From selenite to selenium nanoparticle: comparative biosynthetic mechanisms across Bacillus and a roadmap for Bacillus licheniformis.

Selenium nanoparticles (SeNPs) are increasingly studied as biologically active selenium forms with potential applications in biotechnology, medicine, agriculture, and environmental remediation. Microbial synthesis offers an alternative approach for producing SeNPs under biological conditions. Molecular mechanisms governing selenium transformation, nanoparticle assembly, and extracellular stabilisation remain incompletely understood, particularly in Bacillus licheniformis, an industrially relevant selenium-reducing bacterium. This review examines the mechanisms proposed for bacterial SeNP formation across Bacillus species, with emphasis on B. licheniformis and other microorganisms with better characterised selenium transformation. The review discusses major steps associated with SeNP formation, including selenium oxyanion uptake, intracellular reduction, redox regulation, nucleation, extracellular transport, and biomolecular stabilisation. Comparative analysis highlights recurring features, such as thiol-dependent reduction systems, oxidative stress responses, protein-associated nucleation and extracellular polymeric substances, while also showing that mechanisms identified in one bacterial species cannot be directly transferred to another. Where evidence is available, experimentally validated mechanisms are discussed separately from proposed mechanisms inferred from related organisms. SeNP formation is not considered here as a well established biosynthetic programme. In most systems, the available data are equally consistent with detoxification and general thiol redox chemistry. It also remains uncertain how far cells exert active control over particle formation. Although B. licheniformis has been shown to produce SeNPs, the molecular mechanisms underlying its selenium uptake, SeNP nucleation, export, and surface functionalisation are not fully understood. Recent progress has enabled an experimental approach to this question: a respiratory selenate reductase operon has been characterised in the genus (in B. selenatarsenatis (selenate reduced), not yet in B. licheniformis). Closed genome sequences are now available for a SeNP-producing B. licheniformis strain and for the sister taxon B. paralicheniformis, and marker-free knockout, CRISPR interference and transposon mutagenesis are established in B. licheniformis. Further targeted multi-omics and loss-of-function research will be required to elucidate the role of these candidate pathways in the formation of SeNP in B. licheniformis and their potential for use in strain improvement.