A matrix-assisted laser desorption/ionization time of flight mass spectrometry reference database for the identification of Histoplasma capsulatum

Abstract

The isolation of the pathogenic fungus Histoplasma capsulatum from cultures together with the visualization of typical intracellular yeast in tissues are the gold standard methods for diagnosis of histoplasmosis. However, cultures are time-consuming, require level 3 containment and experienced personnel, and usually call for an additional confirmation test. Matrix-Assisted Laser Desorption Ionization Time of Flight Mass Spectrometry (MALDI-ToF MS) has been established as a suitable tool for microbial identification in several clinical laboratories. A reference database has been constructed for the identification of H. capsulatum by MALDI-ToF MS by using six H. capsulatum strains previously identified by molecular methods. For validation, 63 fungal strains belonging to the Collection of the Spanish National Centre for Microbiology were tested against the new reference database combined with other commercial and in-house databases. In a blind assay, all H. capsulatum strains (n = 30) were correctly identified by the database and 86.6% had scores above 1.7. Considering both phases of the fungus for the same strain, the most reliable results were obtained with the mycelial phase, with only 13.3% of isolates having scores below 1.7. The new database was able to identify both morphological phases of the fungus. MALDI-ToF technology yields a prompt and simple identification from H. capsulatum yeast forms and early mycelial cultures. It allows for reducing response time and decreasing risk in fungus manipulation.