Multidimensional assessment of in-host fitness costs of echinocandin resistance in the opportunistic fungal pathogen Candida glabrata reveals the niche-specific requirement for FKS1 and FKS2 during infection and gut colonization.

Abstract

Host stresses are often considered a major barrier against the emergence of echinocandin resistance (ECR) in prominent infecting organisms like Candida albicans due to fitness defects. Yet, ECR strains of C. glabrata carrying diverse amino acid changes in Fks1 and Fks2 are increasingly reported as breakthrough infections. Nonetheless, the impact of equivalent mutations in different FKS alleles on fitness has not been systematically studied. Herein, we employed a diverse array of ex vivo and in vivo models to address these questions among clinically relevant ECR mutants. All ECR mutants retained fitness during interaction with THP1 macrophages and neutrophils. Whereas a strain with a Fks2^F659del or fks2Δ showed fitness defects during interaction with macrophages and neutrophils. Fks2^F659del showed a unique susceptibility to numerous stresses, especially the combination of alternative carbon sources, low pH, and H₂O₂. Consistent with failure in mounting adaptive oxidative stress response during exposure to H₂O₂, transcriptomic analysis of intracellular Fks2^F659del highlighted the dysregulation of oxidative stress response genes, whereas intracellular fks2Δ showed hallmarks of metabolic dysregulation. Intriguingly, the Fks2^F659del mutant was outcompeted by wild type and Fks2^F659V in in vivo gut colonization and systemic infection models. Importantly, whereas both FKS1 and FKS2 were required to establish gut colonization, only FKS2 was required for systemic infection. Therefore, our study supports the notion that the prevalence of ECR mutants among C. glabrata strains is likely driven by its ability to retain fitness across diverse niches. Furthermore, we identified that the essentiality of FKS1 and FKS2 is similarly dictated by niche-specific requirements.