Real-time PCR and DNA sequencing for detection and identification of Trichophyton rubrum as a cause of culture negative chronic granulomatous dermatophytosis.

A 31-year-old patient presented with a diagnosis of granulomatous dermatophytosis based on the clinical aspect of the lesions and the rare presence of hyphae on direct microscopic examination of clinical material. A chronic evolution and progression of the disease, its resistance to a wide range of antifungal agents, the occasional presence of hyphae on direct examination but consistently negative cultures over a 5-year period prompted the use of amplification-based DNA analyses of several successive swab samples or skin biopsies. DNA was extracted using a combination of two semi-automated DNA isolation methods (FastPrep preparation and NucliSENS lysis magnetic extraction method). Identification relied both on sequence analysis of amplicons after SYBR Green real-time PCR of the panfungal internal transcribed spacer 1 (ITS1) genetic target, as well as the unique amplicon melting curve profile of positive samples. Accordingly, Trichophyton rubrum was unambiguously identified in several clinical samples collected over a 7-month period. This case illustrates the contribution of DNA-based assays applied directly to sample biopsies for identifying causative agents in cases in which fungal pathogens are highly suspected but culture are repeatedly negative. It also pinpoints the benefit of combining semi-automated DNA preparation methods, analysis of ITS1 amplicon melting curve profiles and sequence analysis on repeated skin biopsy samples for unambiguous identification of the causative fungal species.