A secreted AIP-Like peptide from Helcococcus kunzii inhibits the Agr quorum sensing system of Staphylococcus aureus

Staphylococcus aureus is a major human pathogen whose virulence is tightly regulated by the Agr quorum sensing system. In this study, we investigated the impact of Adh2, a secreted protein from the commensal bacterium Helcococcus kunzii, on S. aureus physiology and pathogenicity. Adh2 shares structural similarity with native auto-inducing peptides (AIPs), including the conserved CDFIM motif characteristic of Agr group I. We hypothesized that Adh2 interferes with Agr signaling by competitively binding the AgrC receptor. Exposure to Adh2 significantly repressed agrA and its downstream α-hemolysin hla, while upregulating spa, a gene encoding a surface adhesin. Deletion of an Adh2 region encompassing the conserved CDFIM motif abolished this regulatory effect, indicating that this region is required for Adh2 activity. RNA-Seq analysis revealed global transcriptional reprogramming, with downregulation of virulence and metabolic genes. Proteomic profiling corroborated these findings, showing reduced abundance of proteins involved in metabolic pathways (e.g. carbohydrate, lipid, and nucleotide metabolism), consistent with a shift toward a low-energy, colonization-oriented state. Importantly, Adh2 did not impair S. aureus growth across a wide concentration range (0.01-10 g/L) but significantly enhanced biofilm formation. In vivo, Adh2 administration significantly improved survival in zebrafish embryos infected with S. aureus, validating its anti-virulence potential. Together, these findings demonstrate that Adh2 suppresses Agr signaling and virulence gene expression while promoting a persistent phenotype. By shifting S. aureus toward a metabolically reduced and less pathogenic state, Adh2 emerges as a promising candidate for therapeutic modulation of bacterial behavior, particularly in the context of chronic wound infections.