An undescribed Microsporum species was isolated from skin scales recovered from a 40-mm large, annular, scaling lesion on the wrist of a 46-year-old woman. The risk factors for dermatophyte infection in the patient were frequent work in the garden, hunting, and contact with dogs and horses. Direct microscopic examination of the scales revealed the presence of dermatophyte hyphae; when the samples were cultured, a morphologically similar fungus grew on all slants in pure culture. Both of these findings strongly suggested that the isolate was the true causal agent of infection. The possible geophilic nature of the species was based on phylogenetic analysis (internal transcribed spacer region of rDNA and β-tubulin gene) that placed it in between species of the M. gypseum complex. However, its divergencies from all other Microsporum species exceeded 4% base pairs. Based on β-tubulin phylogeny, the isolated species is a sister to M. gypseum. The species produces abundant chlamydospores and clumps of hyphae similar to those of ascomatal primordia but no conidia and ascospores. The species was unable to grow at 37°C and does not grow on T6 basal medium, which is unlike other Microsporum species; hair perforation and urease tests were positive. The addition of histidine to the T6 medium resulted in rapid growth of the fungus. The phylogenetic evidence, morphology, growth parameters, and physiology justified the proposal that the isolate is a new species, M. aenigmaticum, sp. nov.
Keywords: Arthroderma; Onygenales; geophilic dermatophytes; keratinophilic fungi; skin infection; sterile fungi.