A ceramide synthase is important for filamentous fungal biofilm morphology and antifungal drug susceptibility

ABSTRACT The complex structure of fungal biofilms generates microenvironments that impact the fitness of cells within the biofilm community. Contributions to fitness include the development of emergent properties resulting in the tolerance or resistance to external stressors, such as rapid environmental changes and, in the context of an infection, antifungal drug exposure. The biofilm developed by the filamentous fungal pathogen Aspergillus fumigatus develops zones of low oxygen, which contribute to a reduction in antifungal drug susceptibility. The genes and mechanisms involved in driving this biofilm-specific emergent property are ill-defined. In this study, we utilized a transcriptomic approach to probe the biofilm structure in comparison to drug-susceptible planktonic cultures to identify transcriptional patterns and genes unique to the A. fumigatus biofilm. Importantly, we utilized two phenotypically diverse strains that allowed us to identify biofilm-specific gene co-expression networks. One of these networks was highlighted by a gene encoding a ceramide synthase, designated barA , with a striking increase in barA transcript abundance specifically in the biofilm. Null mutants of barA in two strain backgrounds display a stunted biofilm morphology, with some strain-specific differences in the impact of biofilm biomass. Importantly, barA has a role in regulating susceptibility to the ergosterol-targeting antifungal drugs voriconazole and amphotericin B. These data identify biofilm-specific genes in A. fumigatus for further study and highlight the importance of fungal ceramide synthases in mediating antifungal drug susceptibility in infection-relevant biofilms. IMPORTANCE Biofilms are problematic structures in the context of microbial infections due to their ability to resist both host- and drug-mediated attempts at tissue sterilization. Consequently, it is imperative to identify mechanisms underlying the development of these structures and the emergent properties they develop. The filamentous fungal pathogen Aspergillus fumigatus forms robust-structured biofilms that are resistant to contemporary antifungal drug treatments, although the mechanisms are ill-defined. In this study, we compared the transcriptional landscape of two A. fumigatus reference strains grown as biofilms and in planktonic culture conditions to identify biofilm-specific genes and pathways. These analyses and subsequent genetic and phenotypic studies revealed that a ceramide synthase is important for biofilm development and is involved in antifungal drug susceptibility of the biofilm. Consequently, these data support the rationale for targeting fungal lipid homeostasis for antifungal therapeutic development, particularly in the context of biofilm-mediated infections.