Synthesis and Characterisation of Metallic Nanoparticles from Medicinal Plant Extracts and their Application in combating Antibiotic-Resistant Bacterial Pathogens

Abstract

Background: The rise of antibiotic-resistant (ABR) bacterial pathogens is a major global health concern, especially in the case of respiratory tract infections (RTIs), which can make treatment more difficult. According to statistics, more than 2 million lives were lost due to ABR bacterial infections in 2019, highlighting the seriousness of the situation. This study aims to propose a novel approach to combating ABR bacteria by synthesising metallic nanoparticles (MNPs) from medicinal plants. Integrating nanoparticle-based strategies with medicinal plants demonstrates considerable promise in addressing RTIs. Methodology: The present study investigated the extracts of Spirostachys africana (S. africana), a medicinal plant traditionally used by healers to treat various infections. Extracts were prepared using methanol, ethanol, acetone, and distilled water. A comprehensive approach was employed to synthesise and characterise silver (SA-AgNPs) and gold (SAAuNPs) nanoparticles from S. africana extracts. Methods of characterisation included Ultraviolet-Visible spectrophotometry (UV-Vis), X-ray diffraction (XRD), Dynamic light scattering (DLS), Transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FT-IR), and Liquid chromatography and mass spectroscopy (LC-MS). The antimicrobial activity of the nanoparticles and extracts was tested against the World Health Organization (WHO) ABR priority pathogens using agar well diffusion and microdilution assays. We further, assessed the cytotoxicity, anti-inflammatory, and antioxidant activities using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), Griess assays, and 2,2-diphenyl-1-picrylhydrazyl (DPPH) method, respectively. Results: The UV-Vis analysis revealed surface plasmon resonance (SPR) peaks at 480 nm and 541 nm, indicating the formation of SA-AgNPs and SA-AuNPs, respectively. XRD showed face-centred cubic structures with crystalline sizes ranging from 9–19 nm for SA-AgNPs and 9–10 nm for SA-AuNPs. The DLS measurements displayed a polydisperse distribution for SAAgNPs, while a monodisperse distribution was observed for SA-AuNPs. Most NPs aggregated over time, except for bark-methanol conjugated SA-AgNPs, which exhibited more stability with a zeta potential value of -27 mV. The TEM images showed particle core sizes of 5–49.5 nm for SA-AgNPs, predominantly spherical, and 6–32 nm for SA-AuNPs, mainly spheroidal. The FTIR analysis identified functional groups including hydroxyl, carboxyl, and amine groups in both plant extracts and SA-AgNPs/SA-AuNPs. These fuctional groups are involved in the reduction and capping of Ag+ and Au+ ions to form SA-AgNPs and SA-AuNPs, respectively. The LCMS technique identified 23 bioactive compounds from S. africana extracts, with flavonoids being the most dominant. Antimicrobial assays using agar well diffusion and microdilution methods showed SA-AgNPs were more effective than crude extracts and SA-AuNPs against tested ABR bacteria. Acetone-conjugated SA-AgNPs having the highest zone of inhibition (22 mm) against P. aeruginosa. In contrast, methanol/ethanolconjugated SA-AgNPs displayed potent antimicrobial activity (MIC = 0.05 mg/mL) against E. coli, P. aeruginosa, and A. baumannii, respectively. Additionally, Acetone extracts exhibited selective toxicity against MCF-7 cancer cells, while stimulating MCF-10 cells and RAW 264.7 macrophage cells at concentrations of 0.078 mg/mL. Furthermore, ethanol extracts and ethanol-conjugated SA-AgNPs/SA-AuNPs significantly decreased NO production, indicating their potential as anti-inflammatory agents. Acetone extract exhibited excellent antioxidant activity (IC50 = 0.000335 mg/mL). Conclusions: The MNPs synthesised from S. africana extracts offer a promising solution to combat ABR bacterial pathogens, particularly those causing RTIs. The S. africana extracts and MNPs show significant cytotoxic, anti-inflammatory, and antioxidant activities.