Triazole Priming as an Adaptive Response and Gateway to Resistance in Aspergillus fumigatus

Invasive aspergillosis (IA), caused predominantly by Aspergillus fumigatus, is the most common opportunistic mold infection in immunocompromised patients. Resistance of A. fumigatus to triazoles has been increasingly reported, leading to poor outcomes of IA to the front-line azoles. Triazole resistance is in part driven by exposure to agricultural azoles through mechanisms that are poorly understood beyond mutations in ergosterol biosynthetic genes. Priming is defined as a process in which prior exposures to sublethal stressful stimuli, such as antimicrobial drugs, can enhance the ability of pathogens to withstand reexposure to the same or other stressors. Here, we describe, for the first time, triazole priming, where exposure of conidia of three A. fumigatus strains to subinhibitory concentrations of either agricultural (tebuconazole difenoconazole, epoxiconazole) or medical triazoles (voriconazole) increases germination and growth during subsequent reexposure to subinhibitory triazole challenge. We demonstrate that priming in A. fumigatus is class specific to triazoles, is not confined to a particular isolate, and is retained for extended periods in primed dormant conidia, but is not transferred to subsequent generations. Furthermore, azole priming at subinhibitory triazole concentrations increased the frequency of development of stable resistance development at inhibitory triazole exposures. Triazole priming could have far-reaching clinical implications in generating resistance due to the widespread use of agricultural triazoles or breakthrough IA in patients with subtherapeutic serum levels of azoles.