Three bacterial strains efficiently reduce selenite to selenium nanoparticles in cell membranes

Microbial synthesis of selenium nanoparticles (SeNPs) as a fertilizer can promote the development of selenium-rich agricultural products. However, most known selenium-reduction strains exhibit a tolerance to selenite of ≤ 100 mmol/L and possess relatively low reduction efficiency. In this study, three strains capable of tolerating selenite concentrations of > 300 mmol/L were screened from selenium-rich soil in Bama, Guangxi, China. Based on 16 S rRNA gene sequence analysis, the three strains were identified as Citrobacter sp.BM-1, Providencia sp.BM-2, and Brucella sp.BM-3. Notably, Brucella sp.BM-3 represents a novel selenium-reducing bacteria. All three strains reduced SeO₃^2- to SeNPs on the cell membrane and subsequently released these nanoparticles outside the cells, forming spherical SeNPs with a particle size of 210-221 nm. Furthermore, qPCR analysis revealed that the selenium-reduction mechanisms in these strains primarily involve the glutathione pathway, which is catalyzed by nitrate reductase and sulfate reductase. These findings suggest that Citrobacter sp.BM-1, Providencia sp.BM-2, and Brucella sp.BM-3 are promising candidates for the synthesis of SeNPs.