Discovery of a sexual stage in Trichophyton onychocola, a presumed geophilic dermatophyte isolated from toenails of patients with a history of T. rubrum onychomycosis

Trichophyton onychocola is a recently described geophilic dermatophyte that has been isolated from a toenail of Czech patient with a history of onychomycosis due to T. rubrum and clinical suspicion of relapse. In this study, we report a similar case from Denmark in an otherwise healthy 56-year-old man. The patient had a history of great toenail infection caused by T. rubrum in 2004 and presented with suspected relapse in 2011 and 2013. Trichophyton onychocola was the only microbial agent isolated at the second visit in 2013 and the identification was confirmed by DNA sequencing. Direct microscopic nail examination was positive for hyphae, however the etiological significance of T. onychocola was not supported by repeated isolation of the fungus. This new species may be an overlooked geophilic species due to the resemblance to some common species, for example, zoophilic T. interdigitale or some species of geophilic dermatophytes. We included differential diagnosis with phenotypically similar species; however, it is recommended that molecular methods are used for correct identification. The MAT locus of Danish strain was of opposite mating type than in the previously isolated Czech strain and the two isolates were successfully mated. The mating experiments with related heterothallic species T. thuringiense and Arthroderma melis were negative. The sexual state showed all typical signs of arthroderma-morph and is described by using optical as well as scanning electron microscopy. The sexual state was induced on a set of agar media, however low cultivation temperature and the presence of keratin source were crucial for the success rather than formulation of medium.