Myriodontium keratinophilum is a fungus widespread in nature, most abundantly found in keratin-rich environments such as feathers, nails and hair.^[1] Despite its ability to colonize keratinous surfaces of human body, the species has been known to be non-pathogenic in man^[2] and is phylogentically distant to other human pathogenic species,^[1] such as anthropophilic dermatophytes (e.g., Trichophyton rubrum, Trichophyton interdigitale). However, its occasional isolation from clinical specimens along with its keratinolytic properties suggest the possibility it may contribute to disease.

Myriodontium keratinophilum was first isolated in Italy in 1978, during the screening of soil microbes for their ability to produce antibiotic and antiviral substances.^[3] The fungus was described by Luciano Polonelli and Robert A. Samson, and was classified into a separate taxon for its ability to produce multiple asexual spores (conidia) at different points across the surface of the hypha.^[3] Later in 1997, its asexual state (teleomorph) was independently described and named as Neoarachnotheca keratinophilia by Josef Cano and Krzysztof Ulfig.^[1][4] M. keratinophilum is the only member of the genus Myriodontium.^[2]

The vegetative and fertile hyphae are branched, hyaline, septate, smooth-walled, and ranges from 2.5 µm to 6 µm in width.^[2][3][4] The sexual fruiting structure (gymnothecium) consists of loosely interwoven hyphae that are hyaline to yellow in colour,^[2][4] and typically matures within a month, attaining a diameter of 150-700 µm.^[4] The gymnothecium encloses millions of round, hyaline asci that each bear 8 ascospores.^[2] The defining feature of M. keratinophilum is that its ascospores are randomly wrinkled and pitted with an irregular sheath.^[2] The (conidia) are one-celled and emerge at multiple different points across the surface of fertile hyphae.^[3][4] Species of the genera Chromelosporium and Pulchromyces morphologically resemble Myriodontium.^[3]

Myriodontium keratinophilum exhibits keratinolytic properties,^[4][5][6][7] and it is able to grow on and degrade keratinous surfaces such as feathers.^[5][6] The fungus shows moderate to rapid growth on YpSs agar,^[3][4] attaining a diameter of 6 cm in 14 days at 25 °C (77 °F); the colonies are white with a felt base obscured by floccose mycelium.^[3] On hay-infusion agar, the growth rate is slightly reduced with a diameter of 5 cm in 14 days at 25 °C (77 °F).^[3] Similar growth rate is exhibited on oatmeal agar.^[3] On phytone extract agar at 25 °C (77 °F), its colony shows faster growth relative to other medium, spreading up to 2.7 mm per day.^[4] On potato dextrose agar at 25 °C (77 °F), it exhibits a mean daily spread of 1.8-2.3 mm.^[4] On Sabouraud agar at 25 °C (77 °F), the mean daily spread is slightly reduced, ranging from 1.4-2.0 mm.^[4] It shows no growth at 37 °C (99 °F).^[4]

Myriodontium keratinophilum is commonly found in soils^[1][8] and river sediments.^[4][9] M. keratinophilum has been isolated from many different keratinous materials, such as the penis of a bull,^[1] hair of shrews and cats,^[1] and feathers of pigeons.^[6] The fungus also inhabits human hairs as an ectothrix that do not penetrate the cortex.^[3]