# A Cyclic Pentapeptide Inhibits AgrC as a Quorum-Sensing Quenching Agent in Staphylococcus aureus

## TL;DR

A new cyclic pentapeptide was found to reduce the virulence of Staphylococcus aureus by inhibiting its AgrC signaling system without killing the bacteria.

## Contribution

A novel cyclic pentapeptide inhibitor of AgrC is identified as a potential antivirulence agent for Staphylococcus aureus.

## Key findings

- The compound significantly inhibited hemolytic activity by 77.60% at 16 µg/mL.
- The pentapeptide reduced expression of key AgrC-mediated genes like agrC, agrA, and hla.
- The compound does not inhibit bacterial growth but attenuates virulence.

## Abstract

Background/Objectives: Staphylococcus aureus virulence is tightly regulated by the agr (accessory gene regulator) quorum-sensing system. Targeting AgrC, the histidine kinase receptor that serves as a core regulator of agr signaling, represents a promising antivirulence strategy that circumvents conventional bactericidal pressure. Methods: In this study, structure-based virtual screening using AutoDock Vina was performed, followed by molecular dynamics simulations, to identify potent analogs of known AgrC inhibitors. Results: A cyclo[Ala-Phe-OLeu-Phe-D-Leu] exhibiting high binding affinity and stable receptor interaction was selected for further evaluation. Antimicrobial susceptibility testing confirmed that the compound did not inhibit bacterial growth. However, at a concentration of 16 µg/mL, it significantly inhibited hemolytic activity with high reproducibility, and the inhibition rate reached 77.60%. Quantitative reverse transcription PCR (RT-qPCR) demonstrated that the compound decreased some key AgrC-mediated genes, including agrC, agrA, saeS, hla, spa, fnbA, and lukS. Conclusions: These findings identify a promising cyclic pentapeptide inhibitor of AgrC that effectively attenuates S. aureus virulence without exerting bactericidal pressure. This work provides a valuable lead compound and offers novel insights for the development of advanced, safe, and effective antivirulence therapeutics.