Utility of MALDI-ToF MS for Recognition and Antifungal Susceptibility of Nannizzia, an Underestimated Group of Dermatophytes.

BACKGROUND

Geophilic Nannizzia dermatophytes are increasingly implicated in stubborn skin, hair, and nail infections, yet MALDI-TOF MS evaluations and antifungal-susceptibility data have focused almost exclusively on N. gypsea. Biochemical profiles and MICs cut-offs are limited.

OBJECTIVES

To benchmark two commercial MALDI-TOF MS libraries and to determine in vitro activity of eight antifungals against a genus-wide panel of Nannizzia species.

METHODS

One-hundred-and-three ITS-confirmed isolates representing 12 species were grown on potato-dextrose agar (PDA) for 7-14 days. Spectra were acquired with (i) the MSI-2 Dermatophyte Library after 4-14 days' PDA incubation (100 cultures) and (ii) the Bruker MALDI Biotyper Filamentous-Fungi Library 6.0/2023 after ≤ 3 days' growth in Sabouraud-dextrose broth (SDB) (73 cultures). BCCM/IHEM strains could not be evaluated on the Biotyper because of licence restrictions, leaving 73 non-duplicate isolates for direct MSI-2 vs MBT comparison. EUCAST E.Def 11.0 micro-broth dilution determined MICs for eight agents.

RESULTS

MSI-2 achieved its highest accuracy with PDA day-7 cultures (45/73, 62%), whereas the liquid Biotyper protocol yielded 49/73 correct identifications (67%) within four days. Accepting low-confidence scores (≥ 1.7) from either library increased overall accuracy to 73%. MSI-2 remained superior for N. gypsea (73%) and uniquely detected N. nana (50%), which is absent from the current Biotyper release. Conversely, the Biotyper outperformed MSI-2 for N. incurvata, N. fulva, and N. praecox. Six very rare species (N. lorica, N. aenigmatica, N. corniculata, N. duboisii, N. perplicata, N. polymorpha) were not recognised by either database. Terbinafine displayed the lowest geometric mean MIC (0.009 mg/L); fluconazole and griseofulvin showed the highest values, and one US N. fulva isolate exhibited elevated itraconazole/voriconazole MICs (1 mg/L).

CONCLUSIONS

Diagnostic coverage of Nannizzia remains incomplete. Expanding commercial MALDI-ToF MS libraries with spectra from rare species and performing routine susceptibility testing are essential to optimise patient management.