Preparation, characteristic and anti-inflammatory effect of selenium nanoparticle-enriched probiotic strain Enterococcus durans A8-1

Background: Elemental selenium, a new type of selenium supplement, can be biosynthesized via microorganisms. This study is to characterize a patent probiotic bacteria Enterococcus durans A8–1, capable of reducing selenite (Se⁶⁺ or Se⁴⁺) to elemental selenium (Se⁰) with the formation of Se nanoparticles (SeNPs). Methods: The selenium nanoparticles synthesized from A8–1 were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and X-ray photoelectron energy (XPS). The Caco2 cells were used to investigate the effects of Se-enriched A8–1 on the viability, membrane integrity, and the regulation of cellular inflammation through MTT and ELISA assays. The selenium-enriched metabolic function of A8–1 was analyzed by transcriptome sequencing. Results: E. durans A8–1 has the ability to synthesize intracellular SeNPs that are incubated with 60 mg/L sodium selenite for 18 h at 37 °C with 7 % inoculum under aerobic conditions. The selenium-enriched transformation rate increased to 43.46 %. After selenium enrichment, there were no significant morphological changes in E. durans A8–1 cells. The cells also exhibited no cytotoxicity when incubated with Caco-2 cells, and increased cellular proliferation. Furthermore, Se-enriched A8–1 cells antagonize the adhesion of S. typhimurium ATCC14028 onto the surface of Caco-2 cells protecting cell membrane integrity and was assessed by measuring LDH and AKP activities (P <0.001, P <0.001). Moreover, Se-enriched A8–1 could protect Caco-2 cells from inflammation induced by lipopolysaccharide and help the cells alleviate the inflammation through the reduced expression of cytokine IL-8 (P = 0.0012, P <0.001) and TNF-α (P <0.001, P <0.001). Based on transcriptome sequencing in Se-enriched E. durans A8–1 cells, there were 485 up-regulated genes and 322 down-regulated genes (P_adj < 0.05). There were 19 predicted up-regulated genes that are highly related to the potential selenium metabolism pathway, which focuses on the transportation of Na₂SeO₃ by membrane proteins, and gradually reduces Na₂SeO₃ to elemental selenium aggregates that are deposited onto the membrane surface via the intracellular redox response. Conclusion: E. durans A8–1 could convert extracellular selenite into intracellular biological SeNPs via redox pathway with strong selenium-rich metabolism, and its biological SeNPs have anti-inflammatory properties, which have the potential for the development of composite selenium nanomaterials and can be further studied for the function of SeNPs with potential applications.