Elucidating the augmented resistance profile of Scedosporium/Lomentospora species to azoles in a cystic fibrosis mimic environment.

Background: Scedosporium/Lomentospora species are ranked as the second most frequently isolated filamentous fungi from cystic fibrosis (CF) patients. Previously, we demonstrated that the minimum inhibitory concentration (MIC) for voriconazole and posaconazole increased when performed on a mucin-containing synthetic CF sputum medium (SCFM) compared to the standard medium, RPMI-1640. In this study, we have expanded the MIC comparison to four additional azoles and investigated characteristics linked to azole resistance in Scedosporium apiospermum, Scedosporium minutisporum, Scedosporium aurantiacum and Lomentospora prolificans.

Methods: MIC was assayed by CLSI protocol, efflux pump activity was assessed by rhodamine 6G and sterols were analysed by gas chromatography-mass spectrometry (GC-MS).

Results: Overall, MICs for fluconazole, itraconazole, voriconazole, posaconazole, miconazole and ketoconazole increased by least 2-fold when susceptibility tests were performed using SCFM compared to RPMI. The activity of efflux pumps was similar in both media; however, in RPMI, but not in SCFM, the activity was induced by voriconazole and fluconazole. Additionally, MICs for those antifungals decreased more noticeably in SCFM than in RPMI in the presence of the efflux pump inhibitor PaβN. The SCFM-grown cells presented fewer sterols in their composition, and consequently higher membrane fluidity, than RPMI-grown cells. GC-MS analysis demonstrated a remodulation in the sterol profile in SCFM- compared to RPMI-grown cells. Accordingly, when the MIC assay was performed in the presence of the membrane stressor NaCl (3%), the susceptibility to voriconazole and fluconazole increased more in SCFM- than RPMI-grown cells.

Conclusions: Scedosporium/Lomentospora species undergo cellular adaptations in SCFM that favours their growth in face of the challenges imposed by azole antifungals.