Synthesis, Characterisation, and Antimicrobial Evaluation of Spirostachys africana Extract-Based Silver Nanoparticles Against Selected Bacterial Pathogens, Including Antibiotic-Resistant Strains

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 extremely difficult. This study explores the green synthesis, characterization, and antimicrobial activity of silver nanoparticles from the medicinal plant Spirostachys africana extracts (SA-AgNPs) using methanol, ethanol, acetone, and water solvents. Optimized synthesis was achieved with 5 mM silver nitrate and incubation at 4 °C, producing stable nanoparticles. UV-Vis analysis revealed surface plasmon resonance peaks at 480 nm, confirming nanoparticle formation. X-ray diffraction indicated face-centered cubic crystalline structures with sizes ranging from 9 to 19 nm. Transmission electron microscopy revealed predominantly spherical nanoparticles with core sizes between 5 and 136 nm. Antimicrobial assays demonstrated greater activity of the SA-AgNPs than the corresponding crude extracts against selected bacterial pathogens, including strains with documented antibiotic-resistance phenotypes. Notably, acetone-extract conjugated nanoparticles produced the largest inhibition zone of 22 mm against Pseudomonas aeruginosa. Bark methanol, ethanol, and acetone conjugated nanoparticles were highly effective against Acinetobacter baumannii, with minimum inhibitory concentrations as low as 0.08 mg/mL, while the methanol leaf extract showed activity against P. aeruginosa (0.09 mg/mL). This is the first study to synthesize and characterize nanoparticles from S. africana, showing that AgNPs synthesized from the plant offer promising, eco-friendly alternatives for combating antibiotic-resistant respiratory pathogens. Further in vivo studies and mechanistic investigations are recommended to advance their therapeutic development.