Green synthesis of Saussurea costus-mediated silver nanoparticles: Characterization, antibacterial activity against ESBL-producing bacteria, and cytocompatibility

Abstract

The rapid emergence of extended-spectrum β-lactamase (ESBL)-producing Escherichia coli and Klebsiella pneumoniae has significantly limited the effectiveness of conventional antibiotics, highlighting the urgent need for alternative antimicrobial agents. In the present study, silver nanoparticles (AgNPs) were green synthesized using Saussurea costus root extract as a natural reducing and stabilizing agent. The biosynthesized AgNPs were comprehensively characterized by UV–visible spectroscopy, Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM), dynamic light scattering (DLS), zeta potential analysis, and energy-dispersive X-ray spectroscopy (EDX), confirming the successful formation of stable, crystalline, predominantly spherical nanoparticles. TEM and DLS analyses demonstrated nanoscale particle dimensions, while the negative zeta potential indicated favorable colloidal stability. The antibacterial activity of the synthesized AgNPs was evaluated against clinical ESBL-producing E. coli and K. pneumoniae isolates using agar well diffusion, minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and time-kill assays. The AgNPs exhibited pronounced concentration-dependent antibacterial activity, producing clear inhibition zones and low MIC and MBC values against both pathogens. Time-kill analysis further demonstrated rapid bactericidal activity, with increasing nanoparticle concentrations resulting in enhanced bacterial killing over time. Cytocompatibility evaluation using primary human dermal fibroblasts (HDFa) revealed acceptable cell viability within the antibacterial concentration range, supporting the preliminary biocompatibility of the biosynthesized nanoparticles. Collectively, these findings demonstrate that S. costus-mediated AgNPs possess favorable physicochemical properties, potent in vitro antibacterial activity against clinically important ESBL-producing pathogens, and promising preliminary cytocompatibility. The study highlights the potential of S. costus as a sustainable plant source for green nanoparticle synthesis and provides a basis for the future development of plant-mediated antimicrobial nanomaterials. Nevertheless, additional investigations, including mechanistic studies, long-term stability assessment, comprehensive biosafety evaluation, and in vivo validation, are required before biomedical or clinical translation can be considered.