Biomolecular composition of capping layer and stability of biogenic selenium nanoparticles synthesized by five bacterial species

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.

Impact of Biogenic and Chemogenic Selenium Nanoparticles on Model Eukaryotic Lipid Membranes

Microbial nanotechnology is an expanding research area devoted to producing biogenic metal and metalloid nanomaterials (NMs) using microorganisms. Often, biogenic NMs are explored as antimicrobial, anticancer, or antioxidant agents. Yet, most studies focus on their applications rather than the underlying mechanism of action or toxicity. Here, we evaluate the toxicity of our well-characterized biogenic selenium nanoparticles (bSeNPs) produced by the Stenotrophomonas maltophilia strain SeITE02 against the model yeast Saccharomyces cerevisiae comparing it with chemogenic SeNPs (cSeNPs). Knowing from previous studies that the biogenic extract contained bSeNPs in an organic material (OM) and supported here by Fourier transform infrared spectroscopy, we removed and incubated it with cSeNPs (cSeNPs_OM) to assess its influence on the toxicity of these formulations. Specifically, we focused on the first stages of the eukaryotic cell exposure to these samples─i.e., their interaction with the cell lipid membrane, which was mimicked by preparing vesicles from yeast polar lipid extract or phosphatidylcholine lipids. Fluidity changes derived from biogenic and chemogenic samples revealed that the bSeNP extract mediated the overall rigidification of lipid vesicles, while cSeNPs showed negligible effects. The OM and cSeNPs_OM induced similar modifications to the bSeNP extract, reiterating the need to consider the OM influence on the physical-chemical and biological properties of bSeNP extracts.

A Comparative Study of the Synthesis and Characterization of Biogenic Selenium Nanoparticles by Two Contrasting Endophytic Selenobacteria

The present study examined the biosynthesis and characterization of selenium nanoparticles (SeNPs) using two contrasting endophytic selenobacteria, one Gram-positive (Bacillus sp. E5 identified as Bacillus paranthracis) and one Gram-negative (Enterobacter sp. EC5.2 identified as Enterobacter ludwigi), for further use as biofortifying agents and/or for other biotechnological purposes. We demonstrated that, upon regulating culture conditions and selenite exposure time, both strains were suitable "cell factories" for producing SeNPs (B-SeNPs from B. paranthracis and E-SeNPs from E. ludwigii) with different properties. Briefly, dynamic light scattering (DLS), transmission electron microscopy (TEM), and atomic force microscopy (AFM) studies revealed that intracellular E-SeNPs (56.23 ± 4.85 nm) were smaller in diameter than B-SeNPs (83.44 ± 2.90 nm) and that both formulations were located in the surrounding medium or bound to the cell wall. AFM images indicated the absence of relevant variations in bacterial volume and shape and revealed the existence of layers of peptidoglycan surrounding the bacterial cell wall under the conditions of biosynthesis, particularly in the case of B. paranthracis. Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), energy-dispersive X-ray (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) showed that SeNPs were surrounded by the proteins, lipids, and polysaccharides of bacterial cells and that the numbers of the functional groups present in B-SeNPs were higher than in E-SeNPs. Thus, considering that these findings support the suitability of these two endophytic stains as potential biocatalysts to produce high-quality Se-based nanoparticles, our future efforts must be focused on the evaluation of their bioactivity, as well as on the determination of how the different features of each SeNP modulate their biological action and their stability.

Opportunities and obstacles in microbial synthesis of metal nanoparticles

Metallic nanoparticles (MeNPs) are widely used in many areas such as biomedicine, packaging, cosmetics, colourants, agriculture, antimicrobial agents, cleaning products, as components of electronic devices and nutritional supplements. In addition, some MeNPs exhibit quantum properties, making them suitable materials in the photonics, electronic and energy industries. Through the lens of technology, microbes can be considered nanofactories capable of producing enzymes, metabolites and capping materials involved in the synthesis, assembly and stabilization of MeNPs. This bioprocess is considered more ecofriendly and less energy intensive than the current chemical synthesis routes. However, microbial synthesis of MeNPs as an alternative method to the chemical synthesis of nanomaterials still faces some challenges that need to be solved. Some of these challenges are described in this Editorial.

Impact of microbial processes on the safety of deep geological repositories for radioactive waste

To date, the increasing production of radioactive waste due to the extensive use of nuclear power is becoming a global environmental concern for society. For this reason, many countries have been considering the use of deep geological repositories (DGRs) for the safe disposal of this waste in the near future. Several DGR designs have been chemically, physically, and geologically well characterized. However, less is known about the influence of microbial processes for the safety of these disposal systems. The existence of microorganisms in many materials selected for their use as barriers for DGRs, including clay, cementitious materials, or crystalline rocks (e.g., granites), has previously been reported. The role that microbial processes could play in the metal corrosion of canisters containing radioactive waste, the transformation of clay minerals, gas production, and the mobility of the radionuclides characteristic of such residues is well known. Among the radionuclides present in radioactive waste, selenium (Se), uranium (U), and curium (Cm) are of great interest. Se and Cm are common components of the spent nuclear fuel residues, mainly as 79Se isotope (half-life 3.27 × 105 years), 247Cm (half-life: 1.6 × 107 years) and 248Cm (half-life: 3.5 × 106 years) isotopes, respectively. This review presents an up-to-date overview about how microbes occurring in the surroundings of a DGR may influence their safety, with a particular focus on the radionuclide-microbial interactions. Consequently, this paper will provide an exhaustive understanding about the influence of microorganisms in the safety of planned radioactive waste repositories, which in turn might improve their implementation and efficiency.

Biogenic Selenium Nanoparticles in Biomedical Sciences: Properties, Current Trends, Novel Opportunities and Emerging Challenges in Theranostic Nanomedicine

Selenium is an important dietary supplement and an essential trace element incorporated into selenoproteins with growth-modulating properties and cytotoxic mechanisms of action. However, different compounds of selenium usually possess a narrow nutritional or therapeutic window with a low degree of absorption and delicate safety margins, depending on the dose and the chemical form in which they are provided to the organism. Hence, selenium nanoparticles (SeNPs) are emerging as a novel therapeutic and diagnostic platform with decreased toxicity and the capacity to enhance the biological properties of Se-based compounds. Consistent with the exciting possibilities offered by nanotechnology in the diagnosis, treatment, and prevention of diseases, SeNPs are useful tools in current biomedical research with exceptional benefits as potential therapeutics, with enhanced bioavailability, improved targeting, and effectiveness against oxidative stress and inflammation-mediated disorders. In view of the need for developing eco-friendly, inexpensive, simple, and high-throughput biomedical agents that can also ally with theranostic purposes and exhibit negligible side effects, biogenic SeNPs are receiving special attention. The present manuscript aims to be a reference in its kind by providing the readership with a thorough and comprehensive review that emphasizes the current, yet expanding, possibilities offered by biogenic SeNPs in the biomedical field and the promise they hold among selenium-derived products to, eventually, elicit future developments. First, the present review recalls the physiological importance of selenium as an oligo-element and introduces the unique biological, physicochemical, optoelectronic, and catalytic properties of Se nanomaterials. Then, it addresses the significance of nanosizing on pharmacological activity (pharmacokinetics and pharmacodynamics) and cellular interactions of SeNPs. Importantly, it discusses in detail the role of biosynthesized SeNPs as innovative theranostic agents for personalized nanomedicine-based therapies. Finally, this review explores the role of biogenic SeNPs in the ongoing context of the SARS-CoV-2 pandemic and presents key prospects in translational nanomedicine.

Biogenic metallic nanoparticles as enzyme mimicking agents

The use of biological systems such as plants, bacteria, and fungi for the synthesis of nanomaterials has emerged to fill the gap in the development of sustainable methods that are non-toxic, pollution-free, environmentally friendly, and economical for synthesizing nanomaterials with potential in biomedicine, biotechnology, environmental science, and engineering. Current research focuses on understanding the characteristics of biogenic nanoparticles as these will form the basis for the biosynthesis of nanoparticles with multiple functions due to the physicochemical properties they possess. This review briefly describes the intrinsic enzymatic mimetic activity of biogenic metallic nanoparticles, the cytotoxic effects of nanoparticles due to their physicochemical properties and the use of capping agents, molecules acting as reducing and stability agents and which aid to alleviate toxicity. The review also summarizes recent green synthetic strategies for metallic nanoparticles.

Enhancing tellurite and selenite bioconversions by overexpressing a methyltransferase from Aromatoleum sp. CIB

Pollution by metalloids, e.g., tellurite and selenite, is of serious environmental concern and, therefore, there is an increasing interest in searching for ecologically friendly solutions for their elimination. Some microorganisms are able to reduce toxic tellurite/selenite into less toxic elemental tellurium (Te) and selenium (Se). Here, we describe the use of the environmentally relevant β-proteobacterium Aromatoleum sp. CIB as a platform for tellurite elimination. Aromatoleum sp. CIB was shown to tolerate 0.2 and 0.5 mM tellurite at aerobic and anaerobic conditions, respectively. Furthermore, the CIB strain was able to reduce tellurite into elemental Te producing rod-shaped Te nanoparticles (TeNPs) of around 200 nm length. A search in the genome of Aromatoleum sp. CIB revealed the presence of a gene, AzCIB_0135, which encodes a new methyltransferase that methylates tellurite and also selenite. AzCIB_0135 orthologs are widely distributed in bacterial genomes. The overexpression of the AzCIB_0135 gene both in Escherichia coli and Aromatoleum sp. CIB speeds up tellurite and selenite removal, and it enhances the production of rod-shaped TeNPs and spherical Se nanoparticles (SeNPs), respectively. Thus, the overexpression of a methylase becomes a new genetic strategy to optimize bacterial catalysts for tellurite/selenite bioremediation and for the programmed biosynthesis of metallic nanoparticles of biotechnological interest.

Mycosynthesis of highly fluorescent selenium nanoparticles from Fusarium oxysporum, their antifungal activity against black fungus Aspergillus niger, and in-vivo biodistribution studies

In the past few years, photo-luminescent inorganic materials have been studied extensively as fluorescent sensors, and diagnostic and bioimaging tools. The assessment of photoluminescence (PL) properties of selenium nanoparticles (Se NPs), especially mycosynthesized Se NPs, is still in its infancy. Herein, we have biosynthesized highly dispersed fluorescent Se NPs (42 nm) using endophytic fungus Fusarium oxysporum, and fully characterized them using sophisticated instruments like TEM, XRD, UV–Vis spectrophotometer, FTIR, and PL spectrometer. To determine the therapeutic efficacy and side effect profiles, these crystalline Se NPs were radiolabeled with technetium-99m (^99mTc) and their biodistribution and renal clearance times were investigated in the normal Wister rat. The results showed that these Se NPs may be useful for targeting the lungs and liver dysfunction as significant accumulation of these NPs was observed in the liver (approx. 19.47 ± 4%) and lungs (at 6 ± 1%) after 10 min of post-injection. Quick circulation and the presence of Se NPs in kidney (3.8 ± 2%) also suggested the easy excretion of these NPs from the body through urinary tract. Furthermore, the antioxidant activity of Se NPs (IC₅₀, 159.5 μg/mL) has been investigated using DPPH free radical scavenging assay with scavenging efficacy of 80.4% where ascorbic acid (IC₅₀, 5.6 μg/mL) was used as a positive control. Additionally, the microscopic study of the inhibition zone encircled around Se NPs confirmed their strong antifungal and antisporulant activity against the black fungus Aspergillus niger.

Extremophilic Microorganisms for the Green Synthesis of Antibacterial Nanoparticles

The biogenic synthesis of nanomaterials, i.e., synthesis carried out by means of living organisms, is an emerging technique in nanotechnology since it represents a greener and more eco-friendly method for the production of nanomaterials. In this line, in order to find new biological entities capable of biogenic synthesis, we tested the ability of some extremophilic microorganisms to carry out the biogenic production of AgNPs and SeNPs. Silver NPs were produced extracellularly by means of the thermophilic Thermus thermophilus strain SAMU; the haloalkaliphilic Halomonas campaniensis strain 5AG was instead found to be useful for the synthesis of SeNPs. The structural characterization of the biogenic nanoparticles showed that both the Ag and Se NPs possessed a protein coating on their surface and that they were organized in aggregates. Moreover, both types of NPs were found be able to exert an interesting antibacterial effect against either Gram-positive or Gram-negative species. This study confirmed that extremophilic microorganisms can be considered valuable producers of biologically active nanoparticles; nevertheless, further experiments must be performed to improve the synthesis protocols in addition to the downstream processes.

Tellurite and Selenite: how can these two oxyanions be chemically different yet so similar in the way they are transformed to their metal forms by bacteria?

This opinion review explores the microbiology of tellurite, TeO₃²⁻ and selenite, SeO₃²⁻ oxyanions, two similar Group 16 chalcogen elements, but with slightly different physicochemical properties that lead to intriguing biological differences. Selenium, Se, is a required trace element compared to tellurium, Te, which is not. Here, the challenges around understanding the uptake transport mechanisms of these anions, as reflected in the model organisms used by different groups, are described. This leads to a discussion around how these oxyanions are subsequently reduced to nanomaterials, which mechanistically, has controversies between ideas around the molecule chemistry, chemical reactions involving reduced glutathione and reactive oxygen species (ROS) production along with the bioenergetics at the membrane versus the cytoplasm. Of particular interest is the linkage of glutathione and thioredoxin chemistry from the cytoplasm through the membrane electron transport chain (ETC) system/quinones to the periplasm. Throughout the opinion review we identify open and unanswered questions about the microbial physiology under selenite and tellurite exposure. Thus, demonstrating how far we have come, yet the exciting research directions that are still possible. The review is written in a conversational manner from three long-term researchers in the field, through which to play homage to the late Professor Claudio Vásquez.

Preparation and Anti-tumor Study of Dextran 70,000-Selenium Nanoparticles and Poloxamer 188-Selenium Nanoparticles

The anti-tumor effect of selenium nanoparticles (SeNPs) has received more and more attention. However, the clinical application of SeNPs is not optimistic due to the poor stability. To improve the stability of SeNPs, many polymers are used to modify the SeNPs. However, most of the polymers are not approved by FDA. It is significant to develop a SeNPs product with good stability for clinic application. Dextran 70,000 (T70) and poloxamer 188 (P188) are FDA-approved pharmaceutical injection excipients. In this study, we decorate SeNPs with T70 and P188 and assess the physicochemical characterization, storage stability, and anti-tumor activities of T70-SeNPs and P188-SeNPs. Transmission electron microscopy (TEM) shows that T70-SeNPs and P188-SeNPs are spherical particles with particle sizes of 110 nm and 60 nm respectively. Fourier-Transform Infrared Spectra (FT-IR) show that T70 or P188 can interact with SeNPs through hydrogen bonding. Stability study shows that P188-SeNPs freeze-dried powder and T70-SeNPs freeze-dried powder remain stable at 4℃ for 6 months. T70-SeNPs and P188-SeNPs can aggregate in cell matrix and play an anti-tumor role to HepG2 by promoting apoptosis, increasing reactive oxygen species (ROS) content and reducing mitochondrial membrane potential (MMP). This study can provide reference for industrial production of SeNPs products.

Anticancer Activity of Biogenic Selenium Nanoparticles: Apoptotic and Immunogenic Cell Death Markers in Colon Cancer Cells

Colorectal cancer is a health problem with high mortality rates and prevalence. Thus, innovative treatment approaches need to be developed. Biogenic nanoparticles are nanomaterials that can be synthesised in biological systems and, compared to chemically synthesised nanoparticles, have better bioavailability while being more cost-effective, eco-friendlier, and less toxic. In our previous studies, the probiotic strain Lactobacillus casei ATCC 393 was used to synthesise selenium nanoparticles (SeNps), which were shown to inhibit colon cancer cell growth in vitro and in vivo. Herein, we have further investigated SeNps' pro-apoptotic activity and their ability to induce immunogenic cell death (ICD) in colon cancer cells. The SeNps' effect on Caco-2 cells growth was examined along with their potential to induce caspase activation. Moreover, the expression of typical pro-apoptotic and ICD markers were examined in SeNps-treated HT29 and CT26 cells by flow cytometry, Western blot, ELISA and fluorescence microscopy. Elevated caspase-3 activation and surface phosphatyldoserine, that subsided upon co-incubation with a pan-caspase inhibitor, were detected in SeNps-treated cells. Furthermore, nanoparticles induced modulation of the expression of various apoptosis-related proteins. We also report the detection of biomarkers involved in ICD, namely the translocation of calreticulin and ERp57, the release of HMGB1 and ATP, and the secretion of pro-inflammatory cytokines from SeNps-treated cells. Moreover, RAW246.7 macrophages exhibited a higher rate of phagocytosis against treated CT26 when compared to control cells. Taken together, our findings indicate that treatment with SeNps might be an efficient strategy to destroy tumour cells by inducing apoptotic cell death and triggering immune responses.

Untargeted Metabolomics Investigation on Selenite Reduction to Elemental Selenium by Bacillus mycoides SeITE01

Bacillus mycoides SeITE01 is an environmental isolate that transforms the oxyanion selenite (SeO 3 2 - ) into the less bioavailable elemental selenium (Se0) forming biogenic selenium nanoparticles (Bio-SeNPs). In the present study, the reduction of sodium selenite (Na2SeO3) by SeITE01 strain and the effect ofSeO 3 2 - exposure on the bacterial cells was examined through untargeted metabolomics. A time-course approach was used to monitor both cell pellet and cell free spent medium (referred as intracellular and extracellular, respectively) metabolites in SeITE01 cells treated or not withSeO 3 2 - . The results show substantial biochemical changes in SeITE01 cells when exposed toSeO 3 2 - . The initial uptake ofSeO 3 2 - by SeITE01 cells (3h after inoculation) shows both an increase in intracellular levels of 4-hydroxybenzoate and indole-3-acetic acid, and an extracellular accumulation of guanosine, which are metabolites involved in general stress response adapting strategies. Proactive and defensive mechanisms againstSeO 3 2 - are observed between the end of lag (12h) and beginning of exponential (18h) phases. Glutathione and N-acetyl-L-cysteine are thiol compounds that would be mainly involved in Painter-type reaction for the reduction and detoxification ofSeO 3 2 - to Se0. In these growth stages, thiol metabolites perform a dual role, both acting against the toxic and harmful presence of the oxyanion and as substrate or reducing sources to scavenge ROS production. Moreover, detection of the amino acids L-threonine and ornithine suggests changes in membrane lipids. Starting from stationary phase (24 and 48h), metabolites related to the formation and release of SeNPs in the extracellular environment begin to be observed. 5-hydroxyindole acetate, D-[+]-glucosamine, 4-methyl-2-oxo pentanoic acid, and ethanolamine phosphate may represent signaling strategies following SeNPs release from the cytoplasmic compartment, with consequent damage to SeITE01 cell membranes. This is also accompanied by intracellular accumulation of trans-4-hydroxyproline and L-proline, which likely represent osmoprotectant activity. The identification of these metabolites suggests the activation of signaling strategies that would protect the bacterial cells fromSeO 3 2 - toxicity while it is converting into SeNPs.

Selenium respiration in anaerobic bacteria: Does energy generation pay off?

Selenium (Se) respiration in bacteria was revealed for the first time at the end of 1980s. Although thermodynamically-favorable, energy-dense and documented in phylogenetically-diverse bacteria, this metabolic process appears to be accompanied by a number of challenges and numerous unanswered questions. Selenium oxyanions, SeO₄^2- and SeO₃^2-, are reduced to elemental Se (Se⁰) through anaerobic respiration, the end product being solid and displaying a considerable size (up to 500 nm) at the bacterial scale. Compared to other electron acceptors used in anaerobic respiration (e.g. N, S, Fe, Mn, and As), Se is one of the few elements whose end product is solid. Furthermore, unlike other known bacterial intracellular accumulations such as volutin (inorganic polyphosphate), S⁰, glycogen or magnetite, Se⁰ has not been shown to play a nutritional or ecological role for its host. In the context of anaerobic respiration of Se oxyanions, biogenic Se⁰ appears to be a by-product, a waste that needs proper handling, and this raises the question of the evolutionary implications of this process. Why would bacteria use a respiratory substrate that is useful, in the first place, and then highly detrimental? Interestingly, in certain artificial ecosystems (e.g. upflow bioreactors) Se⁰ might help bacterial cells to increase their density and buoyancy and thus avoid biomass wash-out, ensuring survival. This review article provides an in-depth analysis of selenium respiration (model selenium respiring bacteria, thermodynamics, respiratory enzymes, and genetic determinants), complemented by an extensive discussion about the evolutionary implications and the properties of biogenic Se⁰ using published and original/unpublished results.

Microbial Fabricated Nanosystems: Applications in Drug Delivery and Targeting

The emergence of nanosystems for different biomedical and drug delivery applications has drawn the attention of researchers worldwide. The likeness of microorganisms including bacteria, yeast, algae, fungi, and even viruses toward metals is well-known. Higher tolerance to toxic metals has opened up new avenues of designing microbial fabricated nanomaterials. Their synthesis, characterization and applications in bioremediation, biomineralization, and as a chelating agent has been well-documented and reviewed. Further, these materials, due to their ability to get functionalized, can also be used as theranostics i.e., both therapeutic as well as diagnostic agents in a single unit. Current article attempts to focus particularly on the application of such microbially derived nanoformulations as a drug delivery and targeting agent. Besides metal-based nanoparticles, there is enough evidence wherein nanoparticles have been formulated using only the organic component of microorganisms. Enzymes, peptides, polysaccharides, polyhydroxyalkanoate (PHA), poly-(amino acids) are amongst the most used biomolecules for guiding crystal growth and as a capping/reducing agent in the fabrication of nanoparticles. This has promulgated the idea of complete green chemistry biosynthesis of nano-organics that are most sought after in terms of their biocompatibility and bioavailability.

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.