Discrimination of Fungi by MALDI-TOF Mass Spectrometry

Traditionally, fungal identification has largely been based on the subjective micro- and macroscopic examination of morphological and culture characteristics. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF MS) was used to generate reproducible mass spectral "fingerprints" for 76 fungal species, particularly from the medically important genera, Penicillium and Aspergillus. The mass spectra contain abundant mass signals and allow unambiguous discrimination between species. Species identification error rates were determined to be 0% and 1.4% using resubstitution and cross-validation methods, respectively. The ability of MALDI TOP MS to differentiate fungal strains was additionally examined for the aflatoxin producing species, Aspergillus flavus. Identification error rates for 40 tested A. flavus cultures from five unique strains were determined to be 0% and 5% using resubstitution and cross-validation methods, respectively. Analysis of dematiaceous (dark-pigmented) fungi has been observed to yield poor MALDI-TOF mass spectra. Results demonstrate this was due to the presence of melanin in the cell wall of fungal spores and hyphae. Strategies to overcome this limitation are presented. These results indicate that MALDI-TOF MS data may be a useful diagnostic tool and alternative to available immunodiagnostic and molecular methods for the objective identification of environmental, industrial and clinically important fungal species.