BACKGROUND:Onychomycosis is estimated at approximately 10% of the global population with most cases caused by Trichophyton rubrum (T. rubrum). Some of the persistent cases of onychomycosis are caused by mixed infections of T. rubrum and one or more co-infecting nondermtophyte mould (NDM). T. rubrum strain types in onychomycosis may naturally switch and may also be triggered to switch as a response to antifungal therapy. T. rubrum strain types in mixed infections of onychomycosis have not been characterized. METHODS:T. rubrum DNA strains in mixed infections of onychomycosis containing co-infecting NDM(s) were compared to a baseline North American population through polymerase chain reaction amplification of ribosomal DNA tandemly repetitive subelement one and two (TRS1 and TRS2). The baseline strain types consisted of 102 clinical isolates of T. rubrum from onychomycosis patients and the patients with mixed infections of T. rubrum and an NDM(s) consisted of 63 repeat toenail samples from 15 onychomycosis patients with mixed infections of T. rubrum and an NDM(s). RESULTS:Two unique TRS2-types among the clinical isolates contributed to four unique TRS1;TRS2 strain types. Six TRS1;TRS2 strain types represented 92% of the clinical isolates of T. rubrum. Four TRS1;TRS2 strain types accounted for 100% of the T. rubrum within mixed infections.CONCLUSIONS:Four unique North American T. rubrum strains were identified. In support of a shared ancestry, the T. rubrum DNA strain types found in mixed infections with NDMs were among the most abundant types. A population of T. rubrum strains in mixed infections of onychomycosis has been characterized with more than one strain detected within some nails. The presence of a co-infecting NDM in mixed infections may contribute to failed therapy by stabilizing the T. rubrum strain type, possibly preventing antifungal therapy-induced strain type switching observed with infections caused by T. rubrum alone.

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