Study on the Probiotic Properties of Xinjiang-Characteristic Selenium-Enriched Lactic Acid Bacteria and the Distribution of Selenium Element

Selenium, a crucial trace element, has garnered significant attention in functional food development due to its effective conversion into organic forms. This study systematically investigated the selenium enrichment potential and metabolic regulation mechanisms of 50 lactic acid bacteria (LAB) strains from Xinjiang. Through sodium selenite tolerance tests, eight core strains with over 80% selenium enrichment were selected, with optimal enrichment conditions being a 37 °C temperature, 2% sodium chloride concentration, and pH of 6.0 in MRS medium. Functional tests demonstrated that selenium-enriched strains exhibited a significantly enhanced antioxidant capacity (demonstrated by DPPH and ABTS free radical scavenging activities) and improved gastrointestinal fluid tolerance, with strain No.41 showing the most outstanding performance. Scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEM-EDX) revealed nanoscale selenium (1.34 keV) on cell surfaces. Further characterization showed that 68.94% of selenium was incorporated into selenoproteins, 7.61% into nucleic acids, and 7.02% into polysaccharides. Integrated metabolomic and proteomic studies have shown that selenium reduces the content of L-cysteine primarily by replacing sulfur and competing for key sites in cysteine-S-conjugate-β-lyase, S-adenosyl-L-cysteine hydrolase, and homocysteine synthase, ultimately leading to the synthesis of selenocysteine and selenomethionine. A correlation analysis between differential metabolites and proteins revealed selenium's significant impacts on the metabolic networks of LAB, antioxidant mechanisms, energy metabolism, and membrane stability. This research provides new insights for developing selenium-enriched probiotics for functional dairy products and health supplements.