Rapid in-situ hybridization for dematiaceous fungi using a broad-spectrum oligonucleotide DNA probe

Dematiaceous fungi are a diverse group of "darkly" pigmented fungi, which contain melanin in their cell walls and are commonly found in soil worldwide. Although morphology and histochemical stains may aid identification in tissue sections, these means for species identification are not specific. In-situ hybridization (ISH) for abundant fungal rRNA sequences may provide a means for detecting dematiaceous fungi. In this study, a 24-base synthetic biotin-labeled oligonucleotide probe targeting rRNA sequences of a variety of dematiaceous fungi was developed. This probe was tested on a cohort of 29 patients with culture-proven cases of dematiaceous fungal-associated rhinosinusitis (26 allergic fungal sinusitis, 2 fungal ball, and 1 acute invasive fungal sinusitis). Fungal cultures were positive for Alternaria species (10), Bipolaris species (5), Curvularia species (10), Cladosporium species (1), Scedosporium prolificans (1), Scopulariopsis species (1), and dematiaceous species, not otherwise specific (1). ISH showed positivity in fungal organisms in 24 of 29 specimens. ISH was negative in culture-proven examples of Rhizopus species, Aspergillus species, Fusarium species, Paecilomyces species, Histoplasmosis capsulatum, Candida species, and Blastomyces dermatitidis. ISH with a dematiaceous-specific fungal probe may be useful for differentiating dematiaceous fungi from other filamentous fungi in tissues, particularly those responsible for fungal rhinosinusitis.