Exophiala Mold SpeciesExophiala species are causative agents of Mycetoma, Phaeohyphomycosis and Tinea nigra. (Information from www.doctorfungus.org @ 2005)
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Taxonomic Classifications
Kingdom:
Fungi
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Ecology
Exophiala is a cosmopolitan, dematiaceous fungus isolated from soil, decaying wood material, plants, and surfaces in contact with cool, fresh water. In addition to being a saprophyte in nature, it is the causative agent of various human infections particularly involving the feet and nails.
Species
The genus
Exophiala contains numerous species. The most common ones are
Exophiala castellanii, Exophiala jeanselmei (which currently
has two varieties namely: Exophiala jeanselmei var. heteromorpha
and Exophiala jeanselmei var. lecanii-corni), Exophiala
moniliae, Exophiala pisciphila, Exophiala salmonis,
Exophiala spinifera and
Exophiala werneckii.
Pathogenicity and Health Effects
Exophiala species are among the fungi - causing infections referred to as phaeohyphomycosis. Exophiala isolates may cause subcutaneous infections such as mycetoma and chromoblastomycosis. These infections are usually obtained via traumatic implantation and are associated with the existence of local or systemic immunosuppression, such as organ transplantation. Cases such as infection and abscess formation in subcutaneous tissues, prosthetic valvular vegetations, fungemia, and disseminated infections due to Exophiala species have also been reported. Aside from humans, fish are also infected by the neurotropic Exophiala pisciphila.
Macroscopic Appearance
Ø Growth rate is slow and the texture is mucoid becoming velvety due to short, grayish aerial hyphae formation; and Ø Colony surface color is dark brown to olivaceous black while black on the reverse.
Microscopic Appearance
Ø Yeast cells are present at the beginning of colony formation and are either unicellular or bicellular and often appear in long chains; Ø Pale brown, septate hyphae are eventually formed as the culture matures and which bear conidiogenous cells also referred to as the annellides; Ø The annellides are cylindrical or slightly inflated, with brown pigmentation and which typically tapers to form a narrow elongated tip, the entire structure are often slightly differentiated from the vegetative hyphae; and Ø Conidia are hyaline or pale brown, ellipsoidal in shape, size ranging from 3 x 3 to 6 µm, may either be unicellular or bicellular, and accumulate in clusters at the tip of the annellides or at the sides of the conidiophore.
Laboratory Precautions
Only general laboratory precautions are required, no special safety measures needed.
Susceptibility There are a limited data available on a limited number of isolates. Fluconazole yielded very high MICs for Exophiala jeanselmei while flucytosine and miconazole appeared lower than fluconazole but were still relatively high. MICs of amphotericin B, ketoconazole, and voriconazole were similar and relatively low. Itraconazole and terbinafine yielded the lowest MICs. Exophiala jeanselmei showed decreased susceptibility to both amphotericin B deoxycholate and ABLC in other workers’ hands. On the other hand, the echocandin, caspofungin yielded favorable in vitro activity against Exophiala jeanselmei isolates. Furthermore, MICs of amphotericin B, itraconazole, and voriconazole for Exophiala spiniphera were similar and encouragingly low. The correlation of in vitro susceptibility with clinical outcome for Exophiala is not yet known. Combination of surgical and anti – fungal treatment is usually done for subcutaneous Exophiala infections as there is no standard treatment yet. Amphotericin B with or without flucytosine, itraconazole, and terbinafine have been used as well. The combination of amphotericin B, flucytosine and itraconazole appeared effective in a case with subcutaneous infection due to Exophiala jeanselmei.
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