Differentiating Dimorphic Fungi from Saprophytic Molds

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The page below is a sample from the LabCE course Mycology: Yeasts and Dimorphic Pathogens. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Differentiating Dimorphic Fungi from Saprophytic Molds

Several dimorphic fungi have saprophytic counterparts with similar, look-alike microscopic features. These are shown in the table below:

Dimorphic FungusSaprophytic Mold CounterpartCommon Microscopic Morphologic Features
Blastomyces dermatitidisChrysosporium species
Scedosporium species also produces single, oval microconidia similar in appearance to those of B. dermatitidis. However, the conidia of Scedosporium species are slightly larger and have a dark pigmentation.
Production of small, smooth, oval microconidia, each supported by a delicate conidiophore ("lollipop").
Histoplasma capsulatiumSepedonium speciesProduction of large, spherical macroconidia, which, when mature may have prickly surfaces.
Coccidioides immitisMalbranchia speciesProduction of alternate staining arthroconidia.

These methods can be used to differentiate the dimorphic fungi from their saprophytic counterparts:
  • The dimorphic fungus can be converted to a yeast form by incubation at body temperature and the saprophytic mold cannot.
  • Dimorphic fungi will grow in culture media containing cycloheximide and the saprophytic molds will not.
  • Dimorphic fungi will show positive reactions in antigen-specific nucleic acid probe assays.