Bacillus-mediated selenium nanoparticle biosynthesis: reduction pathways, cellular processes, and applications

Selenium nanoparticles (SeNPs) have attracted considerable interest due to their unique physicochemical properties, low toxicity compared with inorganic selenium (Se) compounds, and diverse biological activities. Microorganisms, particularly species of the genus Bacillus, represent efficient biological platforms for SeNP synthesis because of their tolerance to selenium oxyanions and their ability to reduce selenite Se (IV) to elemental selenium (Se⁰) under mild conditions. Selenate-reduction is relatively poorly understood in this genus, so the candidate selenate-reductase system proposed here is inferred from selenium-respiring organisms and has not been genetically confirmed in Bacillus. This review summarizes current knowledge on Bacillus-mediated SeNP biosynthesis with an emphasis on microbial selenium transformation, cellular responses, and biochemical processes involved in nanoparticle formation. The roles of redox-associated pathways, reducing equivalents, candidate enzymes, metabolites, and extracellular biomolecules in selenium reduction, nucleation, and stabilization are critically discussed, while highlighting the limitations of current mechanistic understanding and the need for further genetic validation. In addition, this review examines how bacterial strain characteristics and cultivation conditions influence SeNP properties, including particle size, surface chemistry, stability, and biological activity. The applications of Bacillus-derived SeNPs, including antimicrobial, antioxidant, anticancer, agricultural, and environmental applications, are discussed in relation to the nanoparticle characteristics and biological performance. Throughout the review, evidence is explicitly attributed to the relevant source system, distinguishing among Bacillus-derived, other microbial, chemically synthesized, plant-derived, and general SeNP literature. Current biomedical evidence for Bacillus-mediated SeNPs is based predominantly on in-vitro studies. Finally, emerging approaches for improving microbial SeNP production through strain selection, metabolic engineering, and process optimization are highlighted. This review provides an integrated perspective on Bacillus-mediated SeNP biosynthesis, identifies future research directions and offers insights for researchers in microbiology, bioprocess engineering, animal nutrition and agricultural biotechnology.