Biosynthesis and Export of Membrane-Enveloped Selenium Nanoparticles by Escherichia coli

Bacteria reduce toxic selenium oxyanions, such as selenite, to elemental selenium (Se⁰), forming selenium nanoparticles (SeNPs) either intracellularly or extracellularly. However, the mechanism through which extracellular SeNPs (Ex-SeNPs) are exported remains unclear. In this study, we characterized Ex-SeNPs biosynthesized by Escherichia coli during the aerobic reduction of selenite. The SeNPs appeared within 2 h of exposure, remained extracellular, and displayed a consistent spherical morphology (∼100 nm). Purified Ex-SeNPs consisted of an Se⁰ core enveloped by a membrane-like layer containing lipids, proteins, carbohydrates, peptidoglycan, and lipopolysaccharides. Fluorescence microscopy and gas chromatography-mass spectrometry indicated that the encapsulated membrane originates from the E. coli cell membrane. Notably, mutants deficient in the outer membrane proteins OmpC or TolA failed to excrete SeNPs, resulting in intracellular accumulation despite efficient Se⁰ synthesis. Our findings suggest that E. coli forms SeNPs intracellularly and exports them via an envelope-dependent process, during which the particles may become encapsulated in membrane-like structures. These findings help clarify the mechanism underlying a membrane-dependent pathway for SeNP detoxification and export that had been suggested but not directly demonstrated.