Current Performance of MALDI-TOF Mass Spectrometry Databases for the Identification of Dermatophyte Species

Abstract

The identification of filamentous fungi by matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS) represents a challenge due to their complex taxonomy and the lack of comprehensive databases. The aim of this study was to evaluate the current status of available MALDI-TOF MS databases for the identification of dermatophytes, including commercial, in-house, and web-based databases. We collected 289 dermatophyte strains from different centers and analyzed them using four databases and a combination of them. The combination of commercial and in-house databases was shown to improve the identification rate and accuracy at the species level. For Trichophyton rubrum, the concordance among all databases was above 90.0%. For the T. mentagrophytes group, correct identification at the species level ranged from 30.0 to 78.9%, depending on the database, and showed very low agreement among them. The addition of the novel species T. japonicum to our in-house database resulted in the successful identification of this species. On the other hand, T. interdigitale and T. tonsurans were the species most frequently misidentified by MALDI-TOF MS. Through deep spectra analysis of both species, up to 29 protein peaks were found to be suitable for their differentiation, demonstrating the potential of peak analysis in differentiating closely related species. In conclusion, improvements of the databases with new strains resulted in increased identification accuracy at the species level. This, combined with peak analysis, could improve the overall identification of dermatophytes by MALDI-TOF MS in clinical laboratories.

Keywords: MALDI–TOF MS; dermatophytes; filamentous fungi; identification.

Figure 1

Identification of the most common groups studied (T. rubrum and T. mentagrophytes groups), comparing identification scores and classification results across different databases: (A) MALDI–TOF MS scores for T. rubrum group isolates obtained with each database, MSI not included since it uses different scoring system; (B) identification obtained with each database for T. rubrum group isolates; (C) concordance in species identification among FilFungi4.0, HGM and MSI databases for T. rubrum group isolates; (D) MALDI–TOF MS scores for T. mentagrophytes group isolates obtained with each database. MSI is not included since it uses a different scoring system; (E) identification obtained with each database for T. mentagrophytes group isolates; and (F) concordance in species identification among FilFungi4.0, HGM and MSI databases for T. mentagrophytes group isolates.

Figure 2

Most relevant peak pairs differentiating T. interdigitale and T. tonsurans isolates: (A) m/z region showing the shift between the 2662/2667 m/z peaks: (B) intensity plot of the 2662 m/z peak by species; (C) intensity plot of the 2667 m/z peak by species; (D) m/z region showing the shift between the 5248/5258 m/z peaks; (E) intensity plot of the 5248 m/z peak by species; (F) intensity plot of the 5258 m/z peak by species; (G) m/z region showing the shift of the 7928/7958 m/z peaks; (H) intensity plot of the 7928 m/z peak by species; and (I) intensity plot of the 7958 m/z peak by species.