Abstract
Biotransformation of selenite by microorganisms is an effective detoxification (in cases of dissimilatory reduction, e.g., to Se0) and assimilation process (when Se is assimilated by cells). However, the current knowledge of the molecular mechanism of selenite reduction remains limited. In this study, a selenite-resistant bacterium was isolated and identified as Proteus sp. YS02. Strain YS02 reduced 93.2% of 5.0 mM selenite to selenium nanoparticles (SeNPs) within 24 h, and the produced SeNPs were spherical and localized intracellularly or extracellularly, with an average dimension of 140 ± 43 nm. The morphology and composition of the isolated and purified SeNPs were characterized using dynamic light scattering (DLS), scanning electron microscopy (SEM) with energy-dispersive X-ray (EDX) spectrometry, and Fourier transform infrared (FTIR) spectroscopy. FTIR spectroscopy indicated the presence of proteins, polysaccharides, and lipids on the surface of the isolated SeNPs. Furthermore, the SeNPs showed excellent antimicrobial activity against several Gram-positive and Gram-negative pathogenic bacteria. Comparative transcriptome analysis was performed to elucidate the selenite reduction mechanism and biosynthesis of SeNPs. It is revealed that 197 genes were significantly upregulated, and 276 genes were significantly downregulated under selenite treatment. Gene ontology and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses revealed that genes associated with ABC transporters, sulfur metabolism, pentose phosphate pathway (PPP), and pyruvate dehydrogenase were significantly enhanced, indicating selenite is reduced by sulfite reductase with PPP and pyruvate dehydrogenase supplying reducing equivalents and energy. This work suggests numerous genes are involved in the response to selenite stress, providing new insights into the molecular mechanisms of selenite bioreduction with the formation of SeNPs.
Highlights
Selenium (Se) is a metalloid that plays a vital role in maintaining human health (Chen et al, 2021)
BLAST search results indicated that the 16S rRNA gene sequence of strain YS02 exhibited 99% sequence similarity to that of Proteus mirabilis ATCC 29906(T)
Phylogenetic analysis showed that strain YS02 belongs to the genus Proteus, with the highest similarity to P. mirabilis (Figure 1)
Summary
Selenium (Se) is a metalloid that plays a vital role in maintaining human health (Chen et al, 2021) It can replace sulfur in cysteine and be co-translationally incorporated as selenocysteine (SeCys) in selenoproteins involved in biological processes, including detoxification, anti-inflammation, antioxidant defense, and thyroid functioning. For enzyme-dependent reduction, reductases encompassing sulfite reductase (Huang et al, 2021), glutathione reductase (Wang et al, 2019), thioredoxin reductase (Hunter, 2014), SerT (Tan et al, 2018), flavoprotein CsrF (Xia et al, 2018), and fumarate reductase (Song et al, 2017b) have been reported to be potentially involved in Se(IV) reduction in various bacterial species, suggesting the reduction of selenite occurs through diverse mechanisms. The current knowledge of the molecular mechanism of microbial selenite reduction is still limited It is largely unknown how bacterial cells respond to selenite stress and what molecular mechanism is utilized by microorganisms to reduce selenite. The antibacterial activity of the SeNPs against Gram-negative and Gram-positive bacteria, such as Escherichia coli and Bacillus subtilis, was investigated to explore the potential of the SeNPs as key assets in the future of healthcare
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