Bioremediation and optimization of selenate reduction by a novel Bacillus cabrialesii strain (Se1) producing red selenium nanoparticles with potential antibacterial activity

Abstract

Background: This study aimed to isolate and identify selenate-reducing bacteria from polluted Egyptian soil. The isolated bacteria could be used to bioremediate contaminated soils and wastewater. Materials and methods: A potent selenate-reducing bacterium was isolated for optimum production of selenium nanoparticles using a Box–Behnken design (BBD) of the response surface methodology. Results: A novel selenate-reducing bacterium, designated Se1, was isolated from an industrial effluent soil in Cairo, Egypt. When cultured in enrichment basal medium and then on nutrient agar medium supplemented with 0.945 g L⁻¹ sodium selenate, the isolate showed characteristic circular, dark red and shiny colonies. This coloration indicates the reduction of selenate to elemental selenium (Se⁰), with a production yield of 108.8 ± 1.846 μmol. The formation of Se⁰ was confirmed with UV–Vis spectroscopy, which revealed characteristic peaks at 224, 229, and 231 nm. X-ray diffraction (XRD) pattern confirmed the amorphous nature of the synthesized Se⁰ nanoparticles (Se-NPs). Fourier-transform infrared (FTIR) spectroscopy identified diverse absorption peaks within the 400–4000 cm⁻¹ range, corresponding to various vibrational modes of chemical bonds, including lipids, proteins, polysaccharides, and functional groups that are present in nanoparticles. Additionally, transmission electron microscopy analysis revealed the presence of Se-NPs within bacterial cells. Based on 16S rRNA gene sequencing, the isolate was identified as Bacillus cabrialesii strain Se1 and deposited in GenBank under accession number PP945477. Optimization experiments revealed that the ideal conditions for Se-NPs formation by the isolate were as follows: 3.6 gL⁻¹ sodium lactate, pH 7.8, 31°C incubation temperature, 7.6 gL⁻¹ selenate concentration, and a ten-day incubation period. Under these conditions, the maximum yield of elemental selenium was 151.311 μmol. The biosynthesized Se-NPs showed potent antibacterial activity against two pathogenic bacteria. Conclusion: This study presents the first documented evidence of selenate reduction by Bacillus cabrialesii, highlighting its potential applications.