MALDI-TOF MS identification of Exophiala species isolated in Japan: Library enrichment and faster sample preparation

Exophiala species cause chromoblastomycosis, mycetoma, and phaeohyphomycosis, which are occasionally fatally in immunocompromised patients. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) provides rapid and accurate examination of isolated bacteria and some fungal isolates, but the preparation method for filamentous fungi is complicated. In this study, 31 clinical isolates of Exophiala spp. in Japan were identified by MALDI-TOF MS with a library enriched by adding data. To simplify the sample preparation method, two modified methods were compared with the standard method for filamentous fungi. The agar cultivation sample preparation method reduced the time required for liquid culture and was considered suitable for clinical use. In 30 of 31 clinical isolates of Exophiala spp., the species identified by MALDI-TOF MS with the highest score matched the species identified by sequencing the internal transcribed spacer region. Exophiala dermatitidis, E. lecanii-corni, and E. oligosperma were identified above the genus level, while E. jeanselmei and E. xenobiotica were often not identified at the species level. The identification scores tended to be lower for less-registered strains in the in-house library. It is suggested that library enrichment and the modified preparation method may facilitate early diagnosis of rare fungal infections by Exophiala spp. in clinical laboratories using MALDI-TOF MS.