dermatophyte test medium dermatophyte test medium dermatophyte test medium (dtm) is a specialized agar used in medical mycology. it is based on sabouraud's dextrose agar with added cycloheximide to inhibit saprotrophic growth, antibiotic to inhibit bacterial growth, and phenol red a ph indicator. the ph indicator is useful in distinguishing a dermatophyte fungus, which utilizes nitrogenous material for preferred metabolism, producing alkaline by-products, imparting a red color change to the medium. typical saprotrophic fungi utilize carbohydrates in the medium producing acidic by-products and no red color change. dtm plate on the right showing color change after two days' growth; microsporum canis