Dermatophyte Test Medium: Principle and Colony characteristics

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Dermatophyte test medium agar

Dermatophytes are a distinct group of fungi that infect human and animal hair, skin, and nails, causing a variety of cutaneous infections known as ringworm. Dermatophytes such as Trichophyton, Microsporum, and Epidermatophyton cause the majority of cutaneous fungal infections. Taplin developed Dermatophyte Test Medium Agar Base as a selective and differentiating medium for the detection and identification of dermatophytes. Dermatophytes are identified on this medium based on morphology and the production of alkaline metabolites. Bacteria, saprophytic yeasts, and moulds are inhibited by a combination of three antimicrobial agents (cycloheximide, chlortetracycline, and gentamicin). Dermatophytes are presumed to be identified based on gross morphology and the production of alkaline metabolites, which raise the pH of the medium and cause the phenol red indicator to change color from yellow to pink-red.

Composition of Dermatophyte Test Medium

CompositionGram
Soya peptone10.000
Dextrose (Glucose)10.000
Phenol red0.200
Agar20.000
Final pH ( at 25°C)5.5±0.2

Principle and Interpretation of Dermatophyte Test Medium

  • Soya peptone contains nitrogenous and carbonaceous substances that are required for growth. The energy source is glucose.
  • Phenol red, a pH indicator, is used to detect amine production.
  • Cycloheximide inhibits the majority of saprophytic fungi. Gentamicin inhibits gram-negative bacteria such as Pseudomonas species, whereas chlortetracycline inhibits both gram-positive and gram-negative bacteria.
  • The presence of growth on the medium suggests the presence of dermatophytes. The distinct color change from yellow to red on Dermatophyte Test Medium Agar aids in the isolation and early identification of members of the Microsporum, Trichophyton.
  • Rapidly growing species may undergo a complete color change in 3 days, whereas slow growing species will take a proportionately longer time to change color.
  • Non-Dermatophytes can be recognized by the absence of colour change.
  • A few saprophytes, yeasts and bacteria change the medium from yellow to red, but can be easily distinguished by colonial morphology.
  • Complete classification of Dermatophytes depends on microscopic observations along with biochemical and serological tests.

Preparation procedure of Dermatophyte Test Medium

  • Suspend 20.10 grams in 500 ml purified / distilled water.
  • Heat to boiling to dissolve the medium completely.
  • Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes.
  • Cool to 45-50°C.
  • Aseptically add the rehydrated contents of one vial of Dermato Supplement.
  • Mix well before pouring into sterile Petri plates.

Type of specimen

Clinical samples such as :

  • Skin scrape
  • Hair
  • nail lesion
  • scaling scalp lesions

Quality Control of Dermatophyte Test Medium

AppearanceLight yellow to pink homogeneous free flowing powder
GellingFirm, comparable with 2.0% agar gel
Color and Clarity of prepared mediumOrange red colored, slightly opalescent gel forms in Petri plates
ReactionReaction of 4% w/v aqueous solution at 25°C. pH : 5.5±0.2
pH5.30-5.70
Uninoculated Dermatophyte Test Medium
Uninoculated Dermatophyte Test Medium

Cultural Response on Dermatophyte Test Medium

OrganismGrowthColor of Medium
Aspergillus brasiliensisnone-poor 
Candida albicansgood 
Microsporum audouiniigoodpink-red
Pseudomonas aeruginosanone-poor 
Trichophyton mentagrophytesgoodpink-red

Limitations of Dermatophyte Test Medium

  1. False-positive reactions may result, if interpretations are made beyond 6 days of incubation.
  2. If the abeyant area of an infection is cultured, false-negative reactions may arise.
  3. If the specimen is heavily contaminated, saprophytic fungi may result in a color change on the medium.

Storage and Shelf Life

  • Keep the prepared medium at 2-8°C and the prepared medium at 10-30°C in a tightly closed container.
  • Use before the expiration date printed on the label.
  • Due to the hygroscopic nature of the product, the product should be stored dry after opening and tightly capping the bottle to prevent lump formation. Improper product storage may result in the formation of lumps.
  • Store in a dry, ventilated area away from extremes of temperature and ignition sources.
  • After use, tightly close the container.
  • Use before the expiration date printed on the label.
  • When used within the specified expiry period, the product performs best.

Disposal of Dermatophyte Test Medium

The user must ensure the safe disposal of used or unusable preparations of this product by autoclaving and/or incineration. Infectious materials must be disposed of in accordance with established laboratory procedures, and any material that comes into contact with clinical samples must be decontaminated and disposed of in accordance with current laboratory techniques.

Reference

  • Isenberg, H.D. Clinical Microbiology Procedures Handbook 2nd Edition
  • Jorgensen, J.H., Pfaller, M.A., Carroll, K.C., Funke, G., Landry, M.L., Richter, S.S and Warnock., D.W. (2015) Manual of Clinical Microbiology, 11th Edition. Vol. 1.
  • Kwon-Chung and Bennett, 1992, Medical Mycology, Lea & Febiger, Philadelphia, Pa
  • Murray P. R., Baron J. H., Pfaller M. A., Jorgensen J. H. and Yolken R. H., (Eds.), 2003, Manual of Clinical Microbiology, 8th Ed., American Society for Microbiology, Washington, D.C

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