Biosynthesis mechanisms, antimicrobial effects, and characterization of silver nanoparticles produced by bacterial and actinomycete isolates derived from a gold mine

Abstract Background Silver nanoparticles (AgNPs) are widely studied for their antimicrobial properties, and microbial synthesis offers an eco-friendly and scalable alternative to conventional methods. While Streptomyces spp. are well-known producers, little is known about AgNP biosynthesis by microorganisms adapted to metal-rich environments such as gold mines, which may represent a largely unexplored source of microorganisms with potential for AgNP biosynthesis. This study investigated the biosynthetic potential of bacterial and actinomycete isolates recovered from the Sukari Gold Mine, Egypt, with a focus on optimizing cell-free extract (CFE) preparation, nanoparticle characterization, and understanding the underlying biochemical contributions. Results Out of 57 isolates, three Streptomyces strains showed the highest AgNP biosynthesis capability. Among the tested methods, soaking was selected as the preferred CFE preparation approach because it provided comparable AgNP biosynthesis while offering a simpler and more practical procedure than freeze/thaw and sonication. The biosynthesized AgNPs exhibited antimicrobial activity against tested microorganisms, with enhanced performance relative to commercial AgNPs. Antimicrobial assessment was supported by MIC and MBC determinations. Biochemical analysis of the CFE revealed the presence of phenols, proteins, and nucleic acids (DNA and RNA), while FTIR analysis identified functional groups including hydroxyl, amine, aromatic, alkene, and alkyne groups, suggesting their involvement in reduction and stabilization processes. The napA gene was detected in all active strains, indicating the potential for nitrate reductase production; however, its role in Ag⁺ reduction appears to be contributory rather than definitive. Characterization using UV–Vis spectroscopy, XRD, FTIR, AFM, HR-TEM, and zeta potential analysis confirmed the formation of stable AgNPs with spherical to irregular morphologies, sizes ranging from 5 to 52 nm, and high colloidal stability (zeta potential up to − 37.3 mV). Conclusions These findings highlight gold mine–derived actinomycetes as promising candidates for the green synthesis of stable, bioactive AgNPs and suggest that multiple biomolecules act synergistically during nanoparticle formation.