Amyloids are functional assemblies whose structural plasticity governs biological activity. We identified fibril-forming antimicrobial peptides with reversible structural switching that modulates their function. Extending this concept to bacterial virulence, we show that PSMα3 from Staphylococcus aureus forms a dynamically regulated cross-α amyloid that transitions between soluble, condensate, and fibrillar states. These environmentally tuned states differentially control antimicrobial activity and host immunometabolic responses.
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