from each other.

Subsequently, a previously flamed covering-object is placed on the streaks that have been 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 under the microscope through the cover slip.

Maintenance of fungal strains

For the maintenance of strains, the seeded and grown plates are kept in a refrigerator (4 to 8 ºC). In this way they can last several weeks and be used for teaching or research purposes.

QA

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 can be sown 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 and E. coli. While a 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.

Limitations

A yellow precipitate forms at the bottom of the plate that should not be confused with colonies.

References

1. Neogen Laboratories. Corn Meal Agar. Available at: foodsafety.neogen.com.
2. Culture Media Microkit. Corn Meal Agar. Available at: Medioscultivo.com.
3. Linares M, Solís F. Yeast Identification Guide. Available at: http: //www.guia.revibero.
4. Urcia F, Guevara M. Rev. Perú Med.Exp. Public Health, 2002; 19 (4): 206-208. Available at: Scielo.com
5. Casas-Rincón G. General Mycology. 1994. 2nd Ed. Central University of Venezuela, Library Editions. Venezuela Caracas.
6. Forbes B, Sahm D, Weissfeld A. (2009). Bailey & Scott Microbiological Diagnosis. 12 ed. Editorial Panamericana SA Argentina.
7. Koneman E, Allen S, Janda W, Schreckenberger P, Winn W. (2004). Microbiological Diagnosis. 5th ed. Editorial Panamericana SA Argentina.
8. Castillo E. Comparative study of some macro and microscopic methods for the isolation and recognition of the genus Candida. Colombian Rev. of Pharmaceutical Chemical Sciences. 1970; 3 (1): 33-57. Available at: Ciencias.unal.edu.co