Exophiala sp.
Colonies are slow growing, dark olivaceous-brown to black, often slimy when young, but later becoming suede-like or downy. Vegetative hyphae are pale brown, often forming long chains of swollen cells (torulose hyphae). Conidiophores mostly little differentiated; conidium formation taking place on terminal or lateral conidiogenous pegs (annellides). Conidia are subhyaline, mostly one-celled, elongate and form in a ball at the tip of each annellide.
Culture of Exophiala jeanselmei.
Despite various taxonomic treatments, the black "yeast-like" fungi belonging to Exophiala and related genera are still not clearly defined, and therefore remain difficult to identify. Many authors hold different opinions on the best classification of these fungi, and discussions on isolated taxonomic decisions are continuing. The main source of dispute is the tremendous phenotypic plasticity of these fungi. The individual strains are highly variable, and each strain shows a pattern of variation slightly different from any other strain. It is therefore essential to examine isolates on media like potato dextrose agar or cornmeal agar to promote the formation of hyphal elements, and to use slide culture preparations.
Although microscopic features are principally used to differentiate Exophiala jeanselmei, Hortaea werneckii and Exophiala dermatitidis, growth at different temperatures (2 weeks on PDA at 25, 37, and 40C) and hydrolysis of casein and tyrosine have proved to be useful confirmatory tests. However, it must be stressed that no one test is infallible and some strains will show variation.
| Species | Growth 2wks at 25C |
Growth 2wks at 37C |
Growth 2wks at 40C |
Hydrolysis
of Casein |
Hydrolysis
of Tyrosine |
|---|---|---|---|---|---|
| Exophiala jeanselmei |
Positive | Positive | Negative | Negative | Positive |
| Hortaea werneckii |
Positive | Positive | Positive | Positive | Negative |
| Exophiala dermatitidis |
Positive | Positive | Positive | Negative | Positive |
Clinical significance:
Exophiala species are common environmental fungi often associated with decaying wood and soil enriched with organic wastes. However, several species notably E. jeanselmei, E. moniliae and E. spinifera, are well documented human pathogens. Clinical manifestations include mycetoma (especially for E. jeanselmei), localized cutaneous infections, subcutaneous cysts, endocarditis and cerebral and disseminated infections. Phaeohyphomycosis caused by Exophiala species has been reported in both normal and immunosuppressed patients.
Mycosis: Phaeohyphomycosis
Further reading:
De Hoog, G.S., and E.J. Hermanides-Nijhof. 1977. The black yeasts and allied hyphomycetes. Studies in Mycology No. 15. Centraalbureau voor Schimmelcultures, Baarn, The Netherlands.
Kwon-Chung, K.J. and J.E. Bennett. 1992. Medical Mycology. Lea & Febiger, Philadelphia and London.
