Exophiala phaeomuriformis

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, Takashi 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]

• Exophiala phaeomuriformis in MycoBank.
• Exophiala phaeomuriformis in Index Fungorum