Reduction of selenite by Azospirillum brasilense with the formation of selenium nanoparticles

Selenium availability in tea: Unraveling the role of microbiota assembly and functions

Tea (Camellia sinensis (L.) O. Kuntze) plants have a strong ability to accumulate selenium (Se). However, the question of how tea plants affect Se availability has received little attention. In this study, five tea cultivars, including Soubei (SB), Aolǜ (AL), Longjing43 (LJ), Zhaori (ZR) and Fenglǜ (FL), were chosen for the study. Quantitative Microbial Ecology Chip and high-throughput sequencing were used to explore the effects of five tea cultivars on soil functions, microbial community structures and Se availability. The results showed that the total soil Se content in the FL garden was lower compared to LJ and SB gardens, whereas available Se was highest in the FL garden. Based on the Bray-Curtis distances, tea cultivar was the main factor affecting bacterial and fungal community structures. The abundance of functional genes concerning carbon, nitrogen, phosphorus and sulfur cycling processes varied among tea gardens. The higher soil NH4+ and NO3- contents, and higher abundance of functional genes like nifH, amoA1 and narG, whereas lower total nitrogen in the FL garden than in the AL and LJ tea gardens demonstrated that the FL tea plants induced microbes to accelerate soil nitrogen cycling processes. Dominant microbes that positively related with functional genes like nifH, narG, and amoA1 were also positively related with the available Se content. In conclusion, tea cultivars could regulate soil functions through affecting microbial community structures and then affecting the soil Se availability. The soil nitrogen cycle processes are suggested to be closely related with Se transformation in tea gardens.

Effect of Different Selenium Species on Indole-3-Acetic Acid Activity of Selenium Nanoparticles Producing Strain Bacillus altitudinis LH18

Acting as a growth regulator, Indole-3-acetic acid (IAA) is an important phytohormone that can be produced by several Bacillus species. However, few studies have been published on the comprehensive evaluation of the strains for practical applications and the effects of selenium species on their IAA-producing ability. The present study showed the selenite reduction strain Bacillus altitudinis LH18, which is capable of producing selenium nanoparticles (SeNPs) at a high yield in a cost-effective manner. Bio-SeNPs were systematically characterized by using DLS, zeta potential, SEM, and FTIR. The results showed that these bio-SeNPs were small in particle size, homogeneously dispersed, and highly stable. Significantly, the IAA-producing ability of strain was differently affected under different selenium species. The addition of SeNPs and sodium selenite resulted in IAA contents of 221.7 µg/mL and 91.01 µg/mL, respectively, which were 3.23 and 1.33 times higher than that of the control. This study is the first to examine the influence of various selenium species on the IAA-producing capacity of Bacillus spp., providing a theoretical foundation for the enhancement of the IAA-production potential of microorganisms.

Selenium Nanoparticles: Green Synthesis and Biomedical Application

Selenium nanoparticles (SeNPs) are extremely popular objects in nanotechnology. "Green" synthesis has special advantages due to the growing necessity for environmentally friendly, non-toxic, and low-cost methods. This review considers the biosynthesis mechanism of bacteria, fungi, algae, and plants, including the role of various biological substances in the processes of reducing selenium compounds to SeNPs and their further packaging. Modern information and approaches to the possible biomedical use of selenium nanoparticles are presented: antimicrobial, antiviral, anticancer, antioxidant, anti-inflammatory, and other properties, as well as the mechanisms of these processes, that have important potential therapeutic value.

Comparison of Nutritional Availability of Biogenic Selenium Nanoparticles and Chemically Synthesized Selenium Nanoparticles

Selenium (Se) is an essential micronutrient, and animals biosynthesize selenoproteins from various selenocompounds such as inorganic salts and organic selenocompounds as a Se source. In addition to the inorganic and organic forms of Se, it is also known that elemental Se is biologically synthesized at the nanoscale in nature. Biologically synthesized Se nanoparticles (Se-NPs), i.e., biogenic Se-NPs (Se-BgNPs), have not been fully investigated as a Se source compared with the other forms of Se. In this study, we evaluated the nutritional availability of Se-BgNPs biosynthesized in E. coli and revealed that Se-BgNPs were less assimilated into selenoproteins in rats as a Se source than inorganic Se salt or chemically synthesized Se-NPs. Se-BgNPs showed tolerance toward digestion and low absorbability in gut, which resulted in the low nutritional availability. Se-BgNPs seem to be coated with a biomaterial that functions to reduce their toxicity toward E. coli and at the same time lowers their availability to animals.

Advances in selenium supplementation: From selenium-enriched yeast to potential selenium-enriched insects, and selenium nanoparticles

Selenium (Se) is an essential micronutrient that plays an important role in animal and human development and physiological homoeostasis. This review surveys the role of Se in the environment, plants and animal bodies, and discusses data on Se biofortification with different sources of supplementation, from inorganic to organic forms, with special focus on Se-enriched yeast (Se-yeast). Although Se-yeast remains one of the main sources of organic Se, other emerging and innovative sources are reviewed, such as Se-enriched insects and Se-nanoparticles and their potential use in animal nutrition. Se-enriched insects are discussed as an option for supplying Se in organic form to livestock diets. Se-nanoparticles are also discussed, as they represent a more biocompatible and less toxic source of inorganic Se for animal organisms, compared to selenite and selenate. We also provide up to date information on the legal framework in the EU, USA, and Canada of Se that is contained in feed additives. From the scientific evidence available in the literature, it can be concluded that among the inorganic forms, sodium selenite is still one of the main options, whereas Se-yeast remains the primary organic form. However, other potential sources such as Se-enriched insects and Se-nanoparticles are being investigated as they could potentially combine a high bioavailability and reduced Se emissions in the environment.

Selenium in soil-plant system: Transport, detoxification and bioremediation

Selenium (Se) is an essential micronutrient for humans and a beneficial element for plants. However, high Se doses always exhibit hazardous effects. Recently, Se toxicity in plant-soil system has received increasing attention. This review will summarize (1) Se concentration in soils and its sources, (2) Se bioavailability in soils and influencing factors, (3) mechanisms on Se uptake and translocation in plants, (4) toxicity and detoxification of Se in plants and (5) strategies to remediate Se pollution. High Se concentration mainly results from wastewater discharge and industrial waste dumping. Selenate (Se [VI]) and selenite (Se [IV]) are the two primary forms absorbed by plants. Soil conditions such as pH, redox potential, organic matter and microorganisms will influence Se bioavailability. In plants, excessive Se will interfere with element uptake, depress photosynthetic pigment biosynthesis, generate oxidative damages and cause genotoxicity. Plants employ a series of strategies to detoxify Se, such as activating antioxidant defense systems and sequestrating excessive Se in the vacuole. In order to alleviate Se toxicity to plants, some strategies can be applied, including phytoremediation, OM remediation, microbial remediation, adsorption technique, chemical reduction technology and exogenous substances (such as Methyl jasmonate, Nitric oxide and Melatonin). This review is expected to expand the knowledge of Se toxicity/detoxicity in soil-plant system and offer valuable insights into soils Se pollution remediation strategies.

Antifungal Properties of Biogenic Selenium Nanoparticles Functionalized with Nystatin for the Inhibition of Candida albicans Biofilm Formation

In the present study, biogenic selenium nanoparticles (SeNPs) have been prepared using Paenibacillus terreus and functionalized with nystatin (SeNP@PVP_Nystatin nanoconjugates) for inhibiting growth, morphogenesis, and a biofilm in Candida albicans. Ultraviolet-visible spectroscopy analysis has shown a characteristic absorption at 289, 303, and 318 nm, and X-ray diffraction analysis has shown characteristic peaks at different 2θ values for SeNPs. Electron microscopy analysis has shown that biogenic SeNPs are spherical in shape with a size in the range of 220-240 nm. Fourier transform infrared spectroscopy has confirmed the functionalization of nystatin on SeNPs (formation of SeNP@PVP_Nystatin nanoconjugates), and the zeta potential has confirmed the negative charge on the nanoconjugates. Biogenic SeNPs are inactive; however, nanoconjugates have shown antifungal activities on C. albicans (inhibited growth, morphogenesis, and a biofilm). The molecular mechanism for the action of nanoconjugates via a real-time polymerase chain reaction has shown that genes involved in the RAS/cAMP/PKA signaling pathway play an important role in antifungal activity. In cytotoxic studies, nanoconjugates have inhibited only 12% growth of the human embryonic kidney cell line 293 cells, indicating that the nanocomposites are not cytotoxic. Thus, the biogenic SeNPs produced by P. terreus can be used as innovative and effective drug carriers to increase the antifungal activity of nystatin.

Selenium species transforming along soil-plant continuum and their beneficial roles for horticultural crops

Selenium (Se) acquirement from daily diet can help reduce the risk of many diseases. The edible parts of crop plants are the main source of dietary Se, while the Se content in crops is determined by Se bioavailability in soil. We summarize recent research on the biogeochemical cycle of Se driven by specific microorganisms and emphasize the oxidizing process in the Se cycle. Moreover, we discuss how plant root exudates and rhizosphere microorganisms affect soil Se availability. Finally, we cover beneficial microorganisms, including endophytes, that promote crop quality and improve crop tolerance to environmental stresses. Se availability to plants depends on the balance between adsorption and desorption, reduction, methylation and oxidation, which are determined by interactions among soil properties, microbial communities and plants. Reduction and methylation processes governed by bacteria or fungi lead to declined Se availability, while Se oxidation regulated by Se-oxidizing microorganisms increases Se availability to plants. Despite a much lower rate of Se oxidization compared to reduction and methylation, the potential roles of microbial communities in increasing Se bioavailability are probably largely underestimated. Enhancing Se oxidation and Se desorption are crucial for the promotion of Se bioavailability and uptake, particularly in Se-deficient soils. Beneficial roles of Se are reported in terms of improved crop growth and quality, and enhanced protection against fungal diseases and abiotic stress through improved photosynthetic traits, increased sugar and amino acid contents, and promoted defense systems. Understanding Se transformation along the plant-soil continuum is crucial for agricultural production and even for human health.

Se transformation and removal by a cattail litter treatment system inoculated with sulfur-based denitrification sludge: Role of the microbial community composition under various temperature and aeration conditions

With a narrow margin between deficiency and toxicity, rising levels of selenium (Se) are threatening aquatic ecosystems. To investigate the role of microorganisms in Se bioremediation, a cattail litter system inoculated with the sulfur-based denitrification sludge was conducted. The results show the litter, as a carrier and nutrient source for bacteria, efficiently removed Se by ~ 97.0% during a 12-d treatment with water circulating. As the major removal pathways, immobilization rates of selenite were ~ 2.9-fold higher than selenate, and the volatilization, contributing to ~ 87.7% of the total Se removal, was significantly correlated with temperature (positively) and oxidation-reduction potential (ORP; negatively). Using X-ray absorption spectroscopy to speciate litter-borne Se, more Se⁰ formed without aeration due to abundant Se-reducing bacteria, among which Azospira and Azospirillum were highly related to the removal of both Se oxyanions, while Desulfovibrio, Azoarcus, Sulfurospirillum, Thauera, Geobacter, Clostridium, and Pediococcus were the major contributors to selenate removal. Overall, our study suggests microbial Se metabolism in the litter system was significantly affected by temperature and ORP, which could be manipulated to enhance Se removal efficiency and the transformation of selenate/selenite into low toxic Se⁰ and volatile Se, reducing risks posed by the residual Se in the system.

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.

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.

Highly stable selenium nanoparticles: Assembly and stabilization via flagellin FliC and porin OmpF in Rahnella aquatilis HX2

Microorganisms play a critical role in the reduction of the more toxic selenite and selenate to the less toxic elemental selenium. However, the assembly process and stability of selenium nanoparticles (SeNPs) remain understudied. The plant growth-promoting rhizobacterium Rahnella aquatilis HX2 can reduce selenite to biogenic SeNPs (BioSeNPs). Two main proteins, namely flagellin FliC and porin OmpF were identified in the BioSeNPs. The fliC and ompF gene mutation experiments demonstrated that the FliC and OmpF could control the assembly of BioSeNPs in vivo. At the same time, the expressed and purified FliC and OmpF could control the assembly of SeNPs in vitro. BioSeNPs produced by R. aquatilis HX2 exhibited high stability under various ionic strengths, while the chemically synthesized SeNPs (CheSeNPs) showed a high level of aggregation. The in vitro experiments verified that FliC and OmpF could prevent the aggregation of the CheSeNPs under various ionic strengths. This work reports the preparation of highly stable BioSeNPs produced by strain R. aquatilis HX2 and verifies that FliC and OmpF both could control the assembly and stability of BioSeNPs. BioSeNPs with high stability could be suitable as nutritional supplement to remedy selenium deficiency and in nanomedicine applications.

Bacillus megaterium-Mediated Synthesis of Selenium Nanoparticles and Their Antifungal Activity against Rhizoctonia solani in Faba Bean Plants

Rhizoctonia root-rot disease causes severe economic losses in a wide range of crops, including Vicia faba worldwide. Currently, biosynthesized nanoparticles have become super-growth promoters as well as antifungal agents. In this study, biosynthesized selenium nanoparticles (Se-NPs) have been examined as growth promoters as well as antifungal agents against Rhizoctonia solani RCMB 031001 in vitro and in vivo. Se-NPs were synthesized biologically by Bacillus megaterium ATCC 55000 and characterized by using UV-Vis spectroscopy, XRD, dynamic light scattering (DLS), and transmission electron microscopy (TEM) imaging. TEM and DLS images showed that Se-NPs are mono-dispersed spheres with a mean diameter of 41.2 nm. Se-NPs improved healthy Vicia faba cv. Giza 716 seed germination, morphological, metabolic indicators, and yield. Furthermore, Se-NPs exhibited influential antifungal activity against R. solani in vitro as well as in vivo. Results revealed that minimum inhibition and minimum fungicidal concentrations of Se-NPs were 0.0625 and 1 mM, respectively. Moreover, Se-NPs were able to decrease the pre-and post-emergence of R. solani damping-off and minimize the severity of root rot disease. The most effective treatment method is found when soaking and spraying were used with each other followed by spraying and then soaking individually. Likewise, Se-NPs improve morphological and metabolic indicators and yield significantly compared with infected control. In conclusion, biosynthesized Se-NPs by B. megaterium ATCC 55000 are a promising and effective agent against R. solani damping-off and root rot diseases in Vicia faba as well as plant growth inducer.

Fourier Transform Infrared (FTIR) Spectroscopic Analyses of Microbiological Samples and Biogenic Selenium Nanoparticles of Microbial Origin: Sample Preparation Effects

To demonstrate the importance of sample preparation used in Fourier transform infrared (FTIR) spectroscopy of microbiological materials, bacterial biomass samples with and without grinding and after different drying periods (1.5-23 h at 45 °C), as well as biogenic selenium nanoparticles (SeNPs; without washing and after one to three washing steps) were comparatively studied by transmission FTIR spectroscopy. For preparing bacterial biomass samples, Azospirillum brasilense Sp7 and A. baldaniorum Sp245 (earlier known as A. brasilense Sp245) were used. The SeNPs were obtained using A. brasilense Sp7 incubated with selenite. Grinding of the biomass samples was shown to result in slight downshifting of the bands related to cellular poly-3-hydroxybutyrate (PHB) present in the samples in small amounts (under ~10%), reflecting its partial crystallisation. Drying for 23 h was shown to give more reproducible FTIR spectra of bacterial samples. SeNPs were shown to contain capping layers of proteins, polysaccharides and lipids. The as-prepared SeNPs contained significant amounts of carboxylated components in their bioorganic capping, which appeared to be weakly bound and were largely removed after washing. Spectroscopic characteristics and changes induced by various sample preparation steps are discussed with regard to optimising sample treatment procedures for FTIR spectroscopic analyses of microbiological specimens.

Role of nano-selenium in health and environment

In recent years, researches on selenium nanoparticle have gained more attention due to its important role in many physiological processes. Generally, selenium nanoparticle has a high level of absorption in regular supplementation comparative to selenium. Therefore it is all-important to develop new techniques to elevate the transportation of selenium compounds (selenoproteins, selenoenzymes, etc.) by increasing their bioavailability, bioactivity, and controlled release. SeNPs have special attention regarding their application as food additives and therapeutic agents. Selenium nanoparticle has biomedical and pharmaceutical uses due to its antioxidant, antimicrobial, antidiabetic, and anticancer effects. Selenium nanoparticle is also used to antagonize the toxic effect of chemical and heavy metals. SeNPs are beneficial for the treatment of water and soil contaminated with metals and heavy metals as it has adsorption capability. Selenium nanoparticle is synthesized by the bioreduction of selenium species (sodium selenate, sodium selenite, selenium dioxide, and selenium tetrachloride, etc.) by using bacteria, fungi, plant, and plant extracts, which have given hope for the bioremediation of selenium contaminated water and soils. This article reviews the procedure of selenium nanoparticle synthesis (physical, chemical and biological methods), characterization (UV-vis spectroscopy, transmission electron microscopy, Scanning electron microscopy, electron dispersive X-ray spectroscopy, X-ray diffraction, Fourier transform-infrared spectroscopy, etc.), with the emphasis on its role and application in health and environment.

Fungal Bioremediation of Selenium-Contaminated Industrial and Municipal Wastewaters

Selenium (Se) is an essential element for most organisms yet can cause severe negative biological consequences at elevated levels. The oxidized forms of Se, selenate [Se(VI)] and selenite [Se(IV)], are more mobile, toxic, and bioavailable than the reduced forms of Se such as volatile or solid phases. Thus, selenate and selenite pose a greater threat to ecosystems and human health. As current Se remediation technologies have varying efficiencies and costs, novel strategies to remove elevated Se levels from environments impacted by anthropogenic activities are desirable. Some common soil fungi quickly remove Se (IV and VI) from solution by aerobic reduction to solid or volatile forms. Here, we perform bench-scale culture experiments of two Se-reducing Ascomycota to determine their Se removal capacity in growth media conditions containing either Se(IV) or Se(VI) as well as in Se-containing municipal (∼25 μg/L Se) and industrial (∼2000 μg/L Se) wastewaters. Dissolved Se was measured throughout the experiments to assess Se concentration and removal rates. Additionally, solid-associated Se was quantified at the end of each experiment to determine the amount of Se removed to solid phases (e.g., Se(0) nanoparticles, biomass-adsorbed Se, or internal organic selenoproteins). Results show that under optimal conditions, fungi more efficiently remove Se(IV) from solution compared to Se(VI). Additionally, both fungi remove a higher percentage of Se from the filtered municipal wastewater compared to the industrial wastewater, though cultures in industrial wastewater retained a greater amount of solid-associated Se. Additional wastewater experiments were conducted with supplemental carbohydrate- or glycerin-based carbon products and additional nitrogen- and phosphorous-containing nutrients in some cases to enhance fungal growth. Relative to unamended wastewater experiments, supplemental carbohydrates promote Se removal from municipal wastewater but minimally impact industrial wastewater removal. This demonstrates that carbon availability and source impacts fungal Se reduction and removal from solution. Calculations to assess the leaching potential of solid-associated Se from fungal biomass show that wastewater Se release will not exceed regulatory limits. This study highlights the considerable potential for the mycoremediation of Se-contaminated wastewaters.

Developments in the study and applications of bacterial transformations of selenium species

Microbial bio-transformations of the essential trace element selenium are now recognized to occur among a wide variety of microorganisms. These transformations are used to convert this element into its assimilated form of selenocysteine, which is at the active center of a number of key enzymes, and to produce selenium nanoparticles, quantum dots, metal selenides, and methylated selenium species that are indispensable for biotechnological and bioremediation applications. The focus of this review is to present the state-of-the-art of all aspects of the investigations into the bacterial transformations of selenium species, and to consider the characterization and biotechnological uses of these transformations and their products.

The Collection of Rhizosphere Microorganisms: its importance for the study of associative plant-bacterium interactions

Microbial culture collections are very important components of biological science. They provide researchers with material for studies and preserve biological resources. One such collection is the Collection of Rhizosphere Microorganisms, kept at the Institute of Biochemistry and Physiology of Plants and Microorganisms of the Russian Academy of Sciences, Saratov (IBPPM). Its activity is primarily directed toward the isolation and preservation of microorganisms from the plant root zone. The international research interest in microorganisms from this ecological niche is not waning, because they are very important for plant growth and development and, consequently, for plant breeding. The group of bacteria with properties of significance for plants has been given the name "plant-growth-promoting rhizobacteria" (PGPR). This group includes nitrogen-fixing soil alpha-proteobacteria of the genus Azospirillum, which form the core of the IBPPM collection. First discovered by Brazilian scientists in the 1970s, azospirilla are now a universally recognized model object for studying the molecular mechanisms underlying plant-bacterium interactions. The broad range of useful properties found in these microorganisms, including the fixation of atmospheric nitrogen, production of phytohormones, solubilization of phosphates, control of pathogens, and formation of induced systemic resistance in the colonized plants, make these bacteria an all-purpose tool that has been used for several decades in basic and applied research. This article reviews the current state of Azospirillum research, with emphasis on the results obtained at the IBPPM. Scientific expeditions across the Saratov region undertaken by IBPPM microbiologists in the early 1980s formed the basis for the unique collection of members of this bacterial taxon. Currently, the collection has more than 160 Azospirillum strains and is one of the largest collections in Europe. The research conducted at the IBPPM is centered mostly on the Azospirillum structures involved in associative symbiosis with plants, primarily extracellular polysaccharide-containing complexes and lectins. The development of immunochemical methods contributed much to our understanding of the overall organization of the surface of rhizosphere bacteria. The extensive studies of the Azospirillum genome largely deepened our understanding of the role of the aforesaid bacterial structures, motility, and biofilms in the colonization of host plant roots. Of interest are also applied studies focusing on agricultural and environmental technologies and on the "green" synthesis of Au, Ag, and Se nanoparticles. The Collection of Rhizosphere Microorganisms continues to grow, being continually supplemented with newly isolated strains. The data presented in this article show the great importance of specialized microbial culture repositories, such as the IBPPM collection, for the development and maintenance of the microbial research base and for the effective solution of basic and applied tasks in microbiology.

Selenite reduction by the rhizobacterium Azospirillum brasilense, synthesis of extracellular selenium nanoparticles and their characterisation

Microbial reduction of selenium oxyanions has attracted attention in recent years. In this study, an original and simple method for the synthesis of extracellular selenium nanoparticles (Se NPs) of relatively uniform size has been developed using strains Sp7 and Sp245 of the ubiquitous plant-growth promoting rhizobacterium Azospirillum brasilense, both capable of selenite (SeO₃^2-) reduction. In addition, a reliable purification protocol for the recovery of the Se NPs has been perfected, which could be applied with minor modifications to cultures of other microbial species. Importantly, it was found that, by changing the conditions of bacterial reduction of selenite, extracellularly localised Se NPs can be obtained using bacteria which would otherwise produce intracellular Se NPs. In particular, bacterial cultures grown up to the end of the logarithmic growth phase, washed free of culture medium and then incubated with selenite, were used to obtain extracellular Se NPs. Their sizes depended on the initial selenite concentration (∼25-80 nm in diameter at 50-10 mM selenite, respectively). The Se NPs obtained were characterised by transmission electron microscopy (TEM), dynamic light scattering, as well as Raman and UV-vis spectroscopies. Their zeta potential was found to be negative (ca. minus 21-24 mV). Bacterial selenite reduction was also studied in the presence of the efflux pump inhibitor carbonyl cyanide m-chlorophenylhydrazone (CCCP). In this case, TEM indicated the formation only of intracellular selenium crystallites. The data show that the formation of extracellular Se NPs requires normal bacterial metabolic activity, while CCCP evidently blocks the membrane export of Se⁰ nuclei.

Addressing the presence of biogenic selenium nanoparticles in yeast cells: analytical strategies based on ICP-TQ-MS

Complementary analytical strategies based on ICP-TQ-MS were used for the detection and characterization of selenium-containing nanoparticles in selenized yeast.

Spatially Resolved Chemical Analysis of Geobacter sulfurreducens Cell Surface

Geobacter sulfurreducens is of interest for the highest efficiency of power generation and extremely long extracellular electron transfer (EET) between the bacterium and electrodes. Despite more than 15 years of intensive molecular biological research, there is still no clear answer which molecules are responsible for these processes. In the present work, we look at the problem from another (atomic) perspective and identify the location and shape of the compounds that are known to be conductive, particularly those containing Fe atoms. By using highly sophisticated energy dispersive X-ray spectroscopy combined with high-angle annular dark-field transmission electron microscopy enabling detection, identification, and localization of chemical compounds on the surface at nearly atomic spatial resolution, we analyze Fe spatial distribution within the G. sulfurreducens community. We discover the presence of small Fe-containing particles on the surface of the bacterium cells. The size of the particles (diameter 5.6 nm) is highly reproducible and comparable with the size of a single protein. The particles cover about 2% of the cell surface, which is similar to that expected for molecular conductors responsible for electron transfer through the bacterium cell wall. We find that G. sulfurreducens filaments ("bacterial molecular wires") also contain Fe atoms in their bundles. We observe that the bacterium enable changing the distance between the Fe-containing bundles in the filaments from separated to attached (the latter is needed for the efficient electron transfer between the Fe-containing particles), depending on the bacterium metabolic activity and attachment to extracellular substrates. These results are consistent with the recently published research about the role of Fe atoms in protein molecular conductance ( Phys. Chem. Chem. Phys. , 2018 , 20 , 14072 - 14081 ) and show what type of Fe-containing particles are involved in the bacterial extracellular communication. They can be used for the design and construction of artificial biomolecular wires and bioinorganic interfaces.

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.

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.

FTIR and Raman spectroscopic studies of selenium nanoparticles synthesised by the bacterium Azospirillum thiophilum

Vibrational (Fourier transform infrared (FTIR) and Raman) spectroscopic techniques can provide unique molecular-level information on the structural and compositional characteristics of complicated biological objects. Thus, their applications in microbiology and related fields are steadily increasing. In this communication, biogenic selenium nanoparticles (Se NPs) were obtained via selenite (SeO₃^2-) reduction by the bacterium Azospirillum thiophilum (strain VKM B-2513) for the first time, using an original methodology for obtaining extracellular NPs. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) showed the Se NPs to have average diameters within 160-250nm; their zeta potential was measured to be minus 18.5mV. Transmission FTIR spectra of the Se NPs separated from bacterial cells showed typical proteinacious, polysaccharide and lipid-related bands, in line with TEM data showing a thin layer covering the Se NPs surface. Raman spectra of dried Se NPs layer in the low-frequency region (under 500cm^-1 down to 150cm^-1) showed a single very strong band with a maximum at 250cm^-1 which, in line with its increased width (ca. 30cm^-1 at half intensity), can be attributed to amorphous elementary Se. Thus, a combination of FTIR and Raman spectroscopic approaches is highly informative in non-destructive analysis of structural and compositional properties of biogenic Se NPs.

Biotransformation and detoxification of selenite by microbial biogenesis of selenium-sulfur nanoparticles

This study combines the interaction between the toxic oxyanions selenite and selenate and the plant growth promoting bacterium Azospirillum brasilense with a comprehensive characterization of the formed selenium particles. As selenium is an essential trace element, but also toxic in high concentrations, its state of occurrence in nature is of major concern. Growth of the bacterium was affected by selenite (1-5mM) only, observable as a prolonged growth lag-phase of 3days. Subsequently, selenite reduction occurred under aerobic conditions resulting in extracellularly formed insoluble Se⁰ particles. Complementary studies by microscopic and spectroscopic techniques revealed the particles to be homogeneous and stable Se_8-nS_n structured spheres with an average size of 400nm and highly negative surface charge of -18mV in the neutral pH range. As this is the first study showing Azospirillum brasilense being able to biotransform selenite to selenium particles containing a certain amount of sulfur, even if environmental waters supplemented with selenite were used, they may significantly contribute to the biogeochemical cycling of both elements in soil as well as to their soil-plant transfer. Therefore, microbial biotransformation of selenite under certain circumstances may be used for various bio-remediation and bio-technological applications.

Proteins in microbial synthesis of selenium nanoparticles

Biogenic formation of nano-sized particles composed of various materials (in particular, selenium) by live microorganisms is widespread in nature. This phenomenon has been increasingly attracting the attention of researchers over the last decade not only owing to a range of diverse applications of such nanoparticles (NPs) in nanobiotechnology, but also because of the specificity of methodologies and mechanisms of NPs formation related to "green synthesis". In this mini-review, recent data are discussed on the multifaceted role of proteins in the processes of microbial reduction of selenium oxyanions and the formation of Se NPs. Besides the involvement of proteins in reducing selenites and selenates, their participation in the microbially driven growth and stabilisation of Se NPs is analysed, which results in the formation of unique nanostructured materials differing from those obtained chemically. This mini-review is thus focussed on proteins involved in microbial synthesis of Se NPs and on instrumental analysis of these processes and their products (biogenic nanostructured selenium particles functionalised by a surface-capping layer of various biomacromolecules).

Aerobic and anaerobic biosynthesis of nano-selenium for remediation of mercury contaminated soil

Selenium (Se) nanoparticles are often synthesized by anaerobes. However, anaerobic bacteria cannot be directly applied for bioremediation of contaminated top soil which is generally aerobic. In this study, a selenite-reducing bacterium, Citrobacter freundii Y9, demonstrated high selenite reducing power and produced elemental nano-selenium nanoparticles (nano-Se⁰) under both aerobic and anaerobic conditions. The biogenic nano-Se⁰ converted 45.8-57.1% and 39.1-48.6% of elemental mercury (Hg⁰) in the contaminated soil to insoluble mercuric selenide (HgSe) under anaerobic and aerobic conditions, respectively. Addition of sodium dodecyl sulfonate enhanced Hg⁰ remediation, probably owing to the release of intracellular nano-Se⁰ from the bacterial cells for Hg fixation. The reaction product after remediation was identified as non-reactive HgSe that was formed by amalgamation of nano-Se⁰ and Hg⁰. Biosynthesis of nano-Se⁰ both aerobically and anaerobically therefore provides a versatile and cost-effective remediation approach for Hg⁰-contaminated surface and subsurface soils, where the redox potential often changes dramatically.

Selenite biotransformation and detoxification by Stenotrophomonas maltophilia SeITE02: Novel clues on the route to bacterial biogenesis of selenium nanoparticles

A putative biosynthetic mechanism for selenium nanoparticles (SeNPs) and efficient reduction of selenite (SeO₃^2-) in the bacterial strain Stenotrophomonas maltophilia SeITE02 are addressed here on the basis of information gained by a combined approach relying on a set of physiological, chemical/biochemical, microscopy, and proteomic analyses. S. maltophilia SeITE02 is demonstrated to efficiently transform selenite into elemental selenium (Se°) by reducing 100% of 0.5mM of this toxic oxyanion to Se° nanoparticles within 48h growth, in liquid medium. Since the selenite reducing activity was detected in the cytoplasmic protein fraction, while biogenic SeNPs showed mainly extracellular localization, a releasing mechanism of SeNPs from the intracellular environment is hypothesized. SeNPs appeared spherical in shape and with size ranging from 160nm to 250nm, depending on the age of the cultures. Proteomic analysis carried out on the cytoplasmic fraction identified an alcohol dehydrogenase homolog, conceivably correlated with the biogenesis of SeNPs. Finally, by Fourier Transformed Infrared Spectrometry, protein and lipid residues were detected on the surface of biogenic SeNPs. Eventually, this strain might be efficaciously exploited for the remediation of selenite-contaminated environmental matrices due to its high SeO₃^2- reducing efficiency. Biogenic SeNPs may also be considered for technological applications in different fields.

Production of selenium nanoparticles in Pseudomonas putida KT2440

Selenium (Se) is an essential element for the cell that has multiple applications in medicine and technology; microorganisms play an important role in Se transformations in the environment. Here we report the previously unidentified ability of the soil bacterium Pseudomonas putida KT2440 to synthesize nanoparticles of elemental selenium (nano-Se) from selenite. Our results show that P. putida is able to reduce selenite aerobically, but not selenate, to nano-Se. Kinetic analysis indicates that, in LB medium supplemented with selenite (1 mM), reduction to nano-Se occurs at a rate of 0.444 mmol L⁻¹ h⁻¹ beginning in the middle-exponential phase and with a final conversion yield of 89%. Measurements with a transmission electron microscope (TEM) show that nano-Se particles synthesized by P. putida have a size range of 100 to 500 nm and that they are located in the surrounding medium or bound to the cell membrane. Experiments involving dynamic light scattering (DLS) show that, in aqueous solution, recovered nano-Se particles have a size range of 70 to 360 nm. The rapid kinetics of conversion, easy retrieval of nano-Se and the metabolic versatility of P. putida offer the opportunity to use this model organism as a microbial factory for production of selenium nanoparticles.

Microbial Transformations of Selenium Species of Relevance to Bioremediation

Selenium species, particularly the oxyanions selenite (SeO3 (2-)) and selenate (SeO4 (2-)), are significant pollutants in the environment that leach from rocks and are released by anthropogenic activities. Selenium is also an essential micronutrient for organisms across the tree of life, including microorganisms and human beings, particularly because of its presence in the 21st genetically encoded amino acid, selenocysteine. Environmental microorganisms are known to be capable of a range of transformations of selenium species, including reduction, methylation, oxidation, and demethylation. Assimilatory reduction of selenium species is necessary for the synthesis of selenoproteins. Dissimilatory reduction of selenate is known to support the anaerobic respiration of a number of microorganisms, and the dissimilatory reduction of soluble selenate and selenite to nanoparticulate elemental selenium greatly reduces the toxicity and bioavailability of selenium and has a major role in bioremediation and potentially in the production of selenium nanospheres for technological applications. Also, microbial methylation after reduction of Se oxyanions is another potentially effective detoxification process if limitations with low reaction rates and capture of the volatile methylated selenium species can be overcome. This review discusses microbial transformations of different forms of Se in an environmental context, with special emphasis on bioremediation of Se pollution.

Aspects of a Distinct Cytotoxicity of Selenium Salts and Organic Selenides in Living Cells with Possible Implications for Drug Design

Selenium is traditionally considered as an antioxidant element and selenium compounds are often discussed in the context of chemoprevention and therapy. Recent studies, however, have revealed a rather more colorful and diverse biological action of selenium-based compounds, including the modulation of the intracellular redox homeostasis and an often selective interference with regulatory cellular pathways. Our basic activity and mode of action studies with simple selenium and tellurium salts in different strains of Staphylococcus aureus (MRSA) and Saccharomyces cerevisiae indicate that such compounds are sometimes not particularly toxic on their own, yet enhance the antibacterial potential of known antibiotics, possibly via the bioreductive formation of insoluble elemental deposits. Whilst the selenium and tellurium compounds tested do not necessarily act via the generation of Reactive Oxygen Species (ROS), they seem to interfere with various cellular pathways, including a possible inhibition of the proteasome and hindrance of DNA repair. Here, organic selenides are considerably more active compared to simple salts. The interference of selenium (and tellurium) compounds with multiple targets could provide new avenues for the development of effective antibiotic and anticancer agents which may go well beyond the traditional notion of selenium as a simple antioxidant.