Dermatophyte Test Medium principle and colonies morphology

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Dermatophyte Test Medium MLTGEEKS

Dermatophyte Test Medium abbreviated DTM is  a growth selective growth medium used for the selective isolation of dermatophytes. Dermatophytes are a distinct group of fungi that infect the skin,hair and nails of humans and animals engendering a variety of cutaneous infections known  as ringworm or Tinea. Examples of these dermatophytes like Epidermophyton,Microsporum and Trichophyton are responsible for most of the cutaneous fungal infections like athlete’s foot,nail infection or infected toenail(onychomycosis) caused by tinea unguium and other infections caused by candida albicans

DTM Agar Base was developed as a selective and differential medium for the detection and identification of dermatophytes .The identification of dermatophytes on DTM are predicated on morphology and alkaline metabolites production. An amalgamation of three antimicrobial agents (cycloheximide,chlortetracycline and gentamicin) helps to inhibits bacteria,saprophytic yeasts and moulds. Therefore,dermatophytes are presumptively identified based on their gross morphology and the production of alkaline metabolites, which raise the pH and cause the phenol red indicator to transmute the color of the medium from yellow to pink-red

Principle of Dermatophyte Test Medium

  • The papaic digest of soybean meal provides nitrogenous and carbonaceous substances essential for growth.
  • Glucose is the energy source in the medium
  • The phenol red pH indicator is used to detect amine production.
  • Antibiotics such as Cycloheximide  inhibits most of the saprophytic fungi while Gentamicin inhibits gram-negative bacteria including Pseudomonas species and chlortetracycline inhibits a wide range of gram-positive and gram-negative bacteria.
  • The presence of growth on the medium provides presumptive identification of dermatophytes. D.T.M. Agar  and helps in the isolation and early recognition of members of the Microsporum, Trichophyton by means of the distinct colour change from yellow to red.
  • Rapidly growing species may affect a complete colour change within 3 days while slow growers will change colour in proportionately longer time.
  • As for non-dermatophytes,they 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.
Also read  Sabouraud Dextrose Agar (SDA) Composition, Principle, Uses, Preparation culture and Colony Morphology

Composition of Dermatophyte Test Medium

The following ingredients are used for the composition of Dermatophyte test medium agar

Papaic digest of soybean meal10.000
Phenol red0.200
Final pH at temperature 25°C5.5±0.2
Dermato Supplement1 Vial

Dermato Supplement

It is an antibiotic supplement which is recommended for the selective isolation of pathogenic dermatophytes. 1 vial is sufficient for 500 ml medium

Composition of Dermato Supplement


How to Prepare Dermatophyte Test Medium

  1. Start by suspending 20.10 grams in 500 ml distilled water.
  2. Heat to boiling in order to completely dissolve the medium.
  3. Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes.
  4. Cool the medium to 50°C.
  5. Aseptically add the rehydrated contents of one vial of Dermato Supplement (Rehydrate the contents of 1 vial with 5 ml of 50% v/v acetone then mix well and aseptically add it to 500 ml of the sterile molten and cooled (45-50°C) D.T.M. Agar .
  6. Mix well before pouring into sterile Petri plates.

NB: It is advisable to incubate a petri dish for 24 hour for quality control in order to avoid wastage of resources and ease early detection of contamination.

Appearance of Medium after preparation

The medium appears orange red coloured, slightly opalescent gel forms in Petri plates

Uninoculated DTM Agar
Uninoculated DTM Agar

Type of Inoculums

  • Skin swab
  • Hairs swab
  • Toe swab
  • Mouth swab
  • Other cutaneous sources etc

How to Inoculate on Dermatophyte Test Medium

  1. Collect the specimen following established procedures.
  2. Allow the medium to equilibrate to room temperature prior to use.
  3. The agar surface should be dry before inoculation
  4. Now place the specimen centrally on the surface of the medium then press into the agar to ensure firm contact.
  5. Incubate the inoculated medium in ambient air at a temperature of 25-30°C for up to 14 days. If you are using a tube,Allow the cap on the tube to remain loose so that air may be exchanged during incubation.
  6. Carefully examine the medium at regular intervals for a red color development. On DTM, dermatophytes elaborate alkaline metabolites which elevate the pH of the medium and change the phenol red indicator from yellow to red.
Also read  Introduction to Medical Mycology and importance of Fungi

For presumptive identification of an isolate as a dermatophyte,microscopic examination  such as wet mountand 10 % KOH test is required and other relevant biochemical tests.

Plate reading and Colonies morphology

The following cultural characteristics are observed on DTM with added Dermato Supplement after an incubation at 25-30°C for 6 days.

OrganismsIncubation (Aerobic)Result
Trichophyton mentagrophytes4-7 days at 15-30°CGrowth with white colonies and medium change pink-red
Candida albicans24 hours at 15-30°CGrowth with small white colonies with no color change in the medium
Escherichia coli24 hours at 35°CInhibited
Aspergillus brasiliensisUp to 7 days at 15-30°CInhibited
Staphylococcus aureus24 hours at 35°CInhibited
Pseudomonas aeruginosa24 hours at 35°CInhibited

Growth Pattern from Day 1 – Day 9


  • Isenberg (Eds.), 1992, Clinical Microbiology Procedures Handbook, Vol . 1, American Society for Microbiology, Washington, D.C.
  • Taplin, Zaias, Rebell and Blank, 1969, Arch. Dermatol., 99:203-209.
  • 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.
  • Kwon-Chung and Bennett, 1992, Medical Mycology, Lea & Febiger, Philadelphia, Pa.
  • Rosanthal S., Stritzler R. and Villafane J., 1968, Arch. Dermatol., 97:685.

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About the Author: Arthur Westmann

DEFFE ARTHUR (AMOEBAMANN) is the founder and author of MLTGEEKS and MLTEXPO.He’s from Cameroon and is currently a Final year State Medical Laboratory Technician (MLT MA). Beyond lab works, he’s a passionate internet user with a keen interest in web design and blogging. Furthermore He likes traveling, hanging around with friends and social networking to do in his spare time.


  1. Welldone for your nice job Arthur,
    I need your help about the conditions of incubation.
    I have inoculated nail scrapings on the DTM on the 25 th of July, and incubated on 37 degrees of celsius. The next day I realised a red colour change on the actual nail scrapings, however there is no progression on the growth until today 30th of July. Now I have incubated it on room temperature on moisture and dark place to see if there will be a progressio.
    What do you thing is the early colour change and there after no progression ton the growth.
    Thank you in advance

    1. Hello Panos
      Incubation at 37°c for dermatophyte will only yield growth basically from dimorphic fungi.
      I’ll advice you to follow the SOP and culture at the required temperature 25-30°c. The medium contains a pH indicator which changes red.
      Can you inoculate another sample and incubate according to the guide above?

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