January 2021 • Microbe Online

Dermatophyte Test Medium (DTM) is a specialized selective and differential medium used in medical mycology to differentiate dermatophytes (ringworm) from other fungi. Taplin and colleagues developed it by modifying Sabouraud Dextrose Agar (SDA). DTM uses phenol red as a pH indicator. The medium is yellow 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.). Dermatophyte Test Medium (DTM) Image source:labm.com Contaminating saprophytes, if grown, do not produce alkaline byproducts within a measured time, 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 two weeks of incubation because more prolonged incubation will increase the possibility of false-positive results. Composition Ingredients for Dermatophyte Test Medium IngredientAmount (per liter)Soy Peptone10.0gmDextrose10.0gmCycloheximide0.5gmChloramphenicol0.05gmGentamicin sulfate0.1 gPhenol Red0.2gmAgar20.0gmpH: 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 of 5.6 favors fungal growth. Preparation of Media Depending on the requirements, individual laboratories either prepare 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 using individual ingredients, which is rarely done in most 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 before 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 to detect dermatophytes. Other organisms such as saprophytic fungi and yeast may also grow on DTM and produce alkaline metabolites, thus changing the color of the medium. Saprophytic fungi may grow and change the color of the medium into red. This problem is mainly 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. References and further readings Acharya T., Hare J. (2022) Sabouraud Agar and Other Fungal Growth Media. In: Gupta V.K., Tuohy M. (eds) Laboratory Protocols in Fungal Biology. Fungal Biology. Springer, Cham. https://doi.org/10.1007/978-3-030-83749-5_2