Helcococcus kunzii-derived Adh2 peptide inhibits *S. aureus* Agr quorum sensing, reducing virulence and boosting host survival.

The pathogenicity of Staphylococcus aureus, a major human pathogen, is tightly controlled by the Agr quorum sensing (QS) system. This system coordinates the transition from commensal colonization to a highly virulent state, regulating the production of toxins and biofilm formation. Current antimicrobial strategies often target bacterial viability, contributing to antimicrobial resistance (AMR). There's a critical need for anti-virulence approaches that disarm bacteria without directly killing them, thereby potentially mitigating resistance development. Targeting the Agr QS system offers a promising avenue to modulate bacterial behavior and reduce pathogenicity.

Researchers investigated Adh2, a secreted peptide from Helcococcus kunzii structurally similar to native auto-inducing peptides (AIPs), for its impact on S. aureus. In vitro, S. aureus was exposed to Adh2 across a wide concentration range (0.01-10 g/L). They assessed gene expression using methods like RNA-Seq and qPCR for virulence factors (agrA, hla) and surface adhesins (spa). Proteomic profiling was used to analyze metabolic changes. A specific Adh2 region containing the conserved CDFIM motif was deleted to confirm its necessity. In vivo, zebrafish embryos were infected with S. aureus, and Adh2 was administered to evaluate its anti-virulence potential on host survival.

Exposure to Adh2 significantly repressed agrA and its downstream α-hemolysin hla, while upregulating spa, a gene encoding a surface adhesin. Deletion of the CDFIM motif within Adh2 abolished this regulatory effect, confirming its critical role. 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 the 0.01-10 g/L concentration range but significantly enhanced biofilm formation. > In vivo, Adh2 administration significantly improved survival in zebrafish embryos infected with S. aureus, validating its anti-virulence potential.