Antibacterial effect and MexA efflux pump Inhibition by green-synthesized copper nanoparticles from Mentha piperita

Infections caused by multidrug-resistant (MDR) Pseudomonas aeruginosa pose a significant worldwide health concern, mostly because of the ability of bacteria to expel antibiotics via efflux pumps such as MexA. The green synthesis of nanoparticles may offer a promising alternative to existing antimicrobial protocols. Hence, this study was designed to synthesize copper nanoparticles (Cu-NPs) from Mentha piperita (peppermint) leaf extract, assess their antibacterial activity, mainly against MDR Pseudomonas aeruginosa, and study efflux pump expression.

Cu-NPs were synthesized and characterized via UV-Vis spectroscopy, energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Antibacterial efficacy was assessed by agar well diffusion and minimum inhibitory concentration (MIC) assays against gram-positive and gram-negative bacteria. The effect of Cu-NPs on MexA gene expression in Pseudomonas aeruginosa was measured via real-time PCR. Cytotoxicity was tested in human white blood cells.

The green-synthesized Cu-NPs were spherical (particle sizes ~ 20-92 nm) and exhibited significant antibacterial activity, with the strongest effect detected against Pseudomonas aeruginosa (MIC: 500 µg/ml; inhibition zone > 20 mm). In addition, biogenic Cu-NP treatment downregulated MexA gene expression by approximately 50%, suggesting a role in reversing bacterial drug resistance. Cytotoxicity assays revealed tolerable safety at effective doses in vitro.

This study demonstrated that Mentha piperita-mediated Cu-NPs have significant antibacterial activity and can reduce MexA efflux pump expression, highlighting their potential as adjuncts for managing MDR Pseudomonas aeruginosa infections. Hence, further in vitro and in vivo studies are warranted.