Selenite reduction by the obligate aerobic bacterium Comamonas testosteroni S44 isolated from a metal-contaminated soil

Fe-S cluster homeostasis and beyond: The multifaceted roles of IscR

The role of IscR in regulating the transcription of genes involved in Fe-S cluster homeostasis has been well established for the model organism Escherichia coli K12. In this bacterium, IscR coordinates expression of the Isc and Suf Fe-S cluster assembly pathways to meet cellular Fe-S cluster demands shaped by a variety of environmental cues. However, since its initial discovery nearly 25 years ago, there has been growing evidence that IscR function extends well beyond Fe-S cluster homeostasis, not only in E. coli, but in bacteria of diverse lifestyles. Notably, pathogenic bacteria have exploited the ability of IscR to respond to changes in oxygen tension, oxidative and nitrosative stress, and iron availability to navigate their trajectory in their respective hosts as changes in these cues are frequently encountered during host infection. In this review, we highlight these broader roles of IscR in different cellular processes and, in particular, discuss the importance of IscR as a virulence factor for many bacterial pathogens.

Electroactive Brevundimonas diminuta consortium mediated selenite bioreduction, biogenesis of selenium nanoparticles and bio-electricity generation

In this study, highly selenite-resistant strains belonging to Brevundimonas diminuta (OK287021, OK287022) genus were isolated from previously operated single chamber microbial fuel cell (SCMFC). The central composite design showed that the B. diminuta consortium could reduce selenite. Under optimum conditions, 15.38 Log CFU mL-1 microbial growth, 99.08% Se(IV) reduction, and 89.94% chemical oxygen demand (COD) removal were observed. Moreover, the UV-visible spectroscopy (UV) and Fourier transform infrared spectroscopy (FTIR) analyses confirmed the synthesis of elemental selenium nanoparticles (SeNPs). In addition, transmission electron microscopy (TEM) and scanning electron microscope (SEM) revealed the formation of SeNPs nano-spheres. Besides, the bioelectrochemical performance of B. diminuta in the SCMFC illustrated that the maximum power density was higher in the case of selenite SCMFCs than those of the sterile control SCMFCs. Additionally, the bioelectrochemical impedance spectroscopy and cyclic voltammetry characterization illustrated the production of definite extracellular redox mediators that might be involved in the electron transfer progression during the reduction of selenite. In conclusion, B. diminuta whose electrochemical activity has never previously been reported could be a suitable and robust biocatalyst for selenite bioreduction along with wastewater treatment, bioelectricity generation, and economical synthesis of SeNPs in MFCs.

The actinomycete Kitasatospora sp. SeTe27, subjected to adaptive laboratory evolution (ALE) in the presence of selenite, varies its cellular morphology, redox stability, and tolerance to the toxic oxyanion

The effects of oxyanions selenite (SeO32-) in soils are of high concern in ecotoxicology and microbiology as they can react with mineral particles and microorganisms. This study investigated the evolution of the actinomycete Kitasatospora sp. SeTe27 in response to selenite. To this aim, we used the Adaptive Laboratory Evolution (ALE) technique, an experimental approach that mimics natural evolution and enhances microbial fitness for specific growth conditions. The original strain (wild type; WT) isolated from uncontaminated soil gave us a unique model system as it has never encountered the oxidative damage generated by the prooxidant nature of selenite. The WT strain exhibited a good basal level of selenite tolerance, although its growth and oxyanion removal capacity were limited compared to other environmental isolates. Based on these premises, the WT and the ALE strains, the latter isolated at the end of the laboratory evolution procedure, were compared. While both bacterial strains had similar fatty acid profiles, only WT cells exhibited hyphae aggregation and extensively produced membrane-like vesicles when grown in the presence of selenite (challenged conditions). Conversely, ALE selenite-grown cells showed morphological adaptation responses similar to the WT strain under unchallenged conditions, demonstrating the ALE strain improved resilience against selenite toxicity. Whole-genome sequencing revealed specific missense mutations in genes associated with anion transport and primary and secondary metabolisms in the ALE variant. These results were interpreted to show that some energy-demanding processes are attenuated in the ALE strain, prioritizing selenite bioprocessing to guarantee cell survival in the presence of selenite. The present study indicates some crucial points for adapting Kitasatospora sp. SeTe27 to selenite oxidative stress to best deal with selenium pollution. Moreover, the importance of exploring non-conventional bacterial genera, like Kitasatospora, for biotechnological applications is emphasized.

Anaerobic selenite-reducing bacteria and their metabolic potentials in Se-rich sediment revealed by the combination of DNA-stable isotope probing, metagenomic binning, and metatranscriptomics

Microorganisms play a critical role in the biogeochemical cycling of selenium (Se) in aquatic environments, particularly in reducing the toxicity and bioavailability of selenite (Se(IV)). This study aimed to identify putative Se(IV)-reducing bacteria (Se^IVRB) and investigate the genetic mechanisms underlying Se(IV) reduction in anoxic Se-rich sediment. Initial microcosm incubation confirmed that Se(IV) reduction was driven by heterotrophic microorganisms. DNA stable-isotope probing (DNA-SIP) analysis identified Pseudomonas, Geobacter, Comamonas, and Anaeromyxobacter as putative Se^IVRB. High-quality metagenome-assembled genomes (MAGs) affiliated with these four putative Se^IVRB were retrieved. Annotation of functional gene indicated that these MAGs contained putative Se(IV)-reducing genes such as DMSO reductase family, fumarate and sulfite reductases. Metatranscriptomic analysis of active Se(IV)-reducing cultures revealed significantly higher transcriptional levels of genes associated with DMSO reductase (serA/PHGDH), fumarate reductase (sdhCD/frdCD), and sulfite reductase (cysDIH) compared to those in cultures not amended with Se(IV), suggesting that these genes played important roles in Se(IV) reduction. The current study expands our knowledge of the genetic mechanisms involved in less-understood anaerobic Se(IV) bio-reduction. Additinally, the complementary abilities of DNA-SIP, metagenomics, and metatranscriptomics analyses are demonstrated in elucidating the microbial mechanisms of biogeochemical processes in anoxic sediment.

Microbial community composition analysis to decipher the possible role of inherent bacteria for in-situ arsenic (As) bioremediation

Biogeochemical reduction and mobilization of sediment-bound arsenic (As) is the major concern for widespread groundwater As contamination in the middle Gangetic plains. The present work examines a microcosm based bio-stimulation study and substrate amendments over 45 days to analyze the bacterial community structure and distribution to indicate the possible in-situ bioremediation strategy in the area. Initially, Bacterial phyla Proteobacteria was predominantly present in all the samples, followed by Actinobacteria, Bacteroidetes, and Firmicutes whereas Cyanobacteria was noted as the minor group. In genus level, Delftia, Acinetobacter, Lysobacter, Bacillus, and Pseudomonas were the major groups of bacteria in the As-rich aquifer system, while Planctomycetes dominated the bio-stimulated samples, followed by a minute portion of Proteobacteria. Alpha diversity and Chaol curve further determined the species richness in the samples with an As tolerant capacity of 152.28 ppb. The presence of γ-Proteobacteria as the dominating member in high As-content water indicated their predominant role in As mobilization, whereas, dominance of α-Proteobacterial members in low As-content water indicated their involvement in As detoxification. The complete change in microbial community structure within the bio-stimulated conditions indicated the extensive role of arsenite-oxidizing microbial communities within different levels of As-contaminated areas in Bihar that will enlighten the significant role of these communities in As-biogeochemical cycle.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03612-0.

Production of selenium nanoparticles occurs through an interconnected pathway of sulphur metabolism and oxidative stress response in Pseudomonas putida KT2440

The soil bacterium Pseudomonas putida KT2440 has been shown to produce selenium nanoparticles aerobically from selenite; however, the molecular actors involved in this process are unknown. Here, through a combination of genetic and analytical techniques, we report the first insights into selenite metabolism in this bacterium. Our results suggest that the reduction of selenite occurs through an interconnected metabolic network involving central metabolic reactions, sulphur metabolism, and the response to oxidative stress. Genes such as sucA, D2HGDH and PP_3148 revealed that the 2-ketoglutarate and glutamate metabolism is important to convert selenite into selenium. On the other hand, mutations affecting the activity of the sulphite reductase decreased the bacteria's ability to transform selenite. Other genes related to sulphur metabolism (ssuEF, sfnCE, sqrR, sqr and pdo2) and stress response (gqr, lsfA, ahpCF and sadI) were also identified as involved in selenite transformation. Interestingly, suppression of genes sqrR, sqr and pdo2 resulted in the production of selenium nanoparticles at a higher rate than the wild-type strain, which is of biotechnological interest. The data provided in this study brings us closer to understanding the metabolism of selenium in bacteria and offers new targets for the development of biotechnological tools for the production of selenium nanoparticles.

Transcriptional response of the xerotolerant Arthrobacter sp. Helios strain to PEG-induced drought stress

A new bacterial strain has been isolated from the microbiome of solar panels and classified as Arthrobacter sp. Helios according to its 16S rDNA, positioning it in the “Arthrobacter citreus group.” The isolated strain is highly tolerant to desiccation, UV radiation and to the presence of metals and metalloids, while it is motile and capable of growing in a variety of carbon sources. These characteristics, together with observation that Arthrobacter sp. Helios seems to be permanently prepared to handle the desiccation stress, make it very versatile and give it a great potential to use it as a biotechnological chassis. The new strain genome has been sequenced and its analysis revealed that it is extremely well poised to respond to environmental stresses. We have analyzed the transcriptional response of this strain to PEG6000-mediated arid stress to investigate the desiccation resistance mechanism. Most of the induced genes participate in cellular homeostasis such as ion and osmolyte transport and iron scavenging. Moreover, the greatest induction has been found in a gene cluster responsible for biogenic amine catabolism, suggesting their involvement in the desiccation resistance mechanism in this bacterium.

Novel mechanisms of selenite reduction in Bacillus subtilis 168：Confirmation of multiple-pathway mediated remediation based on transcriptome analysis

Selenite biotransformation by microorganisms is an effective detoxification and assimilation process. Bacillus subtilis is a probiotic bacterium that can reduce Se(IV) to SeNPs under aerobic conditions. However, current knowledge on the molecular mechanisms of selenite reduction by B. subtilis remains limited. Here, the reduction of Se(IV) by probiotic bacterium Bacillus subtilis 168 was systematically analysed, and the molecular mechanisms of selenium nanoparticle (SeNPs) formation were characterised in detail. B. subtilis 168 reduced 5.0 mM selenite by nearly 40% in 24 h, and the produced SeNPs were spherical and localised intracellularly or extracellularly. FTIR (Fourier transform infrared) spectroscopy suggested the presence of proteins, lipids, and carbohydrates on the surface of the isolated SeNPs. Transcriptome data analysis revealed that the expression of genes associated with the proline metabolism, glutamate metabolism, and sulfite metabolism pathways was significantly up-regulated. Gene mutation and complementation revealed the ability of PutC, GabD, and CysJI to reduce selenite in vivo. In vitro experiments demonstrated that PutC, GabD, and CysJI could catalyse the reduction of Se(IV) under optimal conditions using NADPH as a cofactor. To the best of our knowledge, our study is the first to demonstrate the involvement of PutC and GabD in selenite reduction. Particularly, our findings demonstrated that the reduction of Se(IV) was mediated by multiple pathways both in vivo and in vitro. Our findings thus provide novel insights into the molecular mechanisms of Se(VI) reduction in aerobic bacteria.

Highly Selenite-Tolerant Strain Proteus mirabilis QZB-2 Rapidly Reduces Selenite to Selenium Nanoparticles in the Cell Membrane

The application of biosynthesized nano-selenium fertilizers to crops can improve their nutrient levels by increasing their selenium content. However, microorganisms with a high selenite tolerance and rapid reduction rate accompanied with the production of selenium nanoparticles (SeNPs) at the same time have seldom been reported. In this study, a bacterial strain showing high selenite resistance (up to 300 mM) was isolated from a lateritic red soil and identified as Proteus mirabilis QZB-2. This strain reduced nearly 100% of 1.0 and 2.0 mM selenite within 12 and 18 h, respectively, to produce SeNPs. QZB-2 isolate reduced SeO₃^2– to Se⁰ in the cell membrane with NADPH or NADH as electron donors. Se⁰ was then released outside of the cell, where it formed spherical SeNPs with an average hydrodynamic diameter of 152.0 ± 10.2 nm. P. mirabilis QZB-2 could be used for SeNPs synthesis owing to its simultaneously high SeO₃^2– tolerance and rapid reduction rate.

Improving selenium accumulation in broilers using Escherichia coli Nissle 1917 with surface-displayed selenite reductase SerV01

Selenium levels have a critical impact on livestock and poultry, and selenium nanoparticles (SeNPs) have shown significant efficiency in supplementation. This study identified a high-efficiency selenite reductase, SerV01, in Staphylococcus aureus LZ-01, which can convert Se₂O₃^2- to SeNPs. Subsequently, SerV01 was introduced into the intestines of the broilers using the surface display-engineered E. coli Nissle 1917 (EcN). The results showed that the engineered bacteria (EcN-IS) significantly increased the selenium content by 0.87 mg kg^-1, 0.52 mg kg^-1, and 6.10 mg L^-1 in the liver, breast muscle, and serum, respectively. With SeNPs + EcN-IS treatment, glutathione peroxidase and thioredoxin reductase levels reached 0.7536 ± 0.03176 U μL^-1 protein and 2.463 ± 0.1685 U μL^-1 protein, respectively. With the modified probiotics, the proportion of beneficial intestinal flora increased, with Lactobacillus and Propionibacterium accounting for 75.85% and 0.19%. This technology provides a novel idea to facilitate the exploitation of selenium in broiler diets and improve antioxidant capability.

An economical strategy towards the managing of selenium pollution from contaminated water: A current state-of-the-art review

During the last few decades, contamination of selenium (Se) in groundwater has turned out to be a major environmental concern to provide safe drinking water. The content of selenium in such contaminated water might range from 400 to 700 μg/L, where bringing it down to a safe level of 40 μg/L for municipal water supply employing appropriate methodologies is a major challenge for the global researcher communities. The current review focuses mostly on the governing selenium remediation technologies such as coagulation-flocculation, electrocoagulation, bioremediation, membrane-based approaches, adsorption, electro-kinetics, chemical precipitation, and reduction methods. This study emphasizes on the development of a variety of low-cost adsorbents and metal oxides for the selenium decontamination from groundwater as a cutting-edge technology development along with their applicability, and environmental concerns. Moreover, after the removal, the recovery methodologies using appropriate materials are analyzed which is the need of the hour for the reutilization of selenium in different processing industries for the generation of high valued products. From the literature survey, it has been found that hematite modified magnetic nanoparticles (MNP) efficiently adsorb Se (IV) (25.0 mg/g) from contaminated groundwater. MNP@hematite reduced Se (IV) concentration from 100 g/L to 10 g/L in 10 min at pH 4-9 using a dosage of 1 g/L. In 15 min, the magnetic adsorbent can be recycled and regenerated using a 10 mM NaOH solution. The adsorption and desorption efficiencies were over 97% and 82% for five consecutive cycles, respectively. To encourage the notion towards scale-up, a techno-economic evaluation with possible environmentally sensitive policy analysis has been introduced in this article to introspect the aspects of sustainability. This type of assessment is anticipated to be extremely encouraging to convey crucial recommendations to the scientific communities in order to produce high efficiency selenium elimination and further recovery from contaminated groundwater.

Isolation of Pb(II)-reducing bacteria and demonstration of biological Pb(II) reduction to metallic Pb

Pb(II) contamination imposes serious threats to human health and the environment. Biological reduction of Pb(II) to metallic Pb is an attractive method for the remediation of Pb(II)-contaminated water and sediments. In this study, Pb(II)-reducing microorganisms were isolated by the dilution-to-extinction (DTE) and streak-plate methods. As a result, Delftia acidovorans, Azonexus caeni, and Comamonas testosteroni were successfully isolated. At a high lead concentration (10 mg-Pb(II)/L), each of the isolated D. acidovorans strain Pb11 and A. caeni strain Pb2 cultures showed successful utilization of Pb(II), resulting in a 5.15- and 8.14-fold growth in 3 days, respectively. Pb(II) reduction to metallic Pb by D. acidovorans strain Pb11 and A. caeni strain Pb2 was confirmed using scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS) was coupled with X-ray photoelectron spectroscopy (XPS). This strategic analysis was necessary to confirm the formation of metallic Pb separately from lead phosphate precipitates which are inevitable in the biological Pb(II) removal experiments. Among the 3 isolated microbes, C. testosteroni strain Pb3 did not leave immobile and detectable Pb solids in SEM-EDS analyses. D. acidovorans and A. caeni are recommended for engineered remediation of Pb(II)-contaminated wastewater and sediments.

Radiolytic synthesis and characterization of selenium nanoparticles: comparative biosafety evaluation with selenite and ionizing radiation

The goal of this work is use a green chemistry route to synthesize selenium nanoparticles (SeNPs) that do not trigger oxidative stress, typical of metallic, oxide metallic and carbonaceous nanostructures, and supply the same beneficial effects as selenium nanostructures. SeNPs were synthesized using a radiolytic method involving irradiating a solution containing sodium selenite (Se⁴⁺) as the precursor in 1% Yeast extract, 2% Peptone, 2% Glucose (YPG) liquid medium with gamma-rays (⁶⁰Cobalt). The method did not employ any hazardous reducing agents. Saccharomyces cerevisiae cells were incubated with 1 mM SeNPs for 24 h and/or then challenged with 400 Gy of ionizing radiation were assessed for viability and biomarkers of oxidative stress: lipid peroxidation, protein carbonylation, free radical generation, and total sulfhydryl content. Spherical SeNPs with variable diameters (from 100 to 200 nm) were formed after reactions of sodium selenite with hydrated electrons (e_aq^-) and hydrogen radicals (H·). Subsequent structural characterizations indicated an amorphous structure composed of elemental selenium (Se⁰). Compared to 1 mM selenite, SeNPs were considered safe and less toxic to Saccharomyces cerevisiae cells as did not elicit significant modifications in cell viability or oxidative stress parameters except for increased protein carbonylation. Furthermore, SeNPs treatment afforded some protection against ionizing radiation exposure. SeNPs produced using green chemistry attenuated the reactive oxygen species generation after in vitro ionizing radiation exposure opens up tremendous possibilities for radiosensitizer development.

Microbial reduction and resistance to selenium: Mechanisms, applications and prospects

Selenium is an essential trace element for humans, animals and microorganisms. Microbial transformations, in particular, selenium dissimilatory reduction and bioremediation applications have received increasing attention in recent years. This review focuses on multiple Se-reducing pathways under anaerobic and aerobic conditions, and the phylogenetic clustering of selenium reducing enzymes that are involved in these processes. It is emphasized that a selenium reductase may have more than one metabolic function, meanwhile, there are several Se(VI) and/or Se(IV) reduction pathways in a bacterial strain. It is noted that Se(IV)-reducing efficiency is inconsistent with Se(IV) resistance in bacteria. Moreover, we discussed the links of selenium transformations to biogeochemical cycling of other elements, roles of Se-reducing bacteria in soil, plant and digestion system, and the possibility of using functional genes involved in Se transformation as biomarker in different environments. In addition, we point out the gaps and perspectives both on Se transformation mechanisms and applications in terms of bioremediation, Se fortification or dietary supplementation.

Xerotolerance: A New Property in Exiguobacterium Genus

The highly xerotolerant bacterium classified as Exiguobacterium sp. Helios isolated from a solar panel in Spain showed a close relationship to Exiguobacterium sibiricum 255-15 isolated from Siberian permafrost. Xerotolerance has not been previously described as a characteristic of the extremely diverse Exiguobacterium genus, but both strains Helios and 255-15 showed higher xerotolerance than that described in the reference xerotolerant model strain Deinococcus radiodurans. Significant changes observed in the cell morphology after their desiccation suggests that the structure of cellular surface plays an important role in xerotolerance. Apart from its remarkable resistance to desiccation, Exiguobacterium sp. Helios strain shows several polyextremophilic characteristics that make it a promising chassis for biotechnological applications. Exiguobacterium sp. Helios cells produce nanoparticles of selenium in the presence of selenite linked to its resistance mechanism. Using the Lactobacillus plasmid pRCR12 that harbors a cherry marker, we have developed a transformation protocol for Exiguobacterium sp. Helios strain, being the first time that a bacterium of Exiguobacterium genus has been genetically modified. The comparison of Exiguobacterium sp. Helios and E. sibiricum 255-15 genomes revealed several interesting similarities and differences. Both strains contain a complete set of competence-related DNA transformation genes, suggesting that they might have natural competence, and an incomplete set of genes involved in sporulation; moreover, these strains not produce spores, suggesting that these genes might be involved in xerotolerance.

TRAP transporter TakP: a key player in the resistance against selenite-induced oxidative stress in Rhodobacter sphaeroides

Almost one-third of all proteins require metal ions as an essential component in key biological processes and approximately half of all enzymes are associated with one or more metal ions. The naturally occurring selenium is very toxic at higher levels, but few bacteria can reduce it into the less toxic insoluble elemental selenium. Selenium is required for the synthesis of selenocysteine, an essential residue involved in the active sites of various enzymes. The purple non-sulphur bacteria, Rhodobacter sphaeroidesis demonstrated for its selenite reduction capacity. The exact mechanism of selenite toxicity is unknown but it reacts with glutathione to form selenodiglutathione, producing the highly toxic compounds namely, H₂O₂and O₂^-. A R. sphaeroidesstrain with mutated takP gene, a member of the TRAP (tripartite ATP-independent periplasmic) family of transporter, was reported to be showing more resistance towards selenite in the growth medium but the reason for the resistance is unknown. TRAP transporters are the best-studied family of substrate-binding protein and in our previous study it was confirmed that the gene takP in R. sphaeroides is down-regulated by a small non-coding RNA SorY, providing more resistance to the bacterium against the oxidative stress. By comparative growth analysis and sensitivity assays in the presence of 2 mM selenite, it was observed that the SorY knockout strain is more sensitive to selenite while overexpression of the sRNA conferred more resistance to the bacterium like the takP mutant strain. TakP is involved in the import of malate into the cell, which under oxidative stress needs to be down-regulated to limit malate flux into the cell. Limited malate flux leads to metabolic rearrangements in the cell to avoid excessive generation of prooxidant NADH and facilitate constant generation of antioxidant NADPH. In the presence and absence of selenite, a drastic increase in the NADPH and decrease in the NADH levels are reported respectively. Accumulation of metallic selenium in the cytoplasm was detected via atomic absorption spectrophotometer and our analysis clearly demonstrated the presence of more selenium in the electron micrographs of the SorY knockout strain compared to the takP mutant grown under dark semi-aerobic growth conditions in the presence of selenite. Hence based on our analysis, it is confirmed that lack of TakP transporter led to reduced selenite influx into the cytoplasm, relieving cells with limited generation of ROS, eventually exhibiting more resistance against selenite-induced oxidative stress.

Selenite bioreduction and biosynthesis of selenium nanoparticles by Bacillus paramycoides SP3 isolated from coal mine overburden leachate

A native strain of Bacillus paramycoides isolated from the leachate of coal mine overburden rocks was investigated for its potential to produce selenium nanoparticles (SeNPs) by biogenic reduction of selenite, one of the most toxic forms of selenium. 16S rDNA sequencing was used to identify the bacterial strain (SP3). The SeNPs were characterized using spectroscopic (UV-Vis absorbance, dynamic light scattering, X-ray diffraction, and Raman), surface charge measurement (zeta potential), and ultramicroscopic (FESEM, EDX, FETEM) analyses. SP3 exhibited extremely high selenite tolerance (1000 mM) and reduced 10 mM selenite under 72 h to produce spherical monodisperse SeNPs with an average size of 149.1 ± 29 nm. FTIR analyses indicated exopolysaccharides coating the surface of SeNPs, which imparted a charge of -29.9 mV (zeta potential). The XRD and Raman spectra revealed the SeNPs to be amorphous. Furthermore, biochemical assays and microscopic studies suggest that selenite was reduced by membrane reductases. This study reports, for the first time, the reduction of selenite and biosynthesis of SeNPs by B. paramycoides, a recently discovered bacterium. The results suggest that B. paramycoides SP3 could be exploited for eco-friendly removal of selenite from contaminated sites with the concomitant biosynthesis of SeNPs.

Biosynthesis of Selenium Nanoparticles (via Bacillus subtilis BSN313), and Their Isolation, Characterization, and Bioactivities

Among the trace elements, selenium (Se) has great demand as a health supplement. Compared to its other forms, selenium nanoparticles have minor toxicity, superior reactivity, and excellent bioavailability. The present study was conducted to produce selenium nanoparticles (SeNPs) via a biosynthetic approach using probiotic Bacillus subtilis BSN313 in an economical and easy manner. The BSN313 exhibited a gradual increase in Se reduction and production of SeNPs up to 5-200 µg/mL of its environmental Se. However, the capability was decreased beyond that concentration. The capacity for extracellular SeNP production was evidenced by the emergence of red color, then confirmed by a microscopic approach. Produced SeNPs were purified, freeze-dried, and subsequently characterized systematically using UV-Vis spectroscopy, FTIR, Zetasizer, SEM-EDS, and TEM techniques. SEM-EDS analysis proved the presence of selenium as the foremost constituent of SeNPs. With an average particle size of 530 nm, SeNPs were shown to have a -26.9 (mV) zeta potential and -2.11 µm cm/Vs electrophoretic mobility in water. SeNPs produced during both the 24 and 48 h incubation periods showed good antioxidant activity in terms of DPPH and ABST scavenging action at a concentration of 150 µg/mL with no significant differences (p > 0.05). Moreover, 200 µg/mL of SeNPs showed antibacterial reactivity against Escherichia coli ATCC 8739, Staphylococcus aureus ATCC 9027, and Pseudomonas aeruginosa ATCC 25923. In the future, this work will be helpful to produce biogenic SeNPs using probiotic Bacillus subtilis BSN313 as biofactories, with the potential for safe use in biomedical and nutritional applications.

Two new selenite reducing bacterial isolates from paddy soil and the potential Se biofortification of paddy rice

Selenium (Se) is an essential element for human health. Se-enriched agricultural products can promote people's intake of Se. Microorganisms play an important role in Se cycling. In this study, two new bacterial strains were isolated from paddy soil and were identified as Chitinophaga sp. and Comamonas testosteroni, respectively. More than 44% and 39% of 1.0 mM selenite were reduced in 84 h by them using yeast extract as carbon source, respectively. Scanning electron microscope (SEM) and Energy dispersive X-ray spectrometry (EDS) results indicated that the reduction product of selenite was nanometer Se. These strains could promote the available Se in soil and the content of Se in rice plants in pot experiments. Organic combined Se in soils was increased up to 35%, accompanied by the 92% and 130% increase of Se in rice plants. To our best knowledge, this is the first report of Se reduction by Chitinophaga. This work might provide a prospective strategy for microbial fortification of Se in corps.

Performance Evaluation of Selenite (SeO32−) Reduction by Enterococcus spp

Lactic acid bacteria (LAB) such as Enterococcus spp. have an advantage over several bacteria because of their ability to easily adapt to extreme conditions which include high temperatures, highly acidic or alkaline conditions and toxic metals. Although many microorganisms have been shown to reduce selenite (SeO32−) to elemental selenium (Se0), not much work has been done on the combined effect of Enterococcus spp. In this study, aerobic batch reduction of different selenite concentrations (1, 3 and 5 mM) was conducted using Enterococcus hermanniensis sp. and Enterococcus gallinarum sp. (3.5 h, 35 ± 2 °C, starting pH > 8.5). Results from the experiments showed that the average reductions rates were 0.608, 1.921 and 3.238 mmol·(L·h)−1, for the 1, 3 and 5 mM SeO32− concentrations respectively. In addition, more selenite was reduced for the 5 mM concentration compared to the 1 and 3 mM concentrations albeit constant biomass being used for all experiments. Other parameters which were monitored were the glucose consumption rate, protein variation, pH and ORP (oxidation reduction potential). TEM analysis was also conducted and it showed the location of electron-dense selenium nanoparticles (SeNPs). From the results obtained in this study, the authors concluded that Enterococcus species’s high adaptability makes it suitable for rapid selenium reduction and biosynthesis of elemental selenium.

Antimicrobial Activity of Se-Nanoparticles from Bacterial Biotransformation

Selenium nanoparticles (SeNPs) are gaining importance in the food and medical fields due to their antibacterial properties. The microbial inhibition of these kinds of particles has been tested in a wide range of Gram (+) and Gram (−) pathogenic bacteria. When SeNPs are synthesized by biological methods, they are called biogenic SeNPs, which have a negative charge caused by their interaction between surface and capping layer (bioorganic material), producing their high stability. This review is focused on SeNPs synthesis by bacteria and summarizes the main factors that influence their main characteristics: shape, size and surface charge, considering the bacteria growth conditions for their synthesis. The different mechanisms of antimicrobial activity are revised, and this review describes several biosynthesis hypotheses that have been proposed due to the fact that the biological mechanism of SeNP synthesis is not fully known.

Biological selenite removal and recovery of selenium nanoparticles by haloalkaliphilic bacteria isolated from the Nakdong River

Microbial selenite reduction has increasingly attracted attention from the scientific community because it allows the separation of toxic Se from waste sources with the concurrent recovery of Se nanoparticles, a multifunctional material in nanotechnology industries. In this study, four selenite-reducing bacteria, isolated from a river water sample, were found to reduce selenite by > 85% within 3 d of incubation, at ambient temperature. Among them, strain NDSe-7, belonging to genus Lysinibacillus, can reduce selenite and produce Se nanospheres in alkaline conditions, up to pH 10.0, and in salinity of up to 7.0%. This strain can reduce 80 mg/L of selenite to elemental Se within 24 h at pH 6.0-8.0, at a temperature of 30-40 °C, and salinity of 0.1-3.5%. Strain NDSe-7 exhibited potential for use in Se removal and recovery from industrial saline wastewater with high alkalinity. This study indicates that extremophilic microorganisms for environmental remediation can be found in a conventional environment.

Phytostimulatory Influence of Comamonas testosteroni and Silver Nanoparticles on Linum usitatissimum L. under Salinity Stress

They were shifting in land use increases salinity stress, significant abiotic stress affecting plant growth, limiting crop productivity. This work aimed to improve Linum usitatissimum L. (linseed) growth under salinity using Comamonas testosteroni and silver nanoparticles (AgNPs). AgNPs were fabricated exploiting Rosmarinus officinalis and monitored by U.V./Vis spectrophotometry scanning electron microscopy (SEM) and Fourier transforms infrared spectroscopy (FTIR). Photosynthetic pigments, enzymatic and nonenzymatic antioxidants of linseed were investigated under salt stress in treated and untreated plants with C. testosteroni alongside AgNPs. Our findings recorded the formation of AgNPs at 457 nm, which were globular and with a diameter of 75 nm. Notably, chlorophyll-a, b, and total chlorophyll reduction while enhanced carotenoids and anthocyanin contents were attained under salinity stress. Total dissoluble sugars, proline, and dissoluble proteins, H₂O₂, malondialdehyde, enzymatic and nonenzymatic antioxidants were significantly elevated in NaCl well. Combined AgNPs and C. testosteroni elevated photosynthetic pigments. Also, they led to the mounting of soluble sugars, proline, and soluble proteins. H₂O₂ and malondialdehyde decreased while enzymatic and nonenzymatic antioxidants increased in response to AgNPs, C. testosteroni, and their combination. Thus, AgNPs and C. testosteroni might bio-fertilizers to improve linseed crop productivity under salinity stress.

NAD(P)H-dependent thioredoxin-disulfide reductase TrxR is essential for tellurite and selenite reduction and resistance in Bacillus sp. Y3

Microbial reduction of selenite [Se(IV)] and tellurite [Te(IV)] to Se(0) and Te(0) can function as a detoxification mechanism and serve in energy conservation. In this study, Bacillus sp. Y3 was isolated and demonstrated to have an ability of simultaneous reduction of Se(IV) and Te(IV) during aerobic cultivation, with reduction efficiencies of 100% and 90%, respectively. Proteomics analysis revealed that the putative thioredoxin disulfide reductase (TrxR) and sulfate and energy metabolic pathway proteins were significantly upregulated after the addition of Se(IV) and Te(IV). qRT-PCR also showed an increased trxR transcription level in the presence of Se(IV) and Te(IV). Compared with a wild-type Escherichia coli strain, the TrxR-overexpressed E. coli strain showed higher Se(IV) and Te(IV) resistance levels and reduction efficiencies. Additionally, the TrxR showed in vitro Se(IV) and Te(IV) reduction activities when NADPH or NADH were present. When NADPH was used as the electron donor, the optimum conditions for enzyme activities were pH 8.0 and 37°C. The Km values of Te(IV) and Se(IV) were 16.31 and 2.91 mM, and the Vmax values of Te(IV) and Se(IV) were 12.23 and 11.20 µM min-1 mg-1, respectively. The discovery of the new reductive enzyme TrxR enriches the repertoire of the bacterial Se(IV) and Te(IV) resistance and reduction mechanisms. Bacillus sp. Y3 can efficiently reduce Se(IV) and Te(IV) simultaneously. Strain Y3 provides potential applications for selenite and tellurite bioremediation. The TrxR enzyme shows high catalytic activity for reducing Se(IV) and Te(IV). The discovery of TrxR enriches the bacterial Se(IV) and Te(IV) reduction mechanisms.

Green Synthesis of Selenium and Tellurium Nanoparticles: Current Trends, Biological Properties and Biomedical Applications

The synthesis and assembly of nanoparticles using green technology has been an excellent option in nanotechnology because they are easy to implement, cost-efficient, eco-friendly, risk-free, and amenable to scaling up. They also do not require sophisticated equipment nor well-trained professionals. Bionanotechnology involves various biological systems as suitable nanofactories, including biomolecules, bacteria, fungi, yeasts, and plants. Biologically inspired nanomaterial fabrication approaches have shown great potential to interconnect microbial or plant extract biotechnology and nanotechnology. The present article extensively reviews the eco-friendly production of metalloid nanoparticles, namely made of selenium (SeNPs) and tellurium (TeNPs), using various microorganisms, such as bacteria and fungi, and plants' extracts. It also discusses the methodologies followed by materials scientists and highlights the impact of the experimental sets on the outcomes and shed light on the underlying mechanisms. Moreover, it features the unique properties displayed by these biogenic nanoparticles for a large range of emerging applications in medicine, agriculture, bioengineering, and bioremediation.

Human health risk of vanadium in farmland soils near various vanadium ore mining areas and bioremediation assessment

Various kinds of vanadium (V) ore mining areas produced serious contamination have been widely recognized, while less relevant research was about the associated health risk and V distribution level for farmland soils around. This study assessed the contamination characteristics and associated human health risk of V in the surface farmland soils near various V ore mining areas. The bioremediation of V contamination by indigenous microbes from them was also evaluated. The farmland soils near stone coal area (Hunan province, China) showed the highest mean concentration of V (543.91 mg/kg), posing high non-carcinogenic risks, with high hazard quotient (HQ) value of 1.29 for children. While, V values of sampled soils near V titanomagnetite, petroleum associated minerals and uvanite areas were lower than that near stone coal area, also with lower HQ values (<1.00). Within 60 h, the removal efficiency of V(V) reached 98.4% with farmland soils near uvanite area, suggesting feasibility of V bioremediation via indigenous microbes. Bacterial communities after long-term cultivation (240 d) with V(V) were dominated by native microbes able to tolerate or reduce the toxicity of V(V), such as Ruminococcaceae_incertae_sedis, Trichococcus and Comamonas. This work is helpful for calling attention to V pollution of farmland near various V ore mining areas and formulating effective strategies for V(V) contamination bioremediation.

Bacillus safensis JG-B5T affects the fate of selenium by extracellular production of colloidally less stable selenium nanoparticles

Understanding the impact of microorganisms on the mobility of selenium (Se) is important for predicting the fate of toxic Se in the environment and improving wastewater treatment technologies. The bacteria strain Bacillus safensis JG-B5T, isolated from soil in a uranium mining waste pile, can influence the Se speciation in the environment and engineered systems. However, the mechanism and conditions of this process remain unknown. This study found that the B. safensis JG-B5T is an obligate aerobic microorganism with an ability to reduce 70% of 2.5 mM selenite to produce red spherical biogenic elemental selenium nanoparticles (BioSeNPs). Only extracellular production of BioSeNPs was observed using transmission electron microscopy. The two-chamber reactor experiments, genome analysis and corona proteins identified on BioSeNPs suggested that the selenite reduction process was primarily mediated through membrane-associated proteins, like succinate dehydrogenase. Extracellular presence and low colloidal stability of BioSeNPs as indicated by ζ-potential measurements, render B. safensis JG-B5T an attractive candidate in wastewater treatment as it provides easy way of recovering Se while maintaining low Se discharge. As this microorganism decreases Se mobility, it will affect Se bioavailability in the environment and decreases its toxicity.

Bioaccumulation of potentially toxic elements by submerged plants and biofilms: A critical review

The accumulation of potentially toxic elements (PTEs) in aquatic ecosystems has become a global concern, as PTEs may exert a wide range of toxicological impacts on aquatic organisms. Submerged plants and the microorganisms attached to their surfaces, however, have displayed great potential as a means of coping with such pollution. Therefore, it is crucial to understand the transport pathways of PTEs across sediment and organisms as well as their accumulation mechanisms in the presence of submerged plants and their biofilms. The majority of previous studies have demonstrated that submerged plants and their biofilms are indicators of PTE pollution in the aquatic environment, yet relatively little is known about PTE accumulation in epiphytic biofilms. In this review, we describe the transport pathways of PTEs in the aquatic environment in order to offer remarkable insights into bioaccumulation mechanisms in submerged plants and their biofilms. Based on the literature cited in this review, the roles of epiphytic biofilms in bioaccumulation and as an indicator of ecosystem health are discussed.

Kinetics of microbial selenite reduction by novel bacteria isolated from activated sludge

A total of three bacteria isolated from activated sludge of a wastewater treatment plant were found to reduce selenite to elemental selenium nanoparticles as both amorphous nanospheres and monoclinic nanocrystals. The three isolated strains, which are potential candidates for bioremediation of selenite-contaminated water sources, were designated as Citrobacter sp. NVK-2, Providencia sp. NVK-2A, and Citrobacter sp. NVK-6 based on 16S rRNA sequencing. Despite belonging to the same genus, the kinetics of selenite reduction by strain NVK-2 (V_max = 58.82 μM h^-1, K_m = 3737.12 μM) completely differed from that of strain NVK-6 (V_max = 19.23 μM h^-1, K_m = 1300.17 μM). The selenite reduction rate by strain NVK-2A (V_max = 9.26 μM h^-1, K_m = 3044.73 μM) was the slowest among the investigated microorganisms. The microbial selenite reduction rates according to various organic sources indicated that simple organic sources such as acetate and lactate were better than more complex organic sources such as propionate, butyrate, and glucose for selenite removal. Interestingly, the selenite reduction rate was significantly enhanced when the organic source was strategically divided into small portions and consecutively supplied to the culture.

Microbial transformation of Se oxyanions in cultures of Delftia lacustris grown under aerobic conditions

Delftia lacustris is reported for the first time as a selenate and selenite reducing bacterium, capable of tolerating and growing in the presence of ≥ 100 mM selenate and 25 mM selenite. The selenate reduction profiles of D. lacustris were investigated by varying selenate concentration, inoculum size, concentration and source of organic electron donor in minimal salt medium. Interestingly, the bacterium was able to reduce both selenate and selenite under aerobic conditions. Although considerable removal of selenate was observed at all concentrations investigated, D. lacustris was able to completely reduce 0.1 mM selenate within 96 h using lactate as the carbon source. Around 62.2% unaccounted selenium (unidentified organo-selenium compounds), 10.9% elemental selenium and 26.9% selenite were determined in the medium after complete reduction of selenate. Studies of the enzymatic activity of the cell fractions show that the selenite/selenate reducing enzymes were intracellular and independent of NADPH availability. D. lacustris shows an unique metabolism of selenium oxyanions to form elemental selenium and possibly also selenium ester compounds, thus a potential candidate for the remediation of selenium-contaminated wastewaters in aerobic environments. This novel finding will advance the field of bioremediation of selenium-contaminated sites and selenium bio-recovery and the production of potentially beneficial organic and inorganic reactive selenium species.

Two selenium tolerant Lysinibacillus sp. strains are capable of reducing selenite to elemental Se efficiently under aerobic conditions

Microbes play important roles in the transport and transformation of selenium (Se) in the environment, thereby influencing plant resistance to Se and Se accumulation in plant. The objectives are to characterize the bacteria with high Se tolerance and reduction capacity and explore the significance of microbial origins on their Se tolerance, reduction rate and efficiency. Two bacterial strains were isolated from a naturally occurred Se-rich soil at tea orchard in southern Anhui Province, China. The reduction kinetics of selenite was investigated and the reducing product was characterized using scanning electron microscopy and transmission electron microscopy-energy dispersive spectroscopy. The bacteria were identified as Lysinibacillus xylanilyticus and Lysinibacillus macrolides, respectively, using morphological, physiological and molecular methods. The results showed that the minimal inhibitory concentrations (MICs) of selenite for L. xylanilyticus and L. macrolides were 120 and 220 mmol/L, respectively, while MICs of selenate for L. xylanilyticus and L. macrolides were 800 and 700 mmol/L, respectively. Both strains aerobically reduced selenite with an initial concentration of 1.0 mmol/L to elemental Se nanoparticles (SeNPs) completely within 36 hr. Biogenic SeNPs were observed both inside and outside the cells suggesting either an intra- or extracellular reduction process. Our study implied that the microbes from Se-rich environments were more tolerant to Se and generally quicker and more efficient than those from Se-free habitats in the reduction of Se oxyanions. The bacterial strains with high Se reduction capacity and the biological synthesized SeNPs would have potential applications in agriculture, food, environment and medicine.

Novel mechanisms of selenate and selenite reduction in the obligate aerobic bacterium Comamonas testosteroni S44

Selenium oxyanion reduction is an effective detoxification or/and assimilation processes in organisms, but little is known the mechanisms in aerobic bacteria. Aerobic Comamonas testosteroni S44 reduces Se(VI)/Se(IV) to less-toxic elemental selenium nanoparticles (SeNPs). For Se(VI) reduction, sulfate and Se(VI) reduction displayed a competitive relationship. When essential sulfate reducing genes were respectively disrupted, Se(VI) was not reduced to red-colored SeNPs. Consequently, Se(VI) reduction was catalyzed by enzymes of the sulfate reducing pathway. For Se(IV) reduction, one of the potential periplasm molybdenum oxidoreductase named SerT was screened and further used to analyze Se(IV) reduction. Compared to the wild type and the complemented mutant strain, the ability of Se(IV) reduction was reduced 75% in the deletion mutant ΔserT. Moreover, the Se(IV) reduction rate was significantly enhanced when the gene serT was overexpressed in Escherichia coli W3110. In addition, Se(IV) was reduced to SeNPs by the purified SerT with the presence of NADPH as the electron donor in vitro, showing a V_max of 61 nmol/min·mg and a K_m of 180 μmol/L. A model of Se(VI)/Se(IV) reduction was generated in aerobic C. testosteroni S44. This work provides new insights into the molecular mechanisms of Se(VI)/Se(IV) reduction activities in aerobic bacteria.

Inhibition of bacteriochlorophyll biosynthesis in the purple phototrophic bacteria Rhodospirillumrubrum and Rhodobacter capsulatus grown in the presence of a toxic concentration of selenite

Background In many works, the chemical composition of bacterially-produced elemental selenium nanoparticles (Se0-nanoparticles) was investigated using electron dispersive X-ray analysis. The results suggest that these particles should be associated with organic compounds. However, a complete analysis of their chemical composition is still missing. Aiming at identifying organic compounds associated with the Se0-nanoparticles produced by the purple phototrophic bacteria Rhodospirillum rubrum and Rhodobacter capsulatus (α group of the proteobacteria), we used MALDI-TOF spectrometry.Results This technic revealed that numerous signals obtained from particles produced by both species of bacteria were from metabolites of the photosynthetic system. Furthermore, not only bacteriochlorophyll a, bacteriopheophytin a, and bacteriopheophorbide a, which are known to accumulate in stationary phase cultures of these bacteria grown phototrophically in the absence of selenite, were identified. The particles were also associated with intermediary metabolites of the bacteriochlorophyll a biosynthesis pathway such as protoporphyrin IX, protoporphyrin IX monomethyl ester, bacteriochlorophyllide a and, most likely, Mg-protoporphyrin IX-monomethyl ester, as well as with oxidation products of the substrates of protochlorophyllide reductase and chlorin reductase.Conclusion Accumulation of intermediary metabolites of the bacteriochlorophyll biosynthesis pathway in these purple phototrophic bacteria was attributed to inhibition of oxygen-sensitive enzymes involved in this pathway. Consistent with this interpretation it has been reported that these bacteria reduce selenite intracellularly, that they contain high levels of glutathione and that the reduction of selenite with glutathione is a very fast reaction accompanied by the production of reactive oxygen species. As many enzymes involved in the biosynthesis of bacteriochlorophyll contain [Fe-S] clusters in their active site, which are known to be degraded in the presence of reactive oxygen species as well as in the presence of molecular oxygen, we concluded that the substrates of these enzymes accumulate in cells during selenite reduction.Association of metabolites of bacteriochlorophyll biosynthesis and degradation with the Se0-nanoparticles produced by Rhodospirillum rubrum and Rhodobacter capsulatus is proposed to result from coating of the nanoparticles with the intracytoplasmic membrane of these bacteria, where the photochemical apparatus is concentrated.

Developing a Synthetic Biology Toolkit for Comamonas testosteroni, an Emerging Cellular Chassis for Bioremediation

Synthetic biology is rapidly evolving into a new phase that emphasizes real-world applications such as environmental remediation. Recently, Comamonas testosteroni has become a promising chassis for bioremediation due to its natural pollutant-degrading capacity; however, its application is hindered by the lack of fundamental gene expression tools. Here, we present a synthetic biology toolkit that enables rapid creation of functional gene circuits in C. testosteroni. We first built a shuttle system that allows efficient circuit construction in E. coli and necessary phenotypic testing in C. testosteroni. Then, we tested a set of wildtype inducible promoters, and further used a hybrid strategy to create engineered promoters to expand expression strength and dynamics. Additionally, we tested the T7 RNA Polymerase-P_T7 promoter system and reduced its leaky expression through promoter mutation for gene expression. By coupling random library construction with FACS screening, we further developed a synthetic T7 promoter library to confer a wider range of expression strength and dynamic characteristics. This study provides a set of valuable tools to engineer gene circuits in C. testosteroni, facilitating the establishment of the organism as a useful microbial chassis for bioremediation purposes.

Selenium contamination, consequences and remediation techniques in water and soils: A review

Selenium (Se) contamination in surface and ground water in numerous river basins has become a critical problem worldwide in recent years. The exposure to Se, either direct consumption of Se or indirectly may be fatal to the human health because of its toxicity. The review begins with an introduction of Se chemistry, distribution and health threats, which are essential to the remediation techniques. Then, the review provides the recent and common removal techniques for Se, including reduction techniques, phytoremediation, bioremediation, coagulation-flocculation, electrocoagulation (EC), electrochemical methods, adsorption, coprecipitation, electrokinetics, membrance technology, and chemical precipitation. Removal techniques concentrate on the advantages, drawbacks and the recent achievements of each technique. The review also takes an overall consideration of experimental conditions, comparison criteria and economic aspects.

Adsorption Removal of Multiple Dyes Using Biogenic Selenium Nanoparticles from an Escherichia coli Strain Overexpressed Selenite Reductase CsrF

Selenite reductase CsrF overexpressed Escherichia coli was used as a microbial factory to produce Se(0) nanoparticles (Bio-SeNPs). The Bio-SeNPs were characterized by transmission electronic microscopy, element mapping, scanning electron microscopy, energy-dispersive X-ray spectrographs, Zeta-potential, dynamic light scattering, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses. The results indicated that Bio-SeNPs are irregular spheres with diameters from 60 to105 nm and mainly consist of Se(0), proteins and lipids. Furthermore, it exhibited maximum adsorption capacity for anionic dye (congo red) at acidic pH and cationic dyes (safranine T and methylene blue) at alkaline pH. To gain more insight, adsorption kinetics, adsorption isotherms and adsorption thermodynamics studies were carried out. These results showed that the adsorption capacities of congo red, safranine T and methylene blue were 1577.7, 1911.0 and 1792.2 mg/g, respectively. These adsorption processes were spontaneous and primarily physical reactions. In addition, Bio-SeNPs can be effectively reused by 200 mmol/L NaCl. To the best of our knowledge, this is the first report of adsorption removal dyes by Bio-SeNPs. The adsorption capacities of Bio-SeNPs for congo red, safranine T and methylene blue were 6.8%, 25.2% and 49.0% higher than that for traditional bio-based materials, respectively.

Proteins enriched in charged amino acids control the formation and stabilization of selenium nanoparticles in Comamonas testosteroni S44

Elemental selenium nanoparticles (SeNPs) are useful in medicine, environmental remediation and in material science. Biosynthesized SeNPs (BioSeNPs) by bacteria are cheap, eco-friendly and have a lower cytotoxicity in comparison with chemically synthesized ones. Organic matters were found to cap on the surface of BioSeNPs, but the functions were still not entirely clear. The purified BioSeNPs were coated in a thick layer of organic substrates observed by transmission electron microscopy (TEM). Fourier Transform Infrared (FT-IR) and quantitative detection of the coating agents showed that one gram of purified BioSeNPs bound 1069 mg proteins, 23 mg carbohydrates and only very limited amounts of lipids. Proteomics of BioSeNPs showed more than 800 proteins bound to BioSeNPs. Proteins enriched in charged amino acids are the major factor thought to govern the formation process and stabilization of BioSeNPs in bacteria. In view of the results reported here, a schematic model for the molecular mechanism of BioSeNPs formation in bacteria is proposed. These findings are helpful for the artificial green synthesis of stable SeNPs under specific condition and guiding the surface modification of SeNPs for medicine application.

Metal-tolerant thermophiles: metals as electron donors and acceptors, toxicity, tolerance and industrial applications

Metal-tolerant thermophiles are inhabitants of a wide range of extreme habitats like solfatara fields, hot springs, mud holes, hydrothermal vents oozing out from metal-rich ores, hypersaline pools and soil crusts enriched with metals and other elements. The ability to withstand adverse environmental conditions, like high temperature, high metal concentration and sometimes high pH in their niche, makes them an interesting subject for understanding mechanisms behind their ability to deal with multiple duress simultaneously. Metals are essential for biological systems, as they participate in biochemistries that cannot be achieved only by organic molecules. However, the excess concentration of metals can disrupt natural biogeochemical processes and can impose toxicity. Thermophiles counteract metal toxicity via their unique cell wall, metabolic factors and enzymes that carry out metal-based redox transformations, metal sequestration by metallothioneins and metallochaperones as well as metal efflux. Thermophilic metal resistance is heterogeneous at both genetic and physiology levels and may be chromosomally, plasmid or transposon encoded with one or more genes being involved. These effective response mechanisms either individually or synergistically make proliferation of thermophiles in metal-rich habitats possibly. This article presents the state of the art and future perspectives of responses of thermophiles to metals at genetic as well as physiological levels.

Correlative Cryogenic Spectromicroscopy to Investigate Selenium Bioreduction Products

Accurate mapping of the composition and structure of minerals and associated biological materials is critical in geomicrobiology and environmental research. Here, we have developed an apparatus that allows the correlation of cryogenic transmission electron microscopy (cryo-TEM) and synchrotron hard X-ray microprobe (SHXM) data sets to precisely determine the distribution, valence state, and structure of selenium in biofilms sampled from a contaminated aquifer near Rifle, CO. Results were replicated in the laboratory via anaerobic selenate-reducing enrichment cultures. 16S rRNA analyses of field-derived biofilm indicated the dominance of Betaproteobacteria from the Comamonadaceae family and uncultivated members of the Simplicispira genus. The major product in field and culture-derived biofilms is ∼25-300 nm red amorphous Se⁰ aggregates of colloidal nanoparticles. Correlative analyses of the cultures provided direct evidence for the microbial dissimilatory reduction of Se(VI) to Se(IV) to Se⁰. Extended X-ray absorption fine-structure spectroscopy showed red amorphous Se⁰ with a first shell Se-Se interatomic distance of 2.339 ± 0.003 Å. Complementary scanning transmission X-ray microscopy revealed that these aggregates are strongly associated with a protein-rich biofilm matrix. These findings have important implications for predicting the stability and mobility of Se bioremediation products and understanding of Se biogeochemical cycling. The approach, involving the correlation of cryo-SHXM and cryo-TEM data sets from the same specimen area, is broadly applicable to biological and environmental samples.

Novel bacterial selenite reductase CsrF responsible for Se(IV) and Cr(VI) reduction that produces nanoparticles in Alishewanella sp. WH16-1

Alishewanella sp. WH16-1 is a facultative anaerobic bacterium isolated from mining soil. Under aerobic conditions, this bacterium efficiently reduces selenite and chromate. A flavoprotein showing 37% amino acid identity to E. coli chromate reductase ChrR was identified from the genome (named CsrF). Gene mutation and complementation along with heterologous expression revealed the ability of CsrF to reduce selenite and chromate in vivo. The purified CsrF was yellow and showed an absorption spectra similar to that of FMN. The molecular weight of CsrF was 23,906 for the monomer and 47,960 for the dimer. In vitro, CsrF catalyzes the reduction of Se(IV) and Cr(VI) using NAD(P)H as cofactors with optimal condition of pH 7.0 and temperature of 30-37°C. This enzyme also catalyze the reduction of sulfate and ferric iron but not arsenate and nitrate. Using NADPH as its electron donor, the Km for the reduction of Se(IV) and Cr(VI) was 204.1±27.91 and 250.6±23.46μmol/L, respectively. Site-directed mutagenesis showed that Arg13 and Gly113 were essential for the reduction of Se(IV) and Cr(VI). The products of the reduction of Se(IV) and Cr(VI) were Se(0)- and Cr(III)-nanoparticles, respectively. To our knowledge, CsrF is a novel and well-characterized bacterial aerobic selenite reductase.

Regulation of Class A β-Lactamase CzoA by CzoR and IscR in Comamonas testosteroni S44

A genomic analysis of Comamonas testosteroni S44 revealed a gene that encodes a LysR family transcriptional regulator (here named czoR, czo for cefazolin) located upstream of a putative class A β-lactamase encoding gene (here named czoA). A putative DNA-binding motif of the Fe-S cluster assembly regulator IscR was identified in the czoR-czoA intergenic region. Real-time RT-PCR and lacZ fusion expression assays indicated that transcription of czoA and czoR were induced by multiple β-lactams. CzoA expressed in Escherichia coli was shown to contribute to susceptibility to a wide range of β-lactams judged from minimum inhibitory concentrations. In vitro enzymatic assays showed that CzoA hydrolyzed seven β-lactams, including benzylpenicillin, ampicillin, cefalexin, cefazolin, cefuroxime, ceftriaxone, and cefepime. Deletion of either iscR or czoR increased susceptibility to cefalexin and cefazolin, while complemented strains restored their wild-type susceptibility levels. Electrophoretic mobility shift assays (EMSA) demonstrated that CzoR and IscR bind to different sites of the czoR-czoA intergenic region. Precise CzoR- and IscR-binding sites were confirmed via DNase I footprinting or short fragment EMSA. When cefalexin or cefazolin was added to cultures, czoR deletion completely inhibited czoA expression but did not affect iscR transcription, while iscR deletion decreased the expressions of both czoR and czoA. These results reveal that CzoR positively affects the expression of czoA with its own expression upregulated by IscR.

Ochrobactrum sp. MPV1 from a dump of roasted pyrites can be exploited as bacterial catalyst for the biogenesis of selenium and tellurium nanoparticles

Background

Bacteria have developed different mechanisms for the transformation of metalloid oxyanions to non-toxic chemical forms. A number of bacterial isolates so far obtained in axenic culture has shown the ability to bioreduce selenite and tellurite to the elemental state in different conditions along with the formation of nanoparticles—both inside and outside the cells—characterized by a variety of morphological features. This reductive process can be considered of major importance for two reasons: firstly, toxic and soluble (i.e. bioavailable) compounds such as selenite and tellurite are converted to a less toxic chemical forms (i.e. zero valent state); secondly, chalcogen nanoparticles have attracted great interest due to their photoelectric and semiconducting properties. In addition, their exploitation as antimicrobial agents is currently becoming an area of intensive research in medical sciences.

Results

In the present study, the bacterial strain Ochrobactrum sp. MPV1, isolated from a dump of roasted arsenopyrites as residues of a formerly sulfuric acid production near Scarlino (Tuscany, Italy) was analyzed for its capability of efficaciously bioreducing the chalcogen oxyanions selenite (SeO₃²⁻) and tellurite (TeO₃²⁻) to their respective elemental forms (Se⁰ and Te⁰) in aerobic conditions, with generation of Se- and Te-nanoparticles (Se- and TeNPs). The isolate could bioconvert 2 mM SeO₃²⁻ and 0.5 mM TeO₃²⁻ to the corresponding Se⁰ and Te⁰ in 48 and 120 h, respectively. The intracellular accumulation of nanomaterials was demonstrated through electron microscopy. Moreover, several analyses were performed to shed light on the mechanisms involved in SeO₃²⁻ and TeO₃²⁻ bioreduction to their elemental states. Results obtained suggested that these oxyanions are bioconverted through two different mechanisms in Ochrobactrum sp. MPV1. Glutathione (GSH) seemed to play a key role in SeO₃²⁻ bioreduction, while TeO₃²⁻ bioconversion could be ascribed to the catalytic activity of intracellular NADH-dependent oxidoreductases. The organic coating surrounding biogenic Se- and TeNPs was also characterized through Fourier-transform infrared spectroscopy. This analysis revealed interesting differences among the NPs produced by Ochrobactrum sp. MPV1 and suggested a possible different role of phospholipids and proteins in both biosynthesis and stabilization of such chalcogen-NPs.

Conclusions

In conclusion, Ochrobactrum sp. MPV1 has demonstrated to be an ideal candidate for the bioconversion of toxic oxyanions such as selenite and tellurite to their respective elemental forms, producing intracellular Se- and TeNPs possibly exploitable in biomedical and industrial applications.

Speeding up bioproduction of selenium nanoparticles by using Vibrio natriegens as microbial factory

Selenium and selenium nanoparticles (SeNPs) are extensively used in biomedicine, electronics and some other industrial applications. The bioproduction of SeNPs is gaining interest as a green method to manufacture these biotechnologically relevant products. Several microorganisms have been used for the production of SeNPs either under aerobic or anaerobic conditions. Vibrio natriegens is a non-pathogenic fast-growing bacterium, easily cultured in different carbon sources and that has recently been engineered for easy genetic manipulation in the laboratory. Here we report that V. natriegens was able to perfectly grow aerobically in the presence of selenite concentrations up to 15 mM with a significant survival still observed at concentrations as high as 100 mM selenite. Electron microscopy and X-ray spectroscopy analyses demonstrate that V. natriegens cells growing aerobically in selenite-containing LB medium at 30 °C produced spherical electron-dense SeNPs whose size ranged from 100-400 nm. Selenite reduction just started at the beginning of the exponential growth phase and the release of SeNPs was observed after cell lysis. Remarkably, V. natriegens produced SeNPs faster than other described microorganisms that were proposed as model bioreactors for SeNPs production. Thus, the fast-growing V. natriegens bacterium becomes a suitable biocatalyst for bioremediation of selenite and for speeding-up the eco-friendly synthesis of SeNPs.

Biosynthesis of selenium-nanoparticles and -nanorods as a product of selenite bioconversion by the aerobic bacterium Rhodococcus aetherivorans BCP1

The wide anthropogenic use of selenium compounds represents the major source of selenium pollution worldwide, causing environmental issues and health concerns. Microbe-based strategies for metal removal/recovery have received increasing interest thanks to the association of the microbial ability to detoxify toxic metal/metalloid polluted environments with the production of nanomaterials. This study investigates the tolerance and the bioconversion of selenite (SeO₃^2-) by the aerobically grown Actinomycete Rhodococcus aetherivorans BCP1 in association with its ability to produce selenium nanoparticles and nanorods (SeNPs and SeNRs). The BCP1 strain showed high tolerance towards SeO₃^2- with a Minimal Inhibitory Concentration (MIC) of 500mM. The bioconversion of SeO₃^2- was evaluated considering two different physiological states of the BCP1 strain, i.e. unconditioned and/or conditioned cells, which correspond to cells exposed for the first time or after re-inoculation in fresh medium to either 0.5 or 2mM of Na₂SeO₃, respectively. SeO₃^2- bioconversion was higher for conditioned grown cells compared to the unconditioned ones. Selenium nanostructures appeared polydisperse and not aggregated, as detected by electron microscopy, being embedded in an organic coating likely responsible for their stability, as suggested by the physical-chemical characterization. The production of smaller and/or larger SeNPs was influenced by the initial concentration of provided precursor, which resulted in the growth of longer and/or shorter SeNRs, respectively. The strong ability to tolerate high SeO₃^2- concentrations coupled with SeNP and SeNR biosynthesis highlights promising new applications of Rhodococcus aetherivorans BCP1 as cell factory to produce stable Se-nanostructures, whose suitability might be exploited for biotechnology purposes.

High-quality-draft genomic sequence of Paenibacillus ferrarius CY1T with the potential to bioremediate Cd, Cr and Se contamination

Paenibacillus ferrarius CY1T (= KCTC 33419T = CCTCC AB2013369T) is a Gram-positive, aerobic, endospore-forming, motile and rod-shaped bacterium isolated from iron mineral soil. This bacterium reduces sulfate (SO42-) to S2-, which reacts with Cd(II) to generate precipitated CdS. It also reduces the toxic chromate [Cr(VI)] and selenite [Se(VI)] to the less bioavailable chromite [Cr(III)] and selenium (Se0), respectively. Thus, strain CY1T has the potential to bioremediate Cd, Cr and Se contamination, which is the main reason for the interest in sequencing its genome. Here we describe the features of strain CY1T, together with the draft genome sequence and its annotation. The 9,184,169 bp long genome exhibits a G + C content of 45.6%, 7909 protein-coding genes and 81 RNA genes. Nine putative Se(IV)-reducing genes, five putative Cr(VI) reductase and nine putative sulfate-reducing genes were identified in the genome.

Aerobic biogenesis of selenium nanoparticles by Enterobacter cloacae Z0206 as a consequence of fumarate reductase mediated selenite reduction

In the present study, we examined the ability of Enterobacter cloacae Z0206 to reduce toxic sodium selenite and mechanism of this process. E. cloacae Z0206 was found to completely reduce up to 10 mM selenite to elemental selenium (Se°) and form selenium nanoparticles (SeNPs) under aerobic conditions. The selenite reducing effector of E. cloacae Z0206 cell was to be a membrane-localized enzyme. iTRAQ proteomic analysis revealed that selenite induced a significant increase in the expression of fumarate reductase. Furthermore, the addition of fumarate to the broth and knockout of fumarate reductase (frd) both significantly decreased the selenite reduction rate, which revealed a previously unrecognized role of E. cloacae Z0206 fumarate reductase in selenite reduction. In contrast, glutathione-mediated Painter-type reactions were not the main pathway of selenite reducing. In conclusion, E. cloacae Z0206 effectively reduced selenite to Se° using fumarate reductase and formed SeNPs; this capability may be employed to develop a bioreactor for treating Se pollution and for the biosynthesis of SeNPs in the future.

Biogenic Nanoselenium Particles Effectively Attenuate Oxidative Stress-Induced Intestinal Epithelial Barrier Injury by Activating the Nrf2 Antioxidant Pathway

In the present study, a new form of selenium nanoparticle (biogenic nanoselenium (BNS) particles) was synthesized using bacteria. The protection of BNS particles against oxidative stress-induced intestinal barrier dysfunction and the inherent mechanisms of this process were investigated, and selenomethionine (SeMet) and chemically synthesized nanoselenium (Nano-Se) particles were used for comparison. Characterization of BNS particles revealed that they were monodispersed and homogeneous spheres, with an average size of 139.43 ± 7.44 nm. In the mouse model of intestinal oxidative stress, BNS particles were found to protect the mouse intestinal barrier function and preserve intestinal redox homeostasis more efficiently than SeMet and Nano-Se. In vitro experiments with porcine jejunum epithelial (IPEC-J2) cells verified the stronger epithelial barrier-protecting effect of BNS particles against oxidative stress, with reduced cell apoptosis and an improved cell redox state. BNS activated the nuclear factor (erythroid-derived-2)-like 2 (Nrf2) and increased the expression of its downstream genes, including thioredoxin reductase (TXNRD)-1, NADPH dehydrogenase (NQO)-1, heme oxygenase (HO)-1, and thioredoxin (Trx), in dose- and time-dependent manners. In contrast, SeMet and Nano-Se merely enhanced the activity of the selenoenzymes TXNRD-1 and glutathione peroxidase (GPx)-1, indicating the role of selenium donors. Moreover, the knock down of Nrf2 significantly blocked the antioxidative effect of BNS, confirming that BNS protects the intestinal barrier from oxidative stress-induced damage by activating Nrf2 and its downstream genes. Our results suggest that BNS is a promising selenium species with potential application in treating oxidative stress-related intestinal diseases.

Screening of bacterial strains isolated from uranium mill tailings porewaters for bioremediation purposes

The present work characterizes at different levels a number of bacterial strains isolated from porewaters sampled in the vicinity of two French uranium tailing repositories. The 16S rRNA gene from 33 bacterial isolates, corresponding to the different morphotypes recovered, was almost fully sequenced. The resulting sequences belonged to 13 bacterial genera comprised in the phyla Firmicutes, Actinobacteria and Proteobacteria. Further characterization at physiological level and metals/metalloid tolerance provided evidences for an appropriate selection of bacterial strains potentially useful for immobilization of uranium and other common contaminants. By using High Resolution Transmission Electron Microscope (HRTEM), this potential ability to immobilize uranium as U phosphate mineral phases was confirmed for the bacterial strains Br3 and Br5 corresponding to Arthrobacter sp. and Microbacterium oxydans, respectively. Scanning Transmission Electron Microscope- High-Angle Annular Dark-Field (STEM-HAADF) analysis showed U accumulates on the surface and within bacterial cytoplasm, in addition to the extracellular space. Energy Dispersive X-ray (EDX) element-distribution maps demonstrated the presence of U and P within these accumulates. These results indicate the potential of certain bacterial strains isolated from porewaters of U mill tailings for immobilizing uranium, likely as uranium phosphates. Some of these bacterial isolates might be considered as promising candidates in the design of uranium bioremediation strategies.