Antimicrobial Activity of Green Synthetized Silver-Nanoparticles on Antibiotic Resistant Pathogenic Escherichia coli, Salmonella typhimurium, Listeria monocytogenes, and Staphylococcus epidermidis

The accelerated spread of multidrug-resistant (MDR) bacteria has emerged as one of the most pressing global health challenges, driving a substantial rise in infection-related morbidity, mortality, and healthcare costs. As the clinical effectiveness of conventional antibiotics declines, there is an urgent demand for innovative and sustainable antimicrobial strategies. Among the most promising approaches, nanoparticle-based materials particularly silver nanoparticles (AgNPs) have attracted growing interest due to their potent and broad-spectrum antibacterial properties. In the present investigation, silver nanoparticles were synthesized via a green approach using aqueous extracts of Boswellia carterii (frankincense), Syzygium aromaticum (clove), Moringa oleifera, and a mixed extract of frankincense and clove. These plant extracts acted simultaneously as reducing and stabilizing agents. Multidrug-resistant isolates of Escherichia coli, Salmonella typhimurium, Listeria monocytogenes, and Staphylococcus epidermidis were phenotypically confirmed to exhibit complete resistance to Ciprofloxacin, Streptomycin, Ceftriaxone, and Erythromycin. The antimicrobial efficacy of these green-synthesized AgNPs was then evaluated against multidrug-resistant isolates. Also, the synergistic effect of the Boswellia carterii AgNPs with antibiotics was also determined. Plant extracts were prepared both with and without heat treatment for nanoparticle synthesis. The formation of AgNPs was confirmed visually by a characteristic color change. Antibacterial activity was assessed using the disc diffusion method, and the most potent formulation frankincense-AgNPs was further characterized by Fourier-transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). TEM analysis revealed highly uniform nanoparticles with sizes ranging from 3.2 to 15.9 nm (average 7.54 nm), offering a high surface area-to-volume ratio favorable for antibacterial interactions. The AgNPs exhibited potent antibacterial effects, while the raw plant extracts showed no significant inhibition when tested using the Well Diffusion Method. Frankincense-AgNPs demonstrated higher antibacterial potency using the macro-dilution method, with lower MBC, MIC, and sub-MIC values of 0.6 μg/ml, 0.3 μg/ml, and 0.15 μg/ml, respectively. Additionally, the synergistic effects of Frankincense-AgNPs with Erythromycin, Ceftriaxone, Ciprofloxacin, and Streptomycin were evaluated using the disc diffusion method. This study demonstrates the effective use of plant extracts as reducing and capping agents for the eco-friendly and low-cost synthesis of AgNPs, highlighting the remarkable bactericidal potential of green-synthesized AgNPs against MDR pathogens. Furthermore, this highlights the greater efficacy of frankincense as a green reducing agent for synthesizing small nanoparticles, and its potential role in addressing the global MDR crisis.