Background and Aim:Recent development in the biosynthesis of nanomaterials have demonstrated the important role of biological systems and microorganisms in nanoscience and nanotechnology.These organisms show a unique potential in environmentally friendly production and accumulation of nanoparticleswith different sizes.Studies have demonstrated that nanoparticles with smaller particle sizes have betterproperties. The aim of this study was to produce selenium nanoparticles by indigenous bacterial isolates andthe comparison the sizes of their produced nanoparticles. Methods:Bacteria were isolated and purified by addition of glass factory sewage samples into nutrient medium supplemented with correct amount of metal salt (100 mg/lit) incubated for 24h.Selenium nanoparticles were extracted from pure culture of screened bacterium using centrifugation, repeated washingprocess and disrupting cells by liquid nitrogen. Disrupted cells were resuspended in Tris HCl buffer and washed in chloroform and ethanol after centrifugation. Finally resuspended cleaned nanoparticles were preserved in deionized water and analyzed with Dynamic light scattering (DLS). Results:At first ability of bacterial isolates for production of selenium nanoparticles were qualitatively confirmed by observation of red colonies on nutrient medium containing SeO2 in comparison with control where just normal yellowish colonies appeared. Then various morphological, biochemical and molecular characteristics of isolates were determined showing that from 4 isolates, 1 of them were gram positive while 3 others were gram negative. Also rod, cocci and coccobacillus were dominant structure among isolates and the strain G8, identified as Acinetobacter junii by 16S rRNA gene sequencing .Finally results showed that biogenic selenium nanoparticle sizes produced by G1, G4, G8, G7was about16,30,13,30 nm respectively indicating G8 could produce smallest particle size. Conclusion:This bacterial isolate has the potential to be used as a bionanofactory for the synthesis of Se nanoparticles with small particle sizes.The synthesis of Se NPs using bacteria appears to be simple and an appropriate method for synthesis of Se NPs with particle size less than 100 nm. Also this approach would be suitable for developing a biotechnological process for largescale production of small Se NPs.

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