Inhibition of agr Quorum Sensing in Staphylococci by Novel Thiazole-Indole Hybrid Compounds

The rise of multidrug-resistant Staphylococcus aureus has prompted the search for innovative, non-bactericidal therapeutic strategies. Anti-virulence approaches targeting quorum sensing (QS) pathways offer a promising alternative by disarming pathogenicity, potentially without exerting selective pressure for resistance. In this study, we report the identification of two novel thiazole-indole hybrid compounds, namely 2-[(6-methoxy-1H-indol-2-yl)methyl]-5-cyano-1,3-thiazol-3-amine (designated C3) and 3-amino-5-cyano-2-(5-chloro-1H-indol-2-yl) thiazole (designated C4), as potent inhibitors of the S. aureus accessory gene regulator (agr) quorum sensing system. Building on two heterocyclic scaffolds commonly used in medicinal chemistry and associated with QS modulation, these synthetic small molecules act on the S. aureus AgrC QS receptor, attenuating virulence without affecting bacterial viability. Both compounds significantly reduced the production of secreted virulence factors without promoting biofilm formation, a known drawback of some other QS inhibitors. Furthermore, their activity extended to Staphylococcus lugdunensis, while sparing the commensal Staphylococcus epidermidis, indicating pathogen-selective QS interference. These findings establish novel thiazole-indole hybrids as a promising chemotype for anti-virulence intervention and provide valuable chemical tools for probing agr-mediated regulation in Staphylococcus species.