Antifungal activity, mechanistic insights, and combinatorial effects of Pycnocycla bashagardiana essential oil against Aspergillus fumigatus

Pycnocycla bashagardiana (Apiaceae family) which grows in the subtropical biome from West Tropical Africa to India has received special attention for its health benefits and therapeutic uses in traditional medicine. The present study aimed to investigate the antifungal activity and mechanism of action of P. bashagardiana essential oil (PBEO) against A. fumigatus as a life-threatening fungal pathogen. The PBEO was prepared by hydrodistillation and analyzed by gas chromatography-mass spectrometry (GC–MS). Antifungal activity of the PBEO (125–8000 µg/mL) was examined by microbioassay technique in 6-well microplates. The ergosterol content of fungal mycelia was measured by spectrophotometry and the ultrastructure of fungal compartments was evaluated using electron microscopy. The synergistic effects of the PBEO with voriconazole and amphotericin B were determined according to the fractional inhibitory concentration index (FICI). The PBEO contained 35 compounds which comprised 95.1 % of the EO with myristicin (31.8 %), cis-Isomyristicin (12.2 %), and (E)-β-Ocimene (8.5 %) as the main constituents. The MIC and IC₅₀ of PBEO against A. fumigatus were determined as 1000 μg/mL and 719.4 μg/mL, respectively. The PBEO strongly inhibited ergosterol synthesis in the range of 13.12–86.24 % and showed synergistic antifungal activity with both voriconazole (FICI=0.3503) and amphotericin B (FICI=0.3757). Ultrastructural results revealed pathologic changes including attenuation of the fungal cell wall, destruction of cell membrane and hyphae organelles such as mitochondria, detachment of cell membrane from the cell wall, shrinkage of fungal hyphae, vacuolation of cytoplasm and depletion of cytoplasmic contents. Concerning the strong antifungal activity of PBEO and synergistic effect with amphotericin B and voriconazole evidenced for the first time in the present work, it could be considered a potential candidate in the treatment of A. fumigatus-induced fungal infections.