Anti-staphylococcal activity of the auranofin-analogous PEt3AuCl: antibacterial, anti-biofilm and anti-virulence effect on clinically relevant staphylococci

Objectives – Gold(I) complexes, such as the drug Auranofin (AF) which is approved for treating rheumatoid arthritis, have attracted significant interest as a potential treatment for bacterial infections due to their promising, broad-spectrum antimicrobial activity. In this study, we investigated the anti-staphylococcal activity of three AF analogues [PEt3AuCl (AF-Cl), PEt3AuI (AF-I) and PPh3AuCl (TPP-AuCl)] with the aim of discovering new weapons in the fight against antibiotic resistance. Methods – The antimicrobial activity and cytotoxicity of the gold compounds were evaluated by broth microdilution and the WST-1 assay, respectively. Time-kill assays were used to investigate killing kinetics, and the crystal violet (CV) assay was used to evaluate biofilm formation. Eradication of mature biofilms was assessed using the crystal violet assay, a plate count of biofilm-associated cells and scanning electron microscopy. The anti-virulence effect was tested by the hemolysis and agar diffusion assays. Results – All of the AF analogues were active against staphylococci, including antibiotic-resistant strains, with minimum inhibitory concentrations (MICs) ranging from 0.063 to 4 µg/mL. Additionally, they exhibited lower toxicity towards the A549 lung cell line and the spontaneously immortalized human keratinocyte line HaCaT than AF. AF-Cl was identified as the most promising compound and was selected for further biological investigations. Time-kill experiments revealed that AF-Cl was rapidly bactericidal against clinical staphylococci, causing at least a 3-log reduction in the number of viable cells within six hours. At sub-inhibitory concentrations, the compound inhibited biofilm formation and reduced the secretion of hemolysins and phospholipases, representing key virulence factors in S. aureus infections. Furthermore, AF-Cl was able to eradicate mature S. aureus biofilms at non-cytotoxic concentrations. Conclusion – Overall, our findings highlight the potential of AF-Cl as a promising candidate for treating staphylococcal infections, including those caused by antibiotic-resistant strains. In addition, the compound exhibited anti-biofilm and anti-virulence properties, which could be advantageous in treating toxin-mediated and biofilm-associated staphylococcal diseases.