Tellurite-mediated thiol oxidation in Escherichia coli

Simultaneous bioreduction of tellurite and selenite by Yarrowia lipolytica, Trichosporon cutaneum, and their co-culture along with characterization of biosynthesized Te-Se nanoparticles

BACKGROUND

Natural and anthropogenic activities, such as weathering of rocks and industrial processes, result in the release of toxic oxyanions such as selenium (Se) and tellurium (Te) into the environment. Due to the high toxicity of these compounds, their removal from the environment is vital.

RESULTS

In this study, two yeast strains, Yarrowia lipolytica and Trichosporon cutaneum, were selected as the superior strains for the bioremediation of tellurium and selenium. The reduction analyses showed that exposure to selenite induced more detrimental effects on the strains compared to tellurite. In addition, co-reduction of pollutants displayed almost the same results in selenite reduction and more than ~ 20% higher tellurite reduction in 50 h, which shows that selenite triggered higher tellurite reduction in both strains. The selenite and tellurite kinetics of removal were consistent with the first-order model because of their inhibitory behavior. The result of several characterization experiments, such as FE-SEM (Field emission scanning electron microscopy), dynamic light scattering (DLS), Fourier-transform infrared spectroscopy (FTIR), X-ray diffractometer (XRD), and dispersive X-ray (EDX) on Te-Se nanoparticles (NPs) revealed that the separated Te-Se NPs were needle-like, spherical, and amorphous, consisted of Te-Se NPs ranging from 25 to 171 nm in size, and their surface was covered with different biomolecules.

CONCLUSIONS

Remarkably, this work shows, for the first time, the simultaneous bioreduction of tellurite and selenite and the production of Te-Se NPs using yeast strains, indicating their potential in this area, which may be applied to the nanotechnology industry and environmental remediation.

Insights into the Synergistic Antibacterial Activity of Silver Nitrate with Potassium Tellurite against Pseudomonas aeruginosa

The constant, ever-increasing antibiotic resistance crisis leads to the announcement of "urgent, novel antibiotics needed" by the World Health Organization. Our previous works showed a promising synergistic antibacterial activity of silver nitrate with potassium tellurite out of thousands of other metal/metalloid-based antibacterial combinations. The silver-tellurite combined treatment not only is more effective than common antibiotics but also prevents bacterial recovery, decreases the risk of future resistance chance, and decreases the effective concentrations. We demonstrate that the silver-tellurite combination is effective against clinical isolates. Further, this study was conducted to address knowledge gaps in the available data on the antibacterial mechanism of both silver and tellurite, as well as to give insight into how the mixture provides synergism as a combination. Here, we defined the differentially expressed gene profile of Pseudomonas aeruginosa under silver, tellurite, and silver-tellurite combination stress using an RNA sequencing approach to examine the global transcriptional changes in the challenged cultures grown in simulated wound fluid. The study was complemented with metabolomics and biochemistry assays. Both metal ions mainly affected four cellular processes, including sulfur homeostasis, reactive oxygen species response, energy pathways, and the bacterial cell membrane (for silver). Using a Caenorhabditis elegans animal model we showed silver-tellurite has reduced toxicity over individual metal/metalloid salts and provides increased antioxidant properties to the host. This work demonstrates that the addition of tellurite would improve the efficacy of silver in biomedical applications. IMPORTANCE Metals and/or metalloids could represent antimicrobial alternatives for industrial and clinical applications (e.g., surface coatings, livestock, and topical infection control) because of their great properties, such as good stability and long half-life. Silver is the most common antimicrobial metal, but resistance prevalence is high, and it can be toxic to the host above a certain concentration. We found that a silver-tellurite composition has antibacterial synergistic effect and that the combination is beneficial to the host. So, the efficacy and application of silver could increase by adding tellurite in the recommended concentration(s). We used different methods to evaluate the mechanism for how this combination can be so incredibly synergistic, leading to efficacy against antibiotic- and silver-resistant isolates. Our two main findings are that (i) both silver and tellurite mostly target the same pathways and (ii) the coapplication of silver with tellurite tends not to target new pathways but targets the same pathways with an amplified change.

Simultaneous bioremediation of phenol and tellurite by Lysinibacillus sp. EBL303 and characterization of biosynthesized Te nanoparticles

Aromatic compounds and metalloid oxyanions are abundant in the environment due to natural resources and industrial wastes. The high toxicity of phenol and tellurite poses a significant threat to all forms of life. A halotolerant bacterium was isolated and identified as Lysinibacillus sp. EBL303. The remediation analysis shows that 500 mg/L phenol and 0.5 mM tellurite can be remediated entirely in separate cultures within 74 and 56 h, respectively. In addition, co-remediation of pollutants resulted in the same phenol degradation and 27% less tellurite reduction within 98 h. Since phenol and tellurite exhibited inhibitory behavior, their removal kinetics fitted well with the first-order model. In the characterization of biosynthesized tellurium nanoparticles (TeNPs), transmission electron microscopy, dynamic light scattering, FE-SEM, and dispersive X-ray (EDX) showed that the separated intracellular TeNPs were spherical and consisted of only tellurium with 22-148 nm in size. Additionally, investigations using X-ray diffraction and Fourier-transform infrared spectroscopy revealed proteins and lipids covering the surface of these amorphous TeNPs. Remarkably, this study is the first report to demonstrate the simultaneous bioremediation of phenol and tellurite and the biosynthesis of TeNPs, indicating the potential of Lysinibacillus sp. EBL303 in this matter, which can be applied to environmental remediation and the nanotechnology industry.

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.

Anaerobic RSH-dependent tellurite reduction contributes to Escherichia coli tolerance against tellurite

BACKGROUND

Tellurium is a rare metalloid that exerts high toxicity on cells, especially on bacteria, partly due to reactive oxygen species (ROS) generation. Moreover, it has also been observed that tellurite can target free cell thiols groups (RSH) (i.e. reduced glutathione (GSH)), enhancing the cellular redox imbalance. Additionally, in vitro experiments have suggested that several enzymes can reduce tellurite (IV) to its elemental form (0); where RSH present on their active sites may be responsible for the process. Nevertheless, the mechanisms implemented by bacteria for tellurite reduction and its role in resistance have not been evaluated in vivo.

RESULTS

This work shows that tellurite reduction to elemental tellurium is increased under anaerobic conditions in E. coli cells. The in vivo tellurite reduction is related to the intracellular concentration of total RSH, in the presence and absence of oxygen. This metabolization of tellurite directly contributes to the resistance of the bacteria to the oxyanion.

CONCLUSIONS

We demonstrated that in vivo tellurite reduction is related to the intracellular thiol concentration, i.e. large availability of cellular RSH groups, results in a more significant reduction of tellurite. Furthermore, we observed that, when the bacterium exhibits less resistance to the oxyanion, a decreased tellurite reduction was seen, affecting the growth fitness. Together, these results let us propose that tellurite reduction and the intracellular RSH content are related to the oxyanion bacterial resistance, this tripartite mechanism in an oxygen-independent anaerobic process.

Tellurite Adsorption onto Bacterial Surfaces

Tellurium (Te) is an emerging contaminant and its chemical transformation in the environment is strongly influenced by microbial processes. In this study, we investigated the adsorption of tellurite [Te(IV), TeO₃^2-] onto the common soil bacterium Bacillus subtilis. Thiol-blocking experiments were carried out to investigate the role of cell surface sulfhydryl sites in tellurite binding, and extended X-ray absorption fine structure (EXAFS) spectroscopy was performed to determine the chemical speciation of the adsorbed tellurite. The results indicate that tellurite reacts with sulfhydryl functional groups in the extracellular polymeric substances (EPS) produced by B. subtilis. Upon binding to sulfhydryl sites in the EPS, the Te changes from Te-O bonds to Te-S coordination. Further analysis of the surface-associated molecules shows that the EPS of B. subtilis contain proteins. Removal of the proteinaceous EPS dramatically decreases tellurite adsorption and the sulfhydryl surface site concentration. These findings indicate that sulfhydryl binding in EPS plays a key role in tellurite adsorption on bacterial surfaces.

Genetic analysis of tellurate reduction reveals the selenate/tellurate reductase genes ynfEF and the transcriptional regulation of moeA by NsrR in Escherichia coli

Several bacteria can reduce tellurate into the less toxic elemental tellurium, but the genes responsible for this process have not yet been identified. In this study, we screened the Keio collection of single-gene knockouts of Escherichia coli responsible for decreased tellurate reduction and found that deletions of 29 genes, including those for molybdenum cofactor (Moco) biosynthesis, iron-sulphur biosynthesis, and the twin-arginine translocation pathway resulted in decreased tellurate reduction. Among the gene knockouts, deletions of nsrR, moeA, yjbB, ynbA, ydaS and yidH affected tellurate reduction more severely than those of other genes. Based on our findings, we determined that the ynfEF genes, which code for the components of the selenate reductase YnfEFGH, are responsible for tellurate reduction. Assays of several molybdoenzymes in the knockouts suggested that nsrR, yjbB, ynbA, ydaS and yidH are essential for the activities of molybdoenzymes in E. coli. Furthermore, we found that the nitric oxide sensor NsrR positively regulated the transcription of the Moco biosynthesis gene moeA. These findings provided new insights into the complexity and regulation of Moco biosynthesis in E. coli.

The role of cysteine in tellurate reduction and toxicity

The tellurium oxyanion tellurate is toxic to living organisms even at low concentrations; however, its mechanism of toxicity is poorly understood. Here, we show that exposure of Escherichia coli K-12 to tellurate results in reduction to elemental tellurium (Te[0]) and the formation of intracellular reactive oxygen species (ROS). Toxicity assays performed with E. coli indicated that pre-oxidation of the intracellular thiol pools increases cellular resistance to tellurate-suggesting that intracellular thiols are important in tellurate toxicity. X-ray absorption spectroscopy experiments demonstrated that cysteine reduces tellurate to elemental tellurium. This redox reaction was found to generate superoxide anions. These results indicate that tellurate reduction to Te(0) by cysteine is a source of ROS in the cytoplasm of tellurate-exposed cells.

Tellurium: A Rare Element with Influence on Prokaryotic and Eukaryotic Biological Systems

Metalloid tellurium is characterized as a chemical element belonging to the chalcogen group without known biological function. However, its compounds, especially the oxyanions, exert numerous negative effects on both prokaryotic and eukaryotic organisms. Recent evidence suggests that increasing environmental pollution with tellurium has a causal link to autoimmune, neurodegenerative and oncological diseases. In this review, we provide an overview about the current knowledge on the mechanisms of tellurium compounds' toxicity in bacteria and humans and we summarise the various ways organisms cope and detoxify these compounds. Over the last decades, several gene clusters conferring resistance to tellurium compounds have been identified in a variety of bacterial species and strains. These genetic determinants exhibit great genetic and functional diversity. Besides the existence of specific resistance mechanisms, tellurium and its toxic compounds interact with molecular systems, mediating general detoxification and mitigation of oxidative stress. We also discuss the similarity of tellurium and selenium biochemistry and the impact of their compounds on humans.

Tellurides bearing benzensulfonamide as carbonic anhydrase inhibitors with potent antitumor activity

We evaluated in vitro a series of telluride containing compounds bearing the benzenesulfonamide group, as effective inhibitors of the physiologically relevant human (h) expressed Carbonic Anhydrase (CA; EC 4.2.1.1) enzymes I, II, IV VII and IX. The potent effects of such compounds against the tumor-associated hCA IX being low nanomolar inhibitors (K_I 2.2 to 2.9 nM) and with good selectivity over the ubiquitous hCA II, gave the possibility to evaluate their lethal effect in vitro against a breast cancer cell line (MDA-MB-231). Among the series, both compounds 3a and 3g induced significant toxic effects against tumor cells after 48 h incubation. Under normoxic condition 3a showed high efficacy killing over 94% of tumor cells at 1 µM, and derivative 3g reached the tumor cell viability under the 5% at 10 µM. In hypoxic condition, these two compounds showed less effective although retained excellent cancer cell killer. These unusual features make them interesting lead compounds acting as antitumor agents also in tumor types not dependent from hCA IX overexpression.

Biogenic Selenium Nanoparticles: A Fine Characterization to Unveil Their Thermodynamic Stability

Among the plethora of available metal(loid) nanomaterials (NMs), those containing selenium are interesting from an applicative perspective, due to their high biocompatibility. Microorganisms capable of coping with toxic Se-oxyanions generate mostly Se nanoparticles (SeNPs), representing an ideal and green alternative over the chemogenic synthesis to obtain thermodynamically stable NMs. However, their structural characterization, in terms of biomolecules and interactions stabilizing the biogenic colloidal solution, is still a black hole that impairs the exploitation of biogenic SeNP full potential. Here, spherical and thermodynamically stable SeNPs were produced by a metal(loid) tolerant Micrococcus sp. Structural characterization obtained by Scanning Electron Microscopy (SEM) revealed that these SeNPs were surrounded by an organic material that contributed the most to their electrosteric stabilization, as indicated by Zeta (ζ) potential measurements. Proteins were strongly adsorbed on the SeNP surface, while lipids, polysaccharides, and nucleic acids more loosely interacted with SeNMs as highlighted by Fourier Transform Infrared Spectroscopy (FTIR) and overall supported by multivariate statistical analysis. Nevertheless, all these contributors were fundamental to maintain SeNPs stable, as, upon washing, the NM-containing extract showed the arising of aggregated SeNPs alongside Se nanorods (SeNRs). Besides, Density Functional Theory (DFT) calculation unveiled how thiol-containing molecules appeared to play a role in SeO₃²⁻ bioreduction, stress oxidative response, and SeNP stabilization.

The putative phosphate transporter PitB (PP1373) is involved in tellurite uptake in Pseudomonas putida KT2440

Tellurium oxyanions are chemical species of great toxicity and their presence in the environment has increased because of mining industries and photovoltaic and electronic waste. Recovery strategies for this metalloid that are based on micro-organisms are of interest, but further studies of the transport systems and enzymes responsible for implementing tellurium transformations are required because many mechanisms remain unknown. Here, we investigated the involvement in tellurite uptake of the putative phosphate transporter PitB (PP1373) in soil bacterium Pseudomonas putida KT2440. For this purpose, through a method based on the CRISPR/Cas9 system, we generated a strain deficient in the pitB gene and characterized its phenotype on exposing it to varied concentrations of tellurite. Growth curves and transmission electronic microscopy experiments for the wild-type and ΔpitB strains showed that both were able to internalize tellurite into the cytoplasm and reduce the oxyanion to black nano-sized and rod-shaped tellurium particles, although the ΔpitB strain showed an increased resistance to the tellurite toxic effects. At a concentration of 100 μM tellurite, where the biomass formation of the wild-type strain decreased by half, we observed a greater ability of ΔpitB to reduce this oxyanion with respect to the wild-type strain (~38 vs ~16 %), which is related to the greater biomass production of ΔpitB and not to a greater consumption of tellurite per cell. The phenotype of the mutant was restored on over-expressing pitB in trans. In summary, our results indicate that PitB is one of several transporters responsible for tellurite uptake in P. putida KT2440.

Extreme Environments and High-Level Bacterial Tellurite Resistance

Bacteria have long been known to possess resistance to the highly toxic oxyanion tellurite, most commonly though reduction to elemental tellurium. However, the majority of research has focused on the impact of this compound on microbes, namely E. coli, which have a very low level of resistance. Very little has been done regarding bacteria on the other end of the spectrum, with three to four orders of magnitude greater resistance than E. coli. With more focus on ecologically-friendly methods of pollutant removal, the use of bacteria for tellurite remediation, and possibly recovery, further highlights the importance of better understanding the effect on microbes, and approaches for resistance/reduction. The goal of this review is to compile current research on bacterial tellurite resistance, with a focus on high-level resistance by bacteria inhabiting extreme environments.

Synthesis of novel tellurides bearing benzensulfonamide moiety as carbonic anhydrase inhibitors with antitumor activity

We have synthetized a novel series of β-hydroxy tellurides bearing the benzenesulfonamide group as potent inhibitors of carbonic anhydrase enzymes. In a one pot procedure, we discovered both the ring opening reaction of the three-membered ring and the cleavage of the sulfonamide protecting moiety at the same time. Moreover, the first X-ray co-crystallographic structure of a β-hydroxy telluride derivative with hCA II is reported. The potent effects of these compounds against the tumor-associated hCA IX with low nanomolar constant inhibition values give the possibility to evaluate their activity in vitro using a breast cancer cell line (MDA-MB-231). Compounds 7e and 7g induced significant toxic effects against tumor cells after 48 h incubation in normoxic conditions killing over 50% of tumor cells at 3 μM, but their efficacy decreased in hypoxic conditions reaching the 50% of the tumor cell viability only at 30 μM. These unusual features make them interesting lead compounds to act as antitumor agents, not only as Carbonic Anhydrase IX inhibitors, but reasonably in different pathways, where hCA IX is not overexpressed.

The tellurite-reducing bacterium Alteromonas macleodii from a culture of the toxic dinoflagellate Prorocentrum foraminosum

The Alteromonas macleodii strain 2328 was isolated from a clonal culture of the toxic dinoflagellate Prorocentrum foraminosum. The strain exhibits a resistance to high K₂TeO₃ concentrations (2500 μg/mL). A study of the growth dynamics of the strain exposed to K₂TeO₃ has shown a longer lag phase and a reduced stationary phase compared to those during cultivation with no toxicant. The fatty acids profile is dominated by 16:1 (n-7), 16:0, 17:1, 15:0, 18:1 (n-7), and 17:0. The 2328 strain belongs to the Gammaproteobacteria and is related to the genus Alteromonas with 99-100% sequence similarity to some intra-genome allele variants (paralogues) of 16S rRNA from A. macleodii. A phylogenetic reconstruction (ML and NJ), based on HyHK amino acid sequences, has revealed that the analyzed 2328 strain forms a common cluster with A. macleodii strains. In the presented work, the ability of A. macleodii to reduce potassium tellurite to elemental tellurium has been recorded for the first time. Bacteria reduce potassium tellurite to Te (0), nanoparticles of which become distributed diffusely and in the form of electron-dense globules in cytoplasm. Large polymorphous metalloid crystals are formed in the extracellular space. Such feature of the A. macleodii strain 2328 makes it quite attractive for biotechnological application as an organism concentrating the rare metalloid.

Influence of Bacterial Physiology on Processing of Selenite, Biogenesis of Nanomaterials and Their Thermodynamic Stability

We explored how Ochrobactrum sp. MPV1 can convert up to 2.5 mM selenite within 120 h, surviving the challenge posed by high oxyanion concentrations. The data show that thiol-based biotic chemical reaction(s) occur upon bacterial exposure to low selenite concentrations, whereas enzymatic systems account for oxyanion removal when 2 mM oxyanion is exceeded. The selenite bioprocessing produces selenium nanomaterials, whose size and morphology depend on the bacterial physiology. Selenium nanoparticles were always produced by MPV1 cells, featuring an average diameter ranging between 90 and 140 nm, which we conclude constitutes the thermodynamic stability range for these nanostructures. Alternatively, selenium nanorods were observed for bacterial cells exposed to high selenite concentration or under controlled metabolism. Biogenic nanomaterials were enclosed by an organic material in part composed of amphiphilic biomolecules, which could form nanosized structures independently. Bacterial physiology influences the surface charge characterizing the organic material, suggesting its diverse biomolecular composition and its involvement in the tuning of the nanomaterial morphology. Finally, the organic material is in thermodynamic equilibrium with nanomaterials and responsible for their electrosteric stabilization, as changes in the temperature slightly influence the stability of biogenic compared to chemogenic nanomaterials.

Reduction of Gold (III) and Tellurium (IV) by Enterobacter cloacae MF01 Results in Nanostructure Formation Both in Aerobic and Anaerobic Conditions

Microorganism survival in the presence of toxic substances such as metal(loid)s lies chiefly on their ability to resist (or tolerate) such elements through specific resistance mechanisms. Among them, toxicant reduction has attracted the attention of researchers because metal(loid)-reducing bacteria are being used to recover and/or decontaminate polluted sites. Particularly, our interest is to analyze the toxicity of gold and tellurium compounds for the environmental microorganism Enterobacter cloacae MF01 and also to explore the generation of nanostructures to be used in future biotechnological processes. Resistance of E. cloacae MF01 to gold and tellurium salts as well as the putative mechanisms involved -both in aerobic and anaerobic growth conditions- was evaluated. These metal(loid)s were selected because of their potential application in biotechnology. Resistance to auric tetrachloride acid (HAuCl₄) and potassium tellurite (K₂TeO₃) was assessed by determining areas of growth inhibition, minimum inhibitory concentrations, and growth curves as well as by viability tests. E. cloacae MF01 exhibited higher resistance to HAuCl₄ and K₂TeO₃ under aerobic and anaerobic conditions, respectively. In general, their toxicity is mediated by the generation of reactive oxygen species and by a decrease of intracellular reduced thiols (RSH). To assess if resistance implies toxicant reduction, intra- and extra-cellular toxicant-reducing activities were evaluated. While E. cloacae MF01 exhibited intra- and extra-cellular HAuCl₄-reducing activity, tellurite reduction was observed only intracellularly. Then, Au- and Te-containing nanostructures (AuNS and TeNS, respectively) were synthesized using crude extracts from E. cloacae MF01 and their size, morphology, and chemical composition was evaluated.

Mitochondrial ribosomal proteins involved in tellurite resistance in yeast Saccharomyces cerevisiae

A considerable body of evidence links together mitochondrial dysfunctions, toxic action of metalloid oxyanions, and system and neurodegenerative disorders. In this study we have used the model yeast Saccharomyces cerevisiae to investigate the genetic determinants associated with tellurite resistance/sensitivity. Nitrosoguanidine-induced K₂TeO₃-resistant mutants were isolated, and one of these mutants, named Sc57-Te₅^R, was characterized. Both random spore analysis and tetrad analysis and growth of heterozygous (Te^S/Te₅^R) diploid from Sc57-Te₅^R mutant revealed that nuclear and recessive mutation(s) was responsible for the resistance. To get insight into the mechanisms responsible for K₂TeO₃-resistance, RNA microarray analyses were performed with K₂TeO₃-treated and untreated Sc57-Te₅^R cells. A total of 372 differentially expressed loci were identified corresponding to 6.37% of the S. cerevisiae transcriptome. Of these, 288 transcripts were up-regulated upon K₂TeO₃ treatment. About half of up-regulated transcripts were associated with the following molecular functions: oxidoreductase activity, structural constituent of cell wall, transporter activity. Comparative whole-genome sequencing allowed us to identify nucleotide variants distinguishing Sc57-Te₅^R from parental strain Sc57. We detected 15 CDS-inactivating mutations, and found that 3 of them affected genes coding mitochondrial ribosomal proteins (MRPL44 and NAM9) and mitochondrial ribosomal biogenesis (GEP3) pointing out to alteration of mitochondrial ribosome as main determinant of tellurite resistance.

Morphology-tunable tellurium nanomaterials produced by the tellurite-reducing bacterium Lysinibacillus sp. ZYM-1

Although tellurite is highly toxic to organisms, elemental tellurium nanomaterials (TeNMs) have many uses. The microbe-mediated reduction of tellurite to Te(0) has been shown to be a green and cost-effective approach for turning waste into wealth. However, it is difficult to tune the morphology of biogenic nanomaterials. In this study, a series of experiments was conducted to investigate the factors influencing tellurite reduction by the tellurite-reducing bacterium Lysinibacillus sp. ZYM-1, including pH, tellurite concentration, temperature, and heavy metal ions. The optimal removal efficiency of tellurite was respectively achieved at pH 8, 0.5 mM tellurite, and 40 °C. All of the tested metal ions retarded the reduction of tellurite, especially Cd²⁺ and Co²⁺, which completely inhibited its reduction. Further characterization of the biogenic TeNMs indicated that their morphology could be tuned by the tellurite concentration, pH, temperature, and organic solvents used. Regular Te nanosheets were produced using 5 mM tellurite. The TeNMs were primarily synthesized in the cell membrane. Hexagonal Te nanoplates, nanorods, nanoflowers, and nanobranches were synthesized when combining membrane fractions with tellurite and NADH. The diverse morphologies are assumed to be induced by the synergy between the reduction kinetics and the protein structure. Therefore, this study confirmed that the bacterium can tune the morphology of TeNMs, broadening the potential application of biogenic TeNMs.

A novel strategy to evaluate the degradation of quantum dots: identification and quantification of CdTe quantum dots and corresponding ionic species by CZE-ICP-MS

In view of the significance and urgency of the speciation analysis of quantum dots (QDs) and their degradation products for clarifying their degradation rules and toxicity mechanisms, a method for the identification and quantification of CdTe QDs and corresponding ionic species in complex matrices was developed using capillary zone electrophoresis (CZE) coupled to inductively coupled plasma-mass spectrometry (ICP-MS). The quality assessment of commercial CdTe QDs and serum pharmacokinetics of synthesized CdTe QDs in rats were successfully undertaken using the developed CZE-ICP-MS method.

Tellurate enters Escherichia coli K-12 cells via the SulT-type sulfate transporter CysPUWA

Soluble forms of tellurium are environmental contaminants that are toxic to microorganisms. While tellurite [Te(IV)] is a well-characterized antimicrobial agent, little is known about the interactions of tellurate [Te(VI)] with bacterial cells. In this study, we investigated the role of sulfate transporters in the uptake of tellurate in Escherichia coli K-12. Mutant strains carrying a deletion of the cysW gene in the CysPUWA sulfate transporter system accumulated less cellular tellurium and exhibited higher resistance to tellurate compared with the wild-type strain. Complementation of the mutation restored tellurate sensitivity and uptake. These results indicate that tellurate enters E. coli cells to cause toxic effects via the CysPUWA sulfate transporter.

Antimicrobial and Antioxidant Activity of the Biologically Synthesized Tellurium Nanorods; A Preliminary In vitro Study

Background: Recent theranostic (therapeutic or diagnostic) applications of tellurium nanoparticles have attracted a great interest for development of different methods for synthesis of this valuable nanostructure, especially via biological resources. Objectives: In the present study, the antimicrobial and antioxidant effects of the tellurium nanorods (Te NRs) biosynthesized by a bacterial strain Pseudomonas pseudoalcaligenes strain Te were evaluated. Materials and Methods: The antimicrobial effect of Te NRs and potassium tellurite against different bacterial and fungal pathogens was assessed by microdilution method. Furthermore, the disk diffusion method was used to evaluate the antibacterial effect of the biogenic Te NRs and potassium tellurite against methicillin-resistant Staphylococcus aureus, alone or in combination with various antibiotics. Also, the biogenic Te NRs were investigated for antioxidant activity using 2, 2-diphenyl- 1-picrylhydrazyl (DPPH) scavenging activity and reducing power assay. Results: Transmission electron micrograph (TEM) of the purified Te NRs showed individual and rod-shaped nanostructure (~22 nm diameter by 185 nm in length). Based on the data obtained from both microdilution and disk diffusion method the K₂ TeO₃ exhibited a higher antibacterial and antifungal activity compared to the Te NRs. The measured IC₅₀ for the biogenic Te NRs (i.e. DPPH radical scavenging activity) was found to be 24.9 μg.mL^-1, while, K ₂ TeO₃ has represented only 17.6 ± 0.8 % DPPH radical scavenging effect at the concentration of 160 μg.mL^-1. The reducing power assay revealed a higher electron-donating activity for Te NRs compared to K₂TeO₃. Conclusions: Based on the data obtained from both microdilution and disk diffusion method the K₂TeO₃ exhibited a higher antimicrobial and antifungal activity than Te NRs. Te NRs didn't show the antibacterial effect against the tested bacterial strain: MRSA and showed an inhibitory effect and antibacterial activity of the effective antibiotics. However, more studies should be performed to explore the action mechanism of the produced biogenic Te NRs.

Accumulation of heme biosynthetic intermediates contributes to the antibacterial action of the metalloid tellurite

The metalloid tellurite is highly toxic to microorganisms. Several mechanisms of action have been proposed, including thiol depletion and generation of hydrogen peroxide and superoxide, but none of them can fully explain its toxicity. Here we use a combination of directed evolution and chemical and biochemical approaches to demonstrate that tellurite inhibits heme biosynthesis, leading to the accumulation of intermediates of this pathway and hydroxyl radical. Unexpectedly, the development of tellurite resistance is accompanied by increased susceptibility to hydrogen peroxide. Furthermore, we show that the heme precursor 5-aminolevulinic acid, which is used as an antimicrobial agent in photodynamic therapy, potentiates tellurite toxicity. Our results define a mechanism of tellurite toxicity and warrant further research on the potential use of the combination of tellurite and 5-aminolevulinic acid in antimicrobial therapy.

The mechanisms of action of the antibacterial metalloid tellurite are unclear. Here, the authors show that tellurite induces an accumulation of hydroxyl radical and intermediates of heme biosynthesis in E. coli, and that the heme precursor 5-aminolevulinic acid potentiates tellurite toxicity.

Rhodococcus aetherivorans BCP1 as cell factory for the production of intracellular tellurium nanorods under aerobic conditions

Background

Tellurite (TeO₃²⁻) is recognized as a toxic oxyanion to living organisms. However, mainly anaerobic or facultative-anaerobic microorganisms are able to tolerate and convert TeO₃²⁻ into the less toxic and available form of elemental Tellurium (Te⁰), producing Te-deposits or Te-nanostructures. The use of TeO₃²⁻-reducing bacteria can lead to the decontamination of polluted environments and the development of “green-synthesis” methods for the production of nanomaterials. In this study, the tolerance and the consumption of TeO₃²⁻ have been investigated, along with the production and characterization of Te-nanorods by Rhodococcus aetherivorans BCP1 grown under aerobic conditions.

Results

Aerobically grown BCP1 cells showed high tolerance towards TeO₃²⁻ with a minimal inhibitory concentration (MIC) of 2800 μg/mL (11.2 mM). TeO₃²⁻ consumption has been evaluated exposing the BCP1 strain to either 100 or 500 μg/mL of K₂TeO₃ (unconditioned growth) or after re-inoculation in fresh medium with new addition of K₂TeO₃ (conditioned growth). A complete consumption of TeO₃²⁻ at 100 μg/mL was observed under both growth conditions, although conditioned cells showed higher consumption rate. Unconditioned and conditioned BCP1 cells partially consumed TeO₃²⁻ at 500 μg/mL. However, a greater TeO₃²⁻ consumption was observed with conditioned cells. The production of intracellular, not aggregated and rod-shaped Te-nanostructures (TeNRs) was observed as a consequence of TeO₃²⁻ reduction. Extracted TeNRs appear to be embedded in an organic surrounding material, as suggested by the chemical–physical characterization. Moreover, we observed longer TeNRs depending on either the concentration of precursor (100 or 500 μg/mL of K₂TeO₃) or the growth conditions (unconditioned or conditioned grown cells).

Conclusions

Rhodococcus aetherivorans BCP1 is able to tolerate high concentrations of TeO₃²⁻ during its growth under aerobic conditions. Moreover, compared to unconditioned BCP1 cells, TeO₃²⁻conditioned cells showed a higher oxyanion consumption rate (for 100 μg/mL of K₂TeO₃) or to consume greater amount of TeO₃²⁻ (for 500 μg/mL of K₂TeO₃). TeO₃²⁻ consumption by BCP1 cells led to the production of intracellular and not aggregated TeNRs embedded in an organic surrounding material. The high resistance of BCP1 to TeO₃²⁻ along with its ability to produce Te-nanostructures supports the application of this microorganism as a possible eco-friendly nanofactory.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0602-8) contains supplementary material, which is available to authorized users.

On the role of a specific insert in acetate permeases (ActP) for tellurite uptake in bacteria: Functional and structural studies

The oxyanion tellurite (TeO₃^2-) is extremely toxic to bacterial cells. In Rhodobacter capsulatus, tellurite enters the cytosol by means of the high uptake-rate acetate permease RcActP2, encoded by one of the three actP genes present in this species (actP1, actP2 and actP3). Conversely, in Escherichia coli a low rate influx of the oxyanion is measured, which depends mainly on the phosphate transporter EcPitA, even though E. coli contains its own EcActP acetate permease. Here we report that when the actP2 gene from R. capsulatus is expressed in wild-type E. coli HB101 and in E. coli JW3460 ΔpitA mutant, the cellular intake of tellurite increases up to four times, suggesting intrinsic structural differences between EcActP and RcActP2. Indeed, a sequence analysis indicated the presence in RcActP2 of an insert of 15-16 residues, located between trans-membrane (TM) helices 6 and 7, which is absent in both EcActP and RcActP1. Based on this observation, the molecular models of homodimeric RcActP1 and RcActP2 were calculated and analyzed. In the RcActP2 model, the insert induces a perturbation in the conformation of the loop between TM helices 6 and 7, located at the RcActP2 dimerization interface. This perturbation opens a cavity on the periplasmic side that is closed, instead, in the RcActP1 model. This cavity also features an increase of the positive electric potential on the protein surface, an effect ascribed to specific residues Lys261, Lys281 and Arg560. We propose that this positively charged patch in RcActP2 is involved in recognition and translocation of the TeO₃^2- anion, attributing to RcActP2 a greater ability as compared to RcActP1 to transport this inorganic poison inside the cells.

Biological synthesis of fluorescent nanoparticles by cadmium and tellurite resistant Antarctic bacteria: exploring novel natural nanofactories

Background

Fluorescent nanoparticles or quantum dots (QDs) have been intensely studied for basic and applied research due to their unique size-dependent properties. There is an increasing interest in developing ecofriendly methods to synthesize these nanoparticles since they improve biocompatibility and avoid the generation of toxic byproducts. The use of biological systems, particularly prokaryotes, has emerged as a promising alternative. Recent studies indicate that QDs biosynthesis is related to factors such as cellular redox status and antioxidant defenses. Based on this, the mixture of extreme conditions of Antarctica would allow the development of natural QDs producing bacteria.

Results

In this study we isolated and characterized cadmium and tellurite resistant Antarctic bacteria capable of synthesizing CdS and CdTe QDs when exposed to these oxidizing heavy metals. A time dependent change in fluorescence emission color, moving from green to red, was determined on bacterial cells exposed to metals. Biosynthesis was observed in cells grown at different temperatures and high metal concentrations. Electron microscopy analysis of treated cells revealed nanometric electron-dense elements and structures resembling membrane vesicles mostly associated to periplasmic space. Purified biosynthesized QDs displayed broad absorption and emission spectra characteristic of biogenic Cd nanoparticles.

Conclusions

Our work presents a novel and simple biological approach to produce QDs at room temperature by using heavy metal resistant Antarctic bacteria, highlighting the unique properties of these microorganisms as potent natural producers of nano-scale materials and promising candidates for bioremediation purposes.

Recovery of Elemental Tellurium Nanoparticles by the Reduction of Tellurium Oxyanions in a Methanogenic Microbial Consortium

This research focuses on the microbial recovery of elemental tellurium (Te(0)) from aqueous streams containing soluble tellurium oxyanions, tellurate (Te(VI)), and tellurite (Te(IV)). An anaerobic mixed microbial culture occurring in methanogenic granular sludge was able to biocatalyze the reduction of both Te oxyanions to produce Te(0) nanoparticles (NPs) in sulfur-free medium. Te(IV) reduction was seven times faster than that of Te(VI), such that Te(IV) did not accumulate to a great extent during Te(VI) reduction. Endogenous substrates in the granular sludge provided the electron equivalents required to reduce Te oxyanions; however, the reduction rates were modestly increased with an exogenous electron donor such as H2. The effect of four redox mediators (anthraquinone-2,6-disulfonate, hydroxocobalamin, riboflavin, and lawsone) was also tested. Riboflavin increased the rate of Te(IV) reduction eleven-fold and also enhanced the fraction Te recovered as extracellular Te(0) NPs from 21% to 64%. Lawsone increased the rate of Te(VI) reduction five-fold, and the fraction of Te recovered as extracellular material increased from 49% to 83%. The redox mediators and electron donors also impacted the morphologies and localization of Te(0) NPs, suggesting that NP production can be tailored for a particular application.

Mercury-mediated cross-resistance to tellurite in Pseudomonas spp. isolated from the Chilean Antarctic territory

Mercury salts and tellurite are among the most toxic compounds for microorganisms on Earth.

Selenite Protection of Tellurite Toxicity Toward Escherichia coli

In this work the influence of selenite on metal resistance in Escherichia coli was examined. Both synergistic and antagonistic resistance and toxicities were found upon co exposure with selenite. In wild type cells co-exposure to selenite had little effect on arsenic resistance, decreased resistance to cadmium and mercury but led to a dramatically increased resistance to tellurite of 32-fold. Due to the potential importance of thiol chemistry in metal biochemistry, deletion strains in γ-glutamylcysteine synthetase (key step in glutathione biosynthesis, encoded by gshA), thioredoxin (trxA), glutaredoxin (grxA), glutathione oxidoreductase (gor), and the periplasmic glutathione transporter (cydD) were also evaluated for resistance to various metals in the presence of selenite. The protective effect of selenite on tellurite toxicity was seen in several of the mutants and was pronounced in the gshA mutant were resistance to tellurite was increased up to 1000-fold relative to growth in the absence of selenite. Thiol oxidation studies revealed a faster rate of loss of reduced thiol content in the cell with selenite than with tellurite, indicating differential thiol reactivity. Selenite addition resulted in reactive oxygen species (ROS) production equivalent to levels associated with H₂O₂ addition. Tellurite addition resulted in considerably lower ROS generation while vanadate and chromate treatment did not increase ROS production above that of background. This work shows increased resistance toward most oxyanions in mutants of thiol redox suggesting that metalloid reaction with thiol components such as glutathione actually enhances toxicity of some metalloids.

The Effect of Tellurite on Highly Resistant Freshwater Aerobic Anoxygenic Phototrophs and Their Strategies for Reduction

Six fresh water aerobic anoxygenic phototrophs (Erythromicrobium ezovicum, strain E1; Erythromicrobium hydrolyticum, E4(1); Erythromicrobium ramosum, E5; Erythromonas ursincola, KR99; Sandaracinobacter sibiricus, RB 16-17; and Roseococcus thiosulfatophilus, RB3) possessing high level resistance to TeO₃²⁻ and the ability to reduce it to elemental Te were studied to understand their interaction with this highly toxic oxyanion. Tested organic carbon sources, pH, and level of aeration all had an impact on reduction. Physiological and metabolic responses of cells to tellurite varied among strains. In its presence, versus absence, cellular biomass either increased (KR99, 66.6% and E5, 21.2%) or decreased (RB3, 66.1%, E1, 57.8%, RB 16-17, 41.5%, and E4(1), 21.3%). The increase suggests a possible benefit from tellurite. Cellular ATP production was similarly affected, resulting in an increase (KR99, 15.2% and E5, 38.9%) or decrease (E4(1), 31.9%; RB 16-17, 48.8%; RB3, 55.9%; E1, 35.9%). Two distinct strategies to tellurite reduction were identified. The first, found in E4(1), requires de novo protein preparations as well as an undisturbed whole cell. The second strategy, in which reduction depended on a membrane associated constitutive reductase, was used by the remaining strains.

Anti-bacterial TeNPs biosynthesized by haloarcheaon Halococcus salifodinae BK3

Microbial synthesis of highly structured metal sulfide and metallic nanoparticles is a benign approach of nanomaterial synthesis. Various microbes have been exploited for nanoparticle synthesis, but nanofabrication using haloarchaea is still in nascent stages. Here, we report the intracellular synthesis of hexagonal needle-shaped tellurium nanoparticles with an aspect ratio of 1:4.4, by the haloarcheon Halococcus salifodinae BK3. The isolate was able to tolerate up to 5.5 mM K2TeO3. The yield of tellurium nanoparticles was highest when the culture was exposed to 3 mM K2TeO3, even though the isolate exhibited slightly decreased growth rate as compared to the culture growing in the absence of K2TeO3. The enzyme tellurite reductase was responsible for tellurite resistance and nanoparticle synthesis in H. salifodinae BK3. These tellurium nanoparticles exhibited anti-bacterial activities against both Gram-positive and Gram-negative bacteria, with higher antibacterial activity towards Gram-negative bacteria. This is the first report on the synthesis of tellurium nanoparticles by Halophilic archaea.

Escherichia coli 6-phosphogluconate dehydrogenase aids in tellurite resistance by reducing the toxicant in a NADPH-dependent manner

Exposure to the tellurium oxyanion tellurite (TeO3(2-)) results in the establishment of an oxidative stress status in most microorganisms. Usually, bacteria growing in the presence of the toxicant turn black because of the reduction of tellurite (Te(4+)) to the less-toxic elemental tellurium (Te(0)). In vitro, at least part of tellurite reduction occurs enzymatically in a nicotinamide dinucleotide-dependent reaction. In this work, we show that TeO3(2-) reduction by crude extracts of Escherichia coli overexpressing the zwf gene (encoding glucose-6-phosphate dehydrogenase) takes place preferentially in the presence of NADPH instead of NADH. The enzyme responsible for toxicant reduction was identified as 6-phosphogluconate dehydrogenase (Gnd). The gnd gene showed a subtle induction at short times after toxicant exposure while strains lacking gnd were more susceptible to the toxicant. These results suggest that both NADPH-generating enzymes from the pentose phosphate shunt may be involved in tellurite detoxification and resistance in E. coli.

Tellurium as a valuable tool for studying the prokaryotic origins of mitochondria

Mitochondria are eukaryotic organelles which contain the own genetic material and evolved from free-living Eubacteria, namely hydrogen-producing Alphaproteobacteria. Since 1965, biologists provided, by research at molecular level, evidence for the prokaryotic origins of mitochondria. However, determining the precise origins of mitochondria is challenging due to inherent difficulties in phylogenetically reconstructing ancient evolutionary events. The use of new tools to evidence the prokaryotic origin of mitochondria could be useful to gain an insight into the bacterial endosymbiotic event that resulted in the permanent acquisition of bacteria, from the ancestral cell, that through time were transformed into mitochondria. Electron microscopy has shown that both proteobacterial and yeast cells during their growth in the presence of increasing amount of tellurite resulted in dose-dependent blackening of the culture due to elemental tellurium (Te(0)) that formed large deposits either along the proteobacterial membrane or along the yeast cell wall and mitochondria. Since the mitochondrial inner membrane composition is similar to that of proteobacterial membrane, in the present work we evidenced the black tellurium deposits on both, cell wall and mitochondria of ρ(+) and respiratory deficient ρ(-) mutants of yeast. A possible role of tellurite in studying the evolutionary origins of mitochondria will be discussed.

Se (IV) triggers faster Te (IV) reduction by soil isolates of heterotrophic aerobic bacteria: formation of extracellular SeTe nanospheres

Background

Selenium and Tellurium have many common chemical properties as both belong to group 16 of the periodic table. High toxicities of Se and Te oxyanions cause environmental problems in contaminated soils and waters. Three strains (C4, C6 and C7) of selenite reducing and nanoparticle forming aerobic bacteria which were isolated from agricultural soils of India containing high concentrations of Se were investigated after 3.5 months of freeze-storage for their resistance against the toxic oxyanion tellurite and its reduction to non toxic elemental form Te⁰ as well as nanoparticles formation.

Results

Strains C4, C6 and C7 reduced tellurite at maximum reduction rates of 2.3, 1.5 and 2.1 mg Te (IV)/L/d, respectively and produced extracellular Te⁰ nanospheres as revealed from SEM-EDX analysis. Production of extracellular Te nanospheres has been described seldom. Further, concurrent reduction of both selenite and tellurite by bacteria was examined as these toxic oxyanions are often present together in natural environments, mine tailings or wastewater from copper refining. Interestingly, bioreduction of 100 mg/L selenite in shake flasks was not much affected by the presence of 10 mg/L tellurite but tellurite reduction rate increased 13 fold with selenite in the medium. The concurrent reduction of these oxyanions resulted in rarely described bioformation of extracellular nanoparticles composed of both Se and Te, reported first time for aerobically growing heterotrophic non-halophilic bacterial cultures. Duganella violacienigra, the closely related strain to C4 was also found to be resistant to oxyanions of Se and Te.

Conclusions

Selenite reducing heterotrophic non-halophilic aerobic bacteria revived from 3.5 months freeze storage could successfully reduce toxic tellurite to non toxic elemental form and produced extracellular nanospheres during detoxification. Presence of relatively less toxic selenite in the medium triggers bioreduction of more toxic tellurite leading to formation of extracellular SeTe nanospheres which are sought by solar and optical recording media industry because of their excellent photovoltaic and optical properties. The bacterial cultures investigated in this study could be exploited commercially to remediate not only selenite and tellurite-contaminated soil and water but also for green synthesis of extracellular Se, Te and Se + Te nanospheres.

Signaling mechanisms and disrupted cytoskeleton in the diphenyl ditelluride neurotoxicity

Evidence from our group supports that diphenyl ditelluride (PhTe)₂ neurotoxicity depends on modulation of signaling pathways initiated at the plasma membrane. The (PhTe)₂-evoked signal is transduced downstream of voltage-dependent Ca²⁺ channels (VDCC), N-methyl-D-aspartate receptors (NMDA), or metabotropic glutamate receptors activation via different kinase pathways (protein kinase A, phospholipase C/protein kinase C, mitogen-activated protein kinases (MAPKs), and Akt signaling pathway). Among the most relevant cues of misregulated signaling mechanisms evoked by (PhTe)₂ is the cytoskeleton of neural cells. The in vivo and in vitro exposure to (PhTe)₂ induce hyperphosphorylation/hypophosphorylation of neuronal and glial intermediate filament (IF) proteins (neurofilaments and glial fibrillary acidic protein, resp.) in different brain structures of young rats. Phosphorylation of IFs at specific sites modulates their association/disassociation and interferes with important physiological roles, such as axonal transport. Disrupted cytoskeleton is a crucial marker of neurodegeneration and is associated with reactive astrogliosis and apoptotic cell death. This review focuses the current knowledge and important results on the mechanisms of (PhTe)₂ neurotoxicity with special emphasis on the cytoskeletal proteins and their differential regulation by kinases/phosphatases and Ca²⁺-mediated mechanisms in developmental rat brain. We propose that the disrupted cytoskeletal homeostasis could support brain damage provoked by this neurotoxicant.

On the mechanism underlying tellurite reduction by Aeromonas caviae ST dihydrolipoamide dehydrogenase

The dihydrolipoamide dehydrogenase (LpdA) from the tellurite-resistant bacterium Aeromonas caviae ST reduces tellurite to elemental tellurium. To characterize this NADH-dependent activity, the A. caviae lpdA gene was subjected to site-directed mutagenesis and genes containing C45A, H322Y and E354K substitutions were individually transformed into Escherichia coli Δlpd. Cells expressing the modified genes exhibited decreased pyruvate dehydrogenase, dihydrolipoamide dehydrogenase and TR activity regarding that observed with the wild type A. caviae lpdA gene. In addition, cells expressing the altered lpdA genes showed increased oxidative stress levels and tellurite sensitivity than those carrying the wild type counterpart. The involvement of Cys residues in LpdA's TR activity was analyzed using specific inhibitors that interact with catalytic cysteines and/or disulfide bridges such as aurothiomalate, zinc or nickel. TR activity of purified LpdA was drastically affected by these compounds. Since LpdA belongs to the flavoprotein family, the involvement of the FAD/NAD(P)(+)-binding domain in TR activity was determined. FAD removal from purified LpdA results in loss of TR activity, which was restored with exogenously added FAD. Substitutions in E354, involved in FAD/NADH binding, resulted in low TR activity because of flavin loss. Finally, changing H322 (involved in NAD(+)/NADH binding) by tyrosine also resulted in altered TR activity.

Acinetobacter baylyi long-term stationary-phase protein StiP is a protease required for normal cell morphology and resistance to tellurite

We investigated the Acinetobacter baylyi gene ACIAD1960, known from previous work to be expressed during long-term stationary phase. The protein encoded by this gene had been annotated as a Conserved Hypothetical Protein, surrounded by putative tellurite resistance ("Ter") proteins. Sequence analysis suggested that the protein belongs to the DUF1796 putative papain-like protease family. Here, we show that the purified protein, subsequently named StiP, has cysteine protease activity. Deletion of stiP causes hypersensitivity to tellurite, altered population dynamics during long-term batch culture, and most strikingly, dramatic alteration of normal cell morphology. StiP and associated Ter proteins (the StiP-Ter cluster) are therefore important for regulating cell morphology, likely in response to oxidative damage or depletion of intracellular thiol pools, triggered artificially by tellurite exposure. Our finding has broad significance because while tellurite is an extremely rare compound in nature, oxidative damage, the need to maintain a particular balance of intracellular thiols, and the need to regulate cell morphology are ubiquitous.

Proteomic differences between tellurite-sensitive and tellurite-resistant E.coli

Tellurite containing compounds are in use for industrial processes and increasing delivery into the environment generates specific pollution that may well result in contamination and subsequent potential adverse effects on public health. It was the aim of the current study to reveal mechanism of toxicity in tellurite-sensitive and tellurite-resistant E. coli at the protein level.

In this work an approach using gel-based mass spectrometrical analysis to identify a differential protein profile related to tellurite toxicity was used and the mechanism of ter operon-mediated tellurite resistance was addressed.

E. coli BL21 was genetically manipulated for tellurite-resistance by the introduction of the resistance-conferring ter genes on the pLK18 plasmid. Potassium tellurite was added to cultures in order to obtain a final 3.9 micromolar concentration. Proteins from tellurite-sensitive and tellurite-resistant E. coli were run on 2-D gel electrophoresis, spots of interest were picked, in-gel digested and subsequently analysed by nano-LC-MS/MS (ion trap). In addition, Western blotting and measurement of enzymatic activity were performed to verify the expression of certain candidate proteins.

Following exposure to tellurite, in contrast to tellurite-resistant bacteria, sensitive cells exhibited increased levels of antioxidant enzymes superoxide dismutases, catalase and oxidoreductase YqhD. Cysteine desulfurase, known to be related to tellurite toxicity as well as proteins involved in protein folding: GroEL, DnaK and EF-Tu were upregulated in sensitive cells. In resistant bacteria, several isoforms of four essential Ter proteins were observed and following tellurite treatment the abovementioned protein levels did not show any significant proteome changes as compared to the sensitive control.

The absence of general defense mechanisms against tellurite toxicity in resistant bacteria thus provides further evidence that the four proteins of the ter operon function by a specific mode of action in the mechanism of tellurite resistance probably involving protein cascades from antioxidant and protein folding pathways.

Ter-dependent stress response systems: novel pathways related to metal sensing, production of a nucleoside-like metabolite, and DNA-processing

The mode of action of the bacterial ter cluster and TelA genes, implicated in natural resistance to tellurite and other xenobiotic toxic compounds, pore-forming colicins and several bacteriophages, has remained enigmatic for almost two decades. Using comparative genomics, sequence-profile searches and structural analysis we present evidence that the ter gene products and their functional partners constitute previously underappreciated, chemical stress response and anti-viral defense systems of bacteria. Based on contextual information from conserved gene neighborhoods and domain architectures, we show that the ter gene products and TelA lie at the center of membrane-linked metal recognition complexes with regulatory ramifications encompassing phosphorylation-dependent signal transduction, RNA-dependent regulation, biosynthesis of nucleoside-like metabolites and DNA processing. Our analysis suggests that the multiple metal-binding and non-binding TerD paralogs and TerC are likely to constitute a membrane-associated complex, which might also include TerB and TerY, and feature several, distinct metal-binding sites. Versions of the TerB domain might also bind small molecule ligands and link the TerD paralog-TerC complex to biosynthetic modules comprising phosphoribosyltransferases (PRTases), ATP grasp amidoligases, TIM-barrel carbon-carbon lyases, and HAD phosphoesterases, which are predicted to synthesize novel nucleoside-like molecules. One of the PRTases is also likely to interact with RNA by means of its Pelota/Ribosomal protein L7AE-like domain. The von Willebrand factor A domain protein, TerY, is predicted to be part of a distinct phosphorylation switch, coupling a protein kinase and a PP2C phosphatase. We show, based on the evidence from numerous conserved gene neighborhoods and domain architectures, that both the TerB and TelA domains have been linked to diverse lipid-interaction domains, such as two novel PH-like and the Coq4 domains, in different bacteria, and are likely to comprise membrane-associated sensory complexes that might additionally contain periplasmic binding-protein-II and OmpA domains. We also show that the TerD and TerB domains and the TerY-associated phosphorylation system are functionally linked to many distinct DNA-processing complexes, which feature proteins with SWI2/SNF2 and RecQ-like helicases, multiple AAA+ ATPases, McrC-N-terminal domain proteins, several restriction endonuclease fold DNases, DNA-binding domains and a type-VII/Esx-like system, which is at the center of a predicted DNA transfer apparatus. These DNA-processing modules and associated genes are predicted to be involved in restriction or suicidal action in response to phages and possibly repairing xenobiotic-induced DNA damage. In some eukaryotes, certain components of the ter system appear to be recruited to function in conjunction with the ubiquitin system and calcium-signaling pathways.

Genetic evidence for a molybdopterin-containing tellurate reductase

The genetic identity and cofactor composition of the bacterial tellurate reductase are currently unknown. In this study, we examined the requirement of molybdopterin biosynthesis and molybdate transporter genes for tellurate reduction in Escherichia coli K-12. The results show that mutants deleted of the moaA, moaB, moaE, or mog gene in the molybdopterin biosynthesis pathway lost the ability to reduce tellurate. Deletion of the modB or modC gene in the molybdate transport pathway also resulted in complete loss of tellurate reduction activity. Genetic complementation by the wild-type sequences restored tellurate reduction activity in the mutant strains. These findings provide genetic evidence that tellurate reduction in E. coli involves a molybdoenzyme.

Enhanced glutathione content allows the in vivo synthesis of fluorescent CdTe nanoparticles by Escherichia coli

The vast application of fluorescent semiconductor nanoparticles (NPs) or quantum dots (QDs) has prompted the development of new, cheap and safer methods that allow generating QDs with improved biocompatibility. In this context, green or biological QDs production represents a still unexplored area. This work reports the intracellular CdTe QDs biosynthesis in bacteria. Escherichia coli overexpressing the gshA gene, involved in glutathione (GSH) biosynthesis, was used to produce CdTe QDs. Cells exhibited higher reduced thiols, GSH and Cd/Te contents that allow generating fluorescent intracellular NP-like structures when exposed to CdCl(2) and K(2)TeO(3). Fluorescence microscopy revealed that QDs-producing cells accumulate defined structures of various colors, suggesting the production of differently-sized NPs. Purified fluorescent NPs exhibited structural and spectroscopic properties characteristic of CdTe QDs, as size and absorption/emission spectra. Elemental analysis confirmed that biosynthesized QDs were formed by Cd and Te with Cd/Te ratios expected for CdTe QDs. Finally, fluorescent properties of QDs-producing cells, such as color and intensity, were improved by temperature control and the use of reducing buffers.

Tellurite resistance gene trgB confers copper tolerance to Rhodobacter capsulatus

To identify copper homeostasis genes in Rhodobacter capsulatus, we performed random transposon Tn5 mutagenesis. Screening of more than 10,000 Tn5 mutants identified tellurite resistance gene trgB as a so far unrecognized major copper tolerance determinant. The trgB gene is flanked by tellurite resistance gene trgA and cysteine synthase gene cysK2. While growth of trgA mutants was only moderately restricted by tellurite, trgB and cysK2 mutants were severely affected by tellurite, which implies that viability under tellurite stress requires increased cysteine levels. Mutational analyses revealed that trgB was the only gene in this chromosomal region conferring cross-tolerance towards copper. Expression of the monocistronic trgB gene required promoter elements overlapping the trgA coding region as shown by nested deletions. Neither copper nor tellurite affected trgB transcription as demonstrated by reverse transcriptase PCR and trgB-lacZ fusions. Addition of tellurite or copper gave rise to increased cellular tellurium and copper concentrations, respectively, as determined by inductively coupled plasma-optical emission spectroscopy. By contrast, cellular iron concentrations remained fairly constant irrespective of tellurite or copper addition. This is the first study demonstrating a direct link between copper and tellurite response in bacteria.