Serum modulates the aggregation - toxicity landscape of the staphylococcal toxin PSMα3

The pathogenicity of Staphylococcus aureus relies on the secretion of various toxins, including phenol-soluble modulins 3 (PSM3), which play key roles in invasion and infection through its cytolytic activities. Recent controversies have focused on whether PSM3 propensity to self-assemble into unique amyloid-like cross- structures would underlie its high cytotoxicity. Here, by integrating Thioflavin T fluorescence, electron microscopy, and cell-based assays, we demonstrate that early soluble entities formed upon fibrillation - rather than mature fibrils - drive PSM3 cytotoxicity under near-physiological conditions. This mirrors the behavior of neurodegenerative disease related peptides, thus pointing towards a general mechanism of toxicity by amyloid forming peptides. Importantly, we elucidate the critical role of serum in this process: lipoproteins inhibit the formation of oligomeric and fibrillar entities, both in a time- and concentration-dependent manner, thereby drastically reducing cytotoxicity. We further reveal the uptake of such soluble entities in cells, where membrane interactions likely initiate and trigger cell death. These findings reconcile conflicting interpretations of PSM3 toxicity, highlighting the importance of the cellular environment in shaping peptide aggregation and biological function. By targeting such key virulence determinants, e.g. though the rational design of aggregation inhibitors, this study could open novel avenues in the quest for novel drugs against S. aureus infections.