Objective(s): Silver nanostructures have gathered remarkable attention due to their applications in diverse fields. Researchers have recently demonstrated that bacterial spores are capable of reducing silver ions to elemental silver leading to formation of nanoparticles. Materials and Methods: In this study, spores of Bacillus subtilis and Geobacillus stearothermophilus were employed to produce silver nanoparticles (SNPs) from silver nitrate (AgNO3) through a green synthesis method. The production of SNPs by spores, heat inactivated spores (microcapsule) and spore extracts was monitored and compared at wavelengths between 300 to 700 nm. The biosynthesized SNPs by spore extracts were characterized and confirmed by XRD and TEM analyses. Results: UV-Visible spectro scopy showed that the spore extracts were able to synthesize more SNPs than the other forms. The XRD pattern also revealed that the silver nanometals have crystalline structure with various topologies. The TEM micrographs showed polydispersed nanocrystal with dimensions ranging from 30 to 90 nm and 15 to 50 nm produced by spore extracts of B. subtilis and G. stearothermophilus, respectively. Moreover, these biologically synthesized nanoparticles exhibited antimicrobial activity against different opportunistic pathogens. Conclusion: This study suggests the bacterial spore extract as a safe, efficient, cost effective and eco-friendly material for biosynthesis of SNPs. Key words: Bacillus subtilis, Geobacil
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