Culture media for growing dermatophytes

Abstract

In addition to being keratinophilic, dermatophytes are characterized by the following: (1) production and tolerance of an alkaline pH in culture media, (2) resistance to 500 p.g/ml or less of cycloheximide, and (3) sensitivity to 20 p.gjml or less of griseofulvin. These properties are useful to devise special culture media for their selective isolation and recognition. Most media for general diagnostic mycology are available commercially. The standard medium for growing dermatophytes is Sabouraud's dextrose agar containing chloramphenicol and cycloheximide, which inhibit bacteria and saprophytic fungi, respectively. Four-percent glucose is added to Sabouraud's dextrose agar. Dermatophytes do not necessarily grow best on this agar, but for consistency, colonial morphology is determined on this medium. The addition of cycloheximide inhibits saprophytic fungi such as Scytalidium and Hendersonula (Natrassia), Scopulariopsis brevicaulis, Candida species, and Cryptococcus neoformans. Therefore a second medium without this antifungal agent should be included to avoid elimination of some relevant fungi, especially when nail specimens are being cultured. Media containing cycloheximide and chloramphenicol are commercially known as Mycobiotic or Mycosel agar (BBL, Baltimore, Md.). For poorly sporulating dermatophytes (for example, Trichophyton rubrum), a I: 10 dilution of 2% Sabouraud's dextrose agar is used to stimulate the pro-