Biosynthesis of silver nanoparticles by thermophilic spore-forming bacilli: screening for high-performance strains and characterization of silver nanoparticles from Anoxybacillus sp. D401a.

Abstract

In this study, thermophilic, spore-forming bacteria from the families Bacillaceae and Anoxybacillaceae were analyzed for their ability to produce silver nanoparticles (AgNPs). Anoxybacillus sp. D401a was selected due to its high AgNP synthesis potential. The culture conditions (NaCl-free nutrient broth, 60 °C, pH 8.5, 48 hr) and synthesis parameters (1.5 mM AgNO₃, 48 hr at 60 °C, pH 8.5) were optimized, resulting in a sevenfold increase in AgNP yield. Scale-up production yielded 139.4 mg of dry AgNPs with strong antimicrobial activity that exhibited significant minimum inhibitory concentration (MIC) values against Gram-positive, Gram-negative and fungal pathogens, outperforming commercial AgNPs. Sub-MIC concentrations of the biosynthesized AgNPs also inhibited biofilm formation and quorum sensing in Chromobacterium violaceum ATCC 12472. Characterization by ultraviolet-visible spectroscopy revealed a surface plasmon resonance peak at 400-450 nm, and Fourier-transform infrared spectroscopy indicated the presence of organic residues that stabilized the particles. Transmission electron microscopy analysis showed predominantly spherical AgNPs (24-57 nm). MTT assays showed a dose-dependent cytotoxicity against human keratinocytes (HaCaT), fibroblasts (HDF, L929), and cancer cells (HT-29, MCF-7), indicating a therapeutic window compared to commercial AgNPs. These results emphasize the potential of Anoxybacillus-derived AgNPs as a sustainable alternative for biomedical applications.