Exophiala dermatitidis

Exophiala dermatitidis is a thermophilic black yeast, and a member of the Herpotrichiellaceae. While the species is only found at low abundance in nature, metabolically active strains are commonly isolated in saunas, steam baths, and dish washers. Exophiala dermatitidis only rarely causes infection in humans, however cases have been reported around the world. In East Asia, the species has caused lethal brain infections in young and otherwise healthy individuals. The fungus has been known to cause cutaneous and subcutaneous phaeohyphomycosis, and as a lung colonist in people with cystic fibrosis in Europe. In 2002, an outbreak of systemic E. dermatitidis infection occurred in women who had received contaminated steroid injections at North Carolina hospitals. Exophiala dermatitidis forms slow growing, brown or black colonies. As is common amongst black yeasts, E. dermatitidis is an anamorphic fungus with multiple conidial forms. This morphological plasticity has complicated taxonomic determination based solely on physical appearance. Young colonies are described as waxy, mucoid, smooth, and yeastlike. Over time pigmented aerial hyphae develop, and older colonies are described as appearing filamentous and velvety. Conidia are between globular and elliptical in shape, and form at the mouth of short annellidic conidiogenous cells. Annellations can only be observed by electron microscopy. Prior to analysis by scanning electron microscope, the condiogenous cells were observed to form from non-annelated phialides and phialides without collarettes. These observations caused the species to be erroneously provided with its own monotypic genus, Wangiella. The species name Wangiella dermatitidis is still commonly used in the scientific literature. The black fungus also takes on diverse morphologies in vivo. Infected tissues contain mixtures of ovoid yeast-like cells, short septate hyphae that may be branched or unbranched, toruloid hyphae, as well as isotopically enlarged sclerotic (muriform-like) cells that resemble those found in chromoblastomycosis. The muriform-like cells isolated from E. dermatitidis infections have thinner walls than those found in chromoblastomycosis, and cells are divided along a single plane. A sexual form of E. dermatitidis has not been observed. However, the occurrence of two mating type idiomorphs in approximately equal numbers among clinical and environmental isolates suggests that E. dermatitidis reproduces sexually. Sequencing of rDNA has shown that clinical isolates of E. dermatitidis are very closely related to Capronia mansonii, and It has been predicted that the ascospores and ascomata of an E. dermatitidis teleomorph would therefore resemble those of C. mansonii. Ideal growth conditions for E. dermatitidis occur between 40 °C (104 °F) and 42 °C (108 °F), however E. dermatitidis can tolerate temperatures as high as 47 °C (117 °F). Metabolically active fungus is highly abundant in Turkish steam baths that routinely reach temperatures of over 60 °C (140 °F), but is typically not found in more hot and dry sauna facilities, or in cool environments surrounding steam baths. It is thought that extracellular polysaccharides may protect the species from stress in hot and moist environments, as this feature is typical of strains isolated from steam baths. Exophiala dermatitidis has carotenoid pigments in its cell wall that may serve a role in protecting E. dermatitidis from UV damage. Thermophilicity, negative potassium nitrate assimilation, negative melezitose assimilation, and an ability to decompose tyrosine are used to distinguish E. dermatitidis from other black yeasts. An exoantigen test is useful in species determination, and DNA analysis can also be performed. The fungus has been selectively isolated using high temperature incubation (at 40 °C) on media containing cycloheximide. Exophiala dermatitidis has been isolated around the world in low abundance from a variety of environmental sources, including soil, decaying timber, and wasp nests. The thermophilicity and acid tolerance of E. dermatitidis suggests passage through warm-blooded animals, and it is hypothesized that its ecological niche might be associated with tropical, frugivorous bird and bat species. An ability to utilize nutrients in diverse environments, to adhere to fruit surfaces, and progress through different morphological phases are considered to provide further evidence for this theory. Clinical isolates tend to harbor strains that are found only rarely in nature. It may be the case that traits linked to halotolerance have predisposed these strains towards infecting humans. While E. dermatitidis has been found only in low abundance in nature, the species is well suited to survive in a number of warm and wet man made niches. Metabolically active strains are isolated in high abundance from surfaces inside saunas, steam baths and humidifiers. E. dermatitidis is one of the most common fungal species to inhabit dishwashers, and has been found in dishwashers around the world.