Dermatophyte Agar

Description

Dermatophyte is a medical term used to designate a particular group of fungi that infect the skin, hair and nails of humans and animals, causing various skin infections commonly known as "tineas". Any filamentous fungus isolated from culture samples of skin, hair and nails must be evaluated to determine the presence of dermatophytes. The dermatophytes are divided into three genera, Microsporum, Trichophyton and Epidermophyton, on the basis of their microscopic morphological differences and modes of sporulation. Species of these genera can be divided into three types by their normal habitat: anthropophilic, which are found only in human hosts and are transmitted from person to person; zoophilic, which are found in animals but can be transmitted to humans; and geophilic, which can be found in the ground and may infect humans and animals.

The growth of most dermatophytes results in the production of alkaline metabolites that cause the indicator to change from yellow to red, but there are non-pathogenic fungi (non-dermatophytes) that cause a colour change. Also there are some strains of Microsporum that grow without altering the appearance of the medium. These other organisms that manage to grow in this medium can be recognized as non-dermatophytes both by their colour and colony morphology. The bacteria and few yeasts that may develop typically produce creamy white colonies. Saprophytic contaminants that sometimes cause colour change in the medium can be disregarded if they produce blackish-green coloured hyphae, since dermatophytes always produce white aerial hyphae.

However, as the final identification of dermatophyte is the microscopic observation of sporangia and verification of colour on the reverse of the colony, it is recommended that along with the Dermatophyte Selective Agar another medium for fungi, e.g. Sabouraud Agar (with or without inhibitors), is inoculated simultaneously in order to verify these characteristics. Thus, DTM is used as an isolation and presumptive identification medium and Sabouraud as an isolation and confirmation medium.

Technique

Skin samples or hair and nail samples are inoculated directly on the surface of agar and incubated at room temperature (25-30 °C) for up to two weeks. Examine daily, noting any possible change of colour in the medium. Most dermatophyte pathogens cause a colour change between the third and sixth day.

The appearance of white aerial hyphae and red coloured medium around the fungal growth should be interpreted as presumptive presence of pathogenic dermatophytes.

If there is growth but no colour change to red, the organism is probably not a dermatophyte but identification must be verified.

If growth appears only in the Sabouraud agar but not in the DTM, it is not a dermatophyte.

If the colonies have green or black hyphae they are not dermatophytes, although they may cause the DTM to turn red.

Physical/chemical control

Microbiological control

Isolation by loop spreading.

Analytical methodology according to ISO 11133:2014/Amd 1:2018; A2:2020.

Aerobiosis. Incubation at 25-30 °C. Reading at 3-5 days.

Sterility control

Incubation 48 h at 30-35 °C and 48 h at 20-25 °C: NO GROWTH.

Check at 7 days after incubation in same conditions.

• ALLEN, A.M., R.A. DREWRY & R.E. WEAWER (1970) Evaluation of two new color indicator media for diagnosis of dermatophytosis. Arch. Derm. 102:68-70.
• FORBES, B.A., D.F. SAHM & A.S. WEISSFELD (1998) Bailey & Scott's Diagnostic Microbiology 10th edition. Mosby, Inc. St Louis. Missouri.
• LARONE, D.H. (2002) Medically Important Fungi: A guide to Identification. 4th edition. ASM Press, Washington D.C.
• MERZ, W.G., C.L. BERGER & M. SILVA-HUTNER (1970) Media with pH-indicator for the isolation of dermatophytes. Arch. Derm. 102:545-547.
• REBELL, E. & D. TAPLIN (1970) Dermatophytes: their recognition and identification. 2nd edition. University of Miami Press. Coral Gables. Fla. USA.
• SALKIN, I.F. (1973) Dermatophyte Test Medium: Evaluation with Nondermatophytic Pathogens. Appl. Microbiol 26(2):134-137.
• SUMMERBELL, R.C. (2003) Trichophyton, Microsporum, Epidermophyton and Agents of Superficial Mycoses, in Manual of Clinical Microbiology 8th edition Murray-Baron-Jorgensen-Pfaller-Yolken (Eds). ASM Press. Washington D.C.
• TAPLIN, D., N. ZAIAS, G. REBELL & H. BLANK (1969) Isolation and recognition of dermatophytes on a new medium (DTM). Arch. Derm. 99:203-209.
• TORRES-RODRIGUEZ, J.M. (1982) Dermatofitos y Dermatofitosis. Curso Intensivo de la AEEM. Labs Dr. Esteve SA. Manresa.
• WEITZMAN, I. & R.C. SUMMERBELL (1995) The Dermatophytes. Clin. Microbiol. Rev. 8(2):240-259.
• WINN, W.C. (Jr), S.D. ALLEN, W.M. JANDA, E.W. KONEMAN, G.W. PROCOP, P.C. SCHRECKENBERGER & G.L. WOODS (2006) Koneman's Color Atlas and Textbook of Diagnostic Microbiology. Lippincott, William & Wilkins. Philadelphia. USA.