A Multifactorial Azole Response in Clinically Relevant Fusarium Species Complexes

Fusarium spp. are clinically relevant filamentous fungi with intrinsically low susceptibility to multiple antifungal agents, particularly azoles, and are included in the World Health Organization fungal priority pathogens list.

To investigate azole resistance-related mechanisms in clinical isolates belonging to the Fusarium solani species complex (FSSC), Fusarium oxysporum species complex (FOSC) and Fusarium fujikuroi species complex (FFSC) recovered in São Paulo, Brazil.

Twenty-six clinical isolates were identified by sequencing of the ITS region, TEF1 and RPB2. Antifungal susceptibility testing, CYP51A amplification and sequencing and expression analysis of CYP51A and ABC1 under voriconazole exposure were performed.

Antifungal susceptibility testing showed broadly elevated azole MICs, with the FSSC displaying the least favourable profile. Comparative CYP51A analysis revealed distinct amino acid signatures among the three species complexes, including residues at positions equivalent to canonical azole resistance hotspots in Aspergillus fumigatus. However, no simple relationship was observed between CYP51A amino acid profiles and azole MIC values. In contrast, voriconazole exposure induced marked CYP51A upregulation in all tested isolates, with substantially higher expression in the higher-MIC FSSC isolate. ABC1 was also upregulated in FSSC isolates, although at markedly lower levels than CYP51A.

These findings support a multifactorial model of azole response in Fusarium species complex strains, involving conserved CYP51A sequence features, strong target-gene induction under antifungal stress, and a lower but detectable contribution of efflux-associated mechanisms.