Biogenic silver nanoparticles synthesized from Pseudomonas fluorescens-mediated olive cake waste: antimicrobial, larvicidal activity against Culex pipiens and cytotoxicity assessment

This study presents an eco-friendly approach for synthesizing silver nanoparticles (AgNPs) using olive cake hydrolysate (OCH), produced through microbial fermentation of olive cake waste by Pseudomonas fluorescens. The OCH was analyzed by gas chromatography-mass spectrometry (GC-MS), revealing the biotransformation of olive cake components into bioactive compounds, including 24-norursa-3,12-diene, methyl esters of 9,12-octadecadienoic acid and 9-octadecenoic acid, and α-sitosterol. The biosynthesized olive cake hydrolysate-silver nanoparticles (OCH-AgNPs) were characterized using ultraviolet-visible (UV-Vis) spectroscopy to confirm surface plasmon resonance at 420 nm; Fourier-transform infrared (FTIR) spectroscopy to identify the involvement of hydroxyl and carbonyl functional groups; X-ray diffraction (XRD) analysis to verify the crystalline structure, revealing prominent (111) lattice planes of face-centered cubic (fcc) silver; transmission electron microscopy (TEM) to assess morphology and particle size, showing spherical nanoparticles with an average diameter of 19.6 ± 6.1 nm; dynamic light scattering (DLS) to measure hydrodynamic diameter, yielding a size of 109.8 nm; and zeta potential analysis to determine surface charge, which indicated high colloidal stability with a zeta potential of - 47.0 mV. OCH-AgNPs exhibited superior antimicrobial activity compared to OCH alone, with low MIC values against P. aeruginosa, Candida albicans, Aspergillus brasiliensis, and Staphylococcus aureus MRSA. Larvicidal activity, optimized via Box-Behnken design, showed 98.86% mortality of Culex pipiens at 1.0 µg/mL (LC₅₀ = 0.40 µg/mL), significantly outperforming OCH (LC₅₀ = 57.22 µg/mL). Histopathological and biochemical analyses of treated larvae revealed structural damage, decreased protein and carbohydrate content, and inhibition of acetylcholinesterase. Cytotoxicity assays on human skin fibroblasts confirmed low toxicity (IC₅₀ >200 µg/mL). Molecular docking identified α-sitosterol as a key bioactive component. These findings underscore the potential of OCH-AgNPs as a sustainable and multifunctional biocontrol agent for microbial and vector management.