Dermatophyte Test Medium (DTM): Composition, Preparation, Uses

Last updated on May 30th, 2021

Dermatophyte Test Medium (DTM) is a specialized selective and differential medium used in medical mycology to differentiate dermatophytes (ringworm) from other fungi. It was developed by Taplin and colleagues by modifying Sabouraud Dextrose Agar (SDA).

Dermatophyte Test Medium Culture Plate
Dermatophyte Test Medium (DTM) Image source:labm.com

DTM uses phenol red as a pH indicator. The medium is yellow in color at pH below 6.8 which turns pink at pH ≥ 8.2 in the presence of alkaline metabolites, produced by dermatophytes (Epidermophyton, Microsporum, and Trichophyton spp.).

Contaminating saprophytes, if grown, do not produce alkaline byproducts within a measured time period, so the medium color will remain the same (typical saprotrophic fungi utilize carbohydrates producing acidic by-products and no red color change). Culture results should be reported within 2 weeks of incubation because longer incubation will increase the possibility of false-positive results.

Composition

Ingredients for Dermatophyte Test Medium

IngredientAmount (per liter)
Soy Peptone10.0gm
Dextrose10.0gm
Cycloheximide0.5gm
Chloramphenicol0.05gm
Gentamicin sulfate0.1 g
Phenol Red0.2gm
Agar20.0gm
pH: 5.6 ± 0.2 at 25°C

Functions of each ingredient

  • Soy Peptone provides nitrogenous and carbonaceous compounds essential for microbial growth.
  • Dextrose serves as the energy source for metabolism.
  • Chloramphenicol acts as a broad-spectrum antimicrobial; which inhibits a wide range of gram-positive and gram-negative bacteria.
  • Cycloheximide, an antifungal agent, is added to inhibit saprophytic fungi.
  • Phenol red is the pH indicator.
  • The acidic pH 5.6 favors fungal growth.

Preparation of Media

Depending on the requirements, individual laboratory either prepared DTM from dehydrated powder supplied by the commercial manufacturer or prepared plates or tubes can also be purchased. Theoretically, it can be made in the laboratory by using individual ingredients, but this is rarely done in most of the laboratories.

Methods of inoculation and incubation

Suitable samples for DTM include plucked hair, skin scrapings, or nail clippings.

  • Allow DTM to equilibrate to room temperature prior to sample inoculation. Make sure the agar surface is dry.
  • Place the sample centrally on the surface of the medium and press it gently to ensure firm contact.
  • Allow the cap on the tube to remain loose to ensure gaseous exchange during incubation.
  • Incubate at 25°C for up to 2 weeks in ambient air.

Culture Results

Dermatophytes give white or buff-colored growth within 5-10 days of incubation along with a red-colored medium. For definitive identification of dermatophytes, additional testing is required. The color change after two weeks of incubation is likely due to contaminating saprophytic fungi.

Dermatophyte test medium (Left- uninoculated vial, middle and right vials showing growth compatible with Trichophyton mentagrophytes (Image source: Jorge Oliveira)

Limitations

  • DTM should be used only as a screening test for the detection of dermatophytes. Other organisms such as saprophytic fungi, yeast may also grow on DTM and produce alkaline metabolite thus changing the color of the medium.
  • Saprophytic fungi may grow and change the color of the medium into the red. This problem is particularly encountered if the sample is contaminated with soils such as samples from feet and or nails.
  • DTM should not be used as the only means of identification of dermatophytes
About Acharya Tankeshwar 473 Articles
Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. I am working as an Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please mention in the comments below.

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