Molecular identification of pathogenic fungi

Abstract

Systemic fungal infections represent a major cause of morbidity and mortality in immunocompromised patients. The ever-increasing number of yeast species associated with human infections that are not covered by conventional identification kits, and the fact that moulds isolated from deep infections are frequently impossible to identify using classical methods due to lack of sporulation, has driven the need for rapid, robust molecular identification techniques. We recently developed a rapid method of preparing fungal genomic DNAs using Whatman FTA filters, which has greatly facilitated molecular identification. Mould isolates cultured from dark grain mycetomas (destructive infections of skin/subcutaneous tissues that progress to involve muscle and bone) invariably fail to produce features by which they can be identified and were taxonomic mysteries. PCR amplification and sequencing of 250 bp of the internal transcribed spacer region 1 (ITS1) allowed us to distinguish between the known agents of mycetoma, to describe three new species associated with this disease and to define phylogenetic relationships. For yeasts, 153 isolates encompassing 47 species that had failed to be identified using classical methods were unambiguously identified by conventional sequencing of 350 bp of the 26S rRNA D1D2 region. These represented 5% of the isolates examined and included common species with atypical biochemical and phenotypic profiles, and rarer species infrequently associated with infection. Our recent studies indicate that FTA extraction coupled with pyrosequencing of 25 bp of ITS2 could potentially identify most common yeast species from pure culture in half a day. Together, these data underscore the importance of molecular techniques for fungal identification.