Exophiala phaeomuriformis
Scientific classification
Domain:	Eukaryota
Kingdom:	Fungi
Division:	Ascomycota
Class:	Eurotiomycetes
Order:	Chaetothyriales
Family:	Herpotrichiellaceae
Genus:	Exophiala
Species:	E. phaeomuriformis
Binomial name
Exophiala phaeomuriformis Matos (2003)
Synonyms
Sarcinomyces phaeomuriformis T. Matsumoto (1986)

Exophiala phaeomuriformis is thermophilic fungus belonging to the genus Exophiala and the family Herpotrichiellaceae. ^{[1] [2]} it is a member of the group of fungi known as black yeasts, and is typically found in hot and humid locations, such as saunas, bathrooms, and dishwashers. ^[3] This species can cause skin infections ^[4] and is typically classified as a Biosafety Risk Group 2 agent. ^[5]

Exophiala phaeomuriformis is a member of the genus Exophiala, described in 1952 based on E. jeanselmei. ^[1] Thirty species of Exophiala are currently recognized ^[6] amongst which Exophiala (Wangiella) dermatitidis is the most common. ^[7] When studying samples of E. dermatitidis, Tadahiko Matsumoto and colleagues observed strains with a granular colonial form and distinctive microscopic morphology. ^[8] Based on the resemblance of these strains to the genus Sarcinomyces, they proposed the new name, S. phaeomuriformis. ^[8] This taxon was transferred to the genus Exophiala by Tiago Matos and co-workers in 2003 because of its yeast-like morphology (rather than the meristematic form characteristic to members of the genus Sarcinomyces), and its closer DNA homology to the genus Exophiala. ^[9]

Exophiala phaeomuriformis is a dematiceous (darkly pigmented) fungus and member of the group of fungi known as the black yeasts. ^{[10] [11]} Black yeasts are an unrelated category of fungi that share yeast-like morphology and possess darkly melanized cell walls. ^[5] Although their DNA sequences are distinctive, E. phaeomuriformis and E. dermatitidis are so closely related that the two cannot be reliably differentiated morphologically or physiologically. ^{[5] [12]} Antigenic cross-reactivity suggests that E. phaeomuriformis may have originated as multicellular variant of E. dematitidis. ^[10]

Like many other black yeasts, Exophiala phaeomuriformis is known only by its asexual form and no sexual form has been found. ^{[4] [5] [8]} It is a thermophilic fungus preferring temperatures between 37–42 °C (99–108 °F) ^[2] but growing at any temperature between 15–42 °C (59–108 °F). ^[3] Exophilala phaeomuriformis is more sensitive than other black yeasts to salt, incapable of growth at concentrations of sodium chloride exceeding 17%. ^[3] Like other members of the genus Exophiala, it is able to tolerate a wide range of pH (2.5–12.5). ^[3]

Colonies of E. phaeomuriformis are hyaline, mycoid, and smooth when young ^[9] but become black, dry, crumbly, raised, and mulberry-like in texture with age. ^{[4] [8]} Some strains fail to undergo this morphological switch and remain yeast-like in age. ^{[5] [8]} By contrast, many strains of E. dermatitidis become mycelial with age. ^[8] Hyphal growth has not been observed in E. phaeomuriformis. ^[8] Instead, colonies develop from loosely packed, single, and rounded budding yeast cells that are either scattered or aggregated in groups. ^{[5] [8]} Vegetative cells can either by unicellular or muriform (septate in all planes) or become divided by transverse septa only. ^{[4] [8]} Yeast cells are thick-walled and spherical or near-spherical in shape. ^{[4] [5]} Budding cells can have broad bases, occur in chains, and be multilateral, budding in different directions. ^[9]

Like other member of the genus Exophiala, E. phaeomuriformis is saprotrophic, obtaining its energy exclusively from non-living organic materials. ^[6] When inoculated on a suitable growth medium under optimal conditions, the growth of E. phaeomuriformis is initiated in roughly 3 days; ^[3] however, when subject to competition, the cells may remain in a stationary state for many weeks prior to the development of visible growth. ^[3] Similar to E. dematitidis, E. phaeomuriformis is unable to assimilate nitrate, nitrite and melibiose; however it differs in it that some strains are unable to metabolize D-gluconate, D-glucuronate, D-galacturonate and glucono-δ-lactone. ^[10]

Exophiala phaeomuriformis has a proclivity for environments rich in mono- and polyaromatic compounds, such as hydrocarbons, where it uses these compounds as sources of energy. ^[3] The species is plurivorous, occurring on a wide range of materials from contaminated soils and toluene rich environments to wild berries and animal feces. ^[3] It is also found in environments containing the preservative creosote, such as railroad ties where it is an important agent of biodeterioration. ^{[3] [13]} In indoor environments, E. phaeomuriformis occurs in warm and moist environments such as toilets, saunas, or dishwashers. ^[2] This species is found world-wide. ^[14]

Exophiala phaeomuriformis is a rare causative agent of phaeohyphomycosis ^[15] in cutaneous, subcutaneous and deep tissues, ^[4] and is responsible for 6.4% of infections caused by black yeasts. ^[7] Infection usually occurs following skin abrasion or penetrating injuries. ^[11] Exophiala phaeomuriformis can also cause corneal infection following eye exposure to contaminated water. ^[1] People with cystic fibrosis (CF) are considered abnormally susceptible to Exophiala infections, including E. phaeomuriformis. ^{[14] [16]} It has been suggested that differences in the microbiota profiles of CF patients may be responsible for this predisposition. ^[17] Treatment of E. phaeomuriformis involves a combination of surgical debridement and antifungal therapy. ^[15] A range of antifungal agents including caspofungin, voriconazole, itraconazole, posaconazole, and amphotericin B are active against this species. ^{[16] [18]} Due to its pathogenic potential, E. phaeomuriformis is regarded as a Biosafety Risk Group 2 agent in the laboratory. ^[5]