Multi‐Halogenated Indoles as Antimicrobial and Antivirulence Agents Against Drug‐Resistant Staphylococcus aureus

ABSTRACT The emergence of multidrug‐resistant Staphylococcus aureus requires the development of novel agents that can target both planktonic cells and persistent biofilms. In this study, a library of multi‐halogenated indoles was evaluated for antibacterial, antibiofilm and antivirulence activities against S. aureus including methicillin‐resistant strains. Two lead compounds, 6‐bromo‐4‐iodoindole and 4‐bromo‐6‐chloroindole, exhibited potent bactericidal activity (MIC = 20–30 μg/mL) comparable to the antibiotic gentamicin, effectively inhibited biofilm formation and persister formation and suppressed key virulence haemolysis. These effects were associated with intracellular ROS generation and transcriptional downregulation of quorum‐sensing genes of agrA and RNAIII and virulence genes of hla and nuc1 . Also, 6‐bromo‐4‐iodoindole synergised with aminoglycoside tobramycin and gentamicin, significantly reducing their effective MICs. Notably, these two multi‐halogenated indoles did not induce drug resistance for 20 days while gentamicin rapidly increased drug resistance. Cytotoxicity assays in HepG2 cells and phytotoxicity tests confirmed a favourable safety profile. Structure–activity relationship identified multi‐halogenation at the C4, C5, C6 and C7 positions of indole as favourable for enhanced activities and also suggested that more halogens could improve the activities. This study highlights multi‐halogenated indoles as promising multi‐target antimicrobial agents with potential therapeutic and environmental applications against S. aureus , including drug‐resistant and biofilm‐forming strains.