Biosynthesis of Selenium Nanoparticles and Selenium-Enriched g-Polyglutamic Acid (Se-PGA) by Bacillus subtilis

Biogenic synthesized selenium nanoparticles (SeNPs) have garnered significant interest in both biomedical and agricultural fields. Encapsulation of SeNPs by natural biopolymers can increase the stability and biocompatibility of the nanoparticles. In this work, synthesis of SeNPs by Bacillus subtilis NT147, g-polyglutamic acid (g-PGA) producing strain and production of Se-enriched g-PGA (Se-PGA) were carried out. B. subtilis NT147 was cultured in Luria Bertani broth supplemented with 0.5 mM Na2SeO3 at 37 ± 2 °C with shaking at 150 rpm. From ICP analysis, the selenium amount in the bacterial cells was highest at 18 h (11.3 mg/g cell DW). The TEM image confirmed that SeNPs were spherical shape and produced mainly in the bacterial cells. Particle size analysis using a laser scattering particle size distribution analyzer indicated that small particle sizes 226 and 259 nm were observed at 6 and 12 h, respectively. Poor dispersion and aggregation of SeNPs were observed after 12 h, resulting in larger particles. Production of g-PGA significantly decreased when Na2SeO3 was presented in the culture media. Therefore, B. subtilis NT147 was cultured in a sucrose yeast extract medium containing 5 % sucrose at 37 ± 2 °C with shaking at 150 rpm for 36 h to maximized  g-PGA yield. Then 2.5 mM Na2SeO3 was added into the culture medium and continued shaking for 4 h. Cells were removed and Se-PGA was precipitated. SEM image showed a spherical shape of SeNPs dispersed in g-PGA and the EDS confirmed the presence of Se, C, N and O in the Se-PGA composite suggesting that SeNPs were mainly came out after cell lysis. FTIR spectra of Se-PGA showed peak, indicating the presence of carbonyl and hydroxyl groups, suggesting proteins and amino acids. The Se-PGA at a concentration of 2.0 g/L exhibited 17.96 % of inhibition against Colletotrichum sp. HIGHLIGHTS Synthesis of selenium nanoparticles (SeNPs) by Bacillus subtilis NT147, a g-polyglutamic acid (g-PGA) producing strain was investigated. Se-enriched g-PGA (Se-PGA) production and physiochemical characterization were carried out. Se-PGA exhibited fungal inhibition activity. GRAPHICAL ABSTRACT