Biogenic synthesis and characterization of selenium nanoparticles by halotolerant Bacillus sonorensis 2MNHR

Selenium nanoparticles (SeNPs) have attracted significant attention owing to their unique physicochemical properties and promising biomedical applications. In the present study, a newly isolated halotolerant marine Bacillus sonorensis 2MNHR strain was employed for the extracellular biosynthesis of SeNPs. Bacterial isolates capable of reducing selenium oxyanions were obtained from seawater samples collected at Alexandria Harbor, Mediterranean Sea, Egypt. The ability of ten morphologically distinct bacterial isolates to reduce sodium selenite and sodium selenate under aerobic conditions was evaluated. Among them, only one isolate consistently reduced sodium selenite, as evidenced by the development of a characteristic red coloration in both liquid and solid media within 72 h, whereas no reduction was observed with sodium selenate. Extracellular reduction was confirmed using a cell-free supernatant assay. UV-Vis and FTIR analyses of the filtrate confirmed the involvement of biomolecules in selenite biotransformation, while GC-MS analysis revealed diverse organic compounds potentially responsible for reduction and stabilization. The biosynthesized SeNPs were comprehensively characterized using UV-visible spectroscopy, Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), zeta potential analysis, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX), and transmission electron microscopy (TEM). These analyses confirmed the formation of spherical, amorphous nanoparticles with sizes ranging from 80 to 200 nm (average ~ 160 nm) and a characteristic UV-Vis peak at 226 nm. A zeta potential of -38 mV indicated high colloidal stability. The SeNPs exhibited strong antimicrobial activity, with inhibition zones ranging from 23 to 32 mm and MIC values from 19.5 to 156.25 µg/mL. MBC/MIC ratios (≤ 2) confirmed bactericidal activity against most tested bacteria, while a ratio of 4 indicated borderline activity against Pseudomonas aeruginosa and fungistatic behavior against Candida albicans. These findings highlight the potential of halotolerant marine bacteria as sustainable platforms for the production of stable and biologically active SeNPs. KEY POINTS: • Isolation of marine Bacillus sonorensis 2MNHR with high selenite-reducing potential • Efficient extracellular biosynthesis of stable selenium nanoparticles (SeNPs) • Integrated physicochemical characterization and antimicrobial evaluation of SeNPs.