Staphylococcus aureus strain heterogeneity modulates virulence traits associated to dual-species biofilms with Candida albicans.

Staphylococcus aureus is a commensal Gram-positive bacterium and opportunistic pathogen that exhibits substantial variation in antimicrobial resistance and virulence traits. S. aureus is frequently co-isolated with the fungus Candida albicans, and together they represent the most common microorganisms recovered from biofilm-associated infections originating from both biotic and abiotic surfaces. Despite this clinical relevance, the impact of bacterial intra-species variation on interkingdom interactions remains poorly understood. This study therefore investigated how S. aureus strain heterogeneity influences C. albicans within dual-species biofilms. The majority of the 60 S. aureus strains tested suppressed fungal growth in a contact-dependent manner across different nutrient conditions and surface types. This growth inhibition was independent of strain-specific bacterial growth rates, secretion of secondary metabolites, or the type VII secretion system. As a result, dual-species biofilms varied significantly in their bacterial-to-fungal cell ratios and in the composition of the biofilm-associated extracellular matrix (ECM). These differences had profound effects on C. albicans susceptibility to antifungal agents, with some S. aureus strains enhancing antifungal resistance while others promoted fungal killing. Similarly, macrophage responses-key innate immune defences against S. aureus and C. albicans infections-varied among the dual-species biofilms, with certain strain combinations eliciting significantly stronger pro-inflammatory responses, as measured by the activation of the transcription of NF-kB and the secretion of interleukin (IL-) 6, than those of others. Collectively, these findings highlight the critical role of patient-associated strain diversity in bacterial-fungal interactions and support its consideration in the development of improved diagnostic and therapeutic strategies for biofilm-associated infections.