Candida albicans activates Staphylococcus aureus virulence regulatory systems to drive toxin-mediated human cell death

Co-infection with Staphylococcus aureus and Candida albicans leads to worsened disease severity compared to mono-microbial infection. Because our understanding of the mechanisms driving enhanced disease severity during co-infection is limited, we sought to evaluate how interactions with C. albicans regulate S. aureus virulence towards host cells. We determined that C. albicans enhances S. aureus cytotoxicity towards murine monocytes via a mechanism requiring the Agr system. Agr is a major regulator of S. aureus virulence factors and was previously shown to be activated by C. albicans, but the Agr-regulated virulence factors driving immune cell death are unknown. We identified that enhanced murine monocyte cell death requires the α-type phenol soluble modulins and ψ-hemolysin. Because several S. aureus toxins have species-specific effects, we also tested how co-culture impacts cytotoxicity towards human monocytes. Unexpectedly, we discovered that C. albicans induces robust cytotoxicity of an S. aureus agr mutant (Δagr), which is completely non-toxic towards murine monocytes. Using reporter strains and combinatorial mutants, we identified that co-culture activates the SaeRS regulatory system in S. aureus, and SaeRS is required for human-specific cytotoxicity. We further discovered that the SaeRS-regulated toxin Panton-Valentine Leukocidin (PVL) drives S. aureus Δagr cytotoxicity following co-culture. Finally, we observed similar cytotoxicity phenotypes using both S. aureus and C. albicans clinical isolates, demonstrating broad conservation of this interaction. Interestingly, the magnitude by which C. albicans isolates induce cytotoxicity of S. aureus Δagr varies among strains tested. Overall, this study identifies that C. albicans activates a major S. aureus virulence regulatory system in a typically non-toxic strain, triggering S. aureus to induce potent human-selective cell death.