The medically important dematiaceous fungi and their identification

Abstract

Dematiaceous fungi include a large group of organisms that are darkly pigmented (dark brown, olivaceous, or black). In most cases the pigment is melanin, and specifically, dihydroxynaphthalene melanin. The diseases produced include chromoblastomycosis, eumycotic mycetoma, and phaeohyphomycosis. Phaeohyphomycosis is a new classification for a diverse group of previously known entities grouped together on the basis of finding dematiaceous hyphal and/or yeast-like forms in tissue; tissue involvement may be superficial, cutaneous and corneal, subcutaneous, or systemic. Identification of these fungi is based mostly upon morphology. Important structures include annellides (Phaeoannellomyces, Exophiala), phialides (Phialophora, Wangiella), adelophialides (Phialemonium without collarettes, Lecythophora with collarettes), differentiation of conidiophores (Xylohypha versus Cladosporium) and conidial hilum, septation and germination (Bipolaris, Drechslera, Exserohilum). Useful laboratory tests include the 12% gelatin test (controversial), nitrate assimilation (W. dermatitidis is negative, most other species are positive), and determination of temperature maxima (especially 37 degrees C for E. jeanselmei, 40 degrees C for W. dermatitidis and B. spicifera, 42 degrees C for X. bantiana, and 45 degrees C for Dactylaria constricta var. gallopava and Scedosporium inflatum).