Biosynthesis and Export of Membrane-Enveloped Selenium Nanoparticles by <i>Escherichia coli</i>

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 <i>Escherichia coli</i> 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 <i>E. coli</i> 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 <i>E. coli</i> 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.