Dermatophyte Test Medium Slant

Principle And Interpretation

The Dermatophytes are a distinct group of fungi that infect the hair, skin and nails of humans and animals producing a variety of cutaneous infections known as ringworm (1). Dermatophytes like Trichophyton, Microsporum and Epidermatophyton are responsible for most of the cutaneous fungal infections (2). On this medium identification of Dermatophytes are based on morphology and alkaline metabolites production. A combination of three antimicrobial agents (cycloheximide, chlortetracycline and gentamicin) inhibits bacteria and saprophytic yeasts and moulds. Dermatophytes are presumptively identified based on gross morphology and the production of alkaline metabolites, which raise the pH and cause the phenol red indicator to change the color of the medium from yellow to pink-red (2-4). Soya peptone provides nitrogenous and carbonaceous substances essential for growth. Glucose is the energy source. The pH indicator, phenol red, is used to detect amine production. Cycloheximide (as FD) inhibits most of the saprophytic fungi. Gentamic in inhibits gram-negative bacteria including Pseudomonas species while 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 helps in 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 effect a complete colour change within 3 days while slow growers will change colour in proportionately longer time.

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.