MALDI - TOF Mass-Spectrometry for Fast and Reliable Identification of Clinical Yeast Isolates

The clinical impact of severe infections with yeasts and yeast-like fungi has increased especially in immunocompromised hosts. In recent years new antifungal agents have become available with different and partially species-specific activity patterns. Therefore, rapid and reliable species identification is essential for antifungal treatment, however, conventional biochemical methods are time consuming and require considerable expertise. We evaluated matrix-assisted laser desorption ionisation-time-of-flight mass spectrometry (MALDI-TOF MS) for the rapid routine identification of clinical yeast isolates. A total of 18 type collection strains and 267 recent clinical isolates of Candida (n=250), Cryptococcus, Saccharomyces, Trichosporon, Geotrichum, Pichia, and Blastoschizomyces spp. were identified employing MALDI-TOF MS. Results were compared with those of conventional phenotyping and biochemical tests including ID-32C (API bioMérieux, Nürtingen, Germany). Starting with cells from single colonies, accurate species identification by MALDI-TOF MS was achieved for 247 of the clinical isolates (92.5%). The remaining 20 isolates required complementation of the reference database with appropriate reference strains which were obtained from type culture collections or identified by 26S rDNA sequencing. The absence of a suitable reference strain from the MALDI-TOF MS database was clearly indicated by too low log score values for the respective clinical isolates, i.e. no false positive identification occurred. After complementation of the database, all isolates were unambiguously identified. The established API-ID-32C biochemical diagnostic system identified 244 isolates in a first round. Overall, MALDITOF MS proved a most rapid and reliable identification tool for yeasts and yeast-like fungi combining lowest expenditure of consumables and easy interpretion, and fast turnaround time.