The perceived importance of tellurium (Te) in biological systems has lagged behind selenium (Se), its lighter sister in the Group 16 chalcogens, because of tellurium's lower crustal abundance, lower oxyanion solubility and biospheric mobility and the fact that, unlike Se, Te has yet to be found to be an essential trace element. Te applications in electronics, optics, batteries and mining industries have expanded during the last few years, leading to an increase in environmental Te contamination, thus renewing biological interest in Te toxicity. This chalcogen is rarely found in the nontoxic, elemental state (Te(0)), but its soluble oxyanions, tellurite (TeO(3)(2-)) and tellurate (TeO(4)(2-)), are toxic for most forms of life even at very low concentrations. Although a number of Te resistance determinants (Tel) have been identified in plasmids or in the bacterial chromosome of different species of bacteria, the genetic and/or biochemical basis underlying bacterial TeO(3)(2-) toxicity is still poorly understood. This review traces the history of Te in its biological interactions, its enigmatic toxicity, importance in cellular oxidative stress, and interaction in cysteine metabolism.

Biochemical, genetic, enzymatic and molecular approaches were used to demonstrate, for the first time, that tellurite (TeO3 2−) toxicity in E. coli involves superoxide formation. This radical is derived, at least in part, from enzymatic TeO3 2− reduction. This conclusion is supported by the following observations made in K2TeO3-treated E. coli BW25113: i) induction of the ibpA gene encoding for the small heat shock protein IbpA, which has been associated with resistance to superoxide, ii) increase of cytoplasmic reactive oxygen species (ROS) as determined with ROS-specific probe 2′7′-dichlorodihydrofluorescein diacetate (H2DCFDA), iii) increase of carbonyl content in cellular proteins, iv) increase in the generation of thiobarbituric acid-reactive substances (TBARs), v) inactivation of oxidative stress-sensitive [Fe-S] enzymes such as aconitase, vi) increase of superoxide dismutase (SOD) activity, vii) increase of sodA, sodB and soxS mRNA transcription, and viii) generation of superoxide radical during in vitro enzymatic reduction of potassium tellurite.

Evidence that Escherichia coli YqhD is involved in bacterial response to compounds that generate membrane lipid peroxidation is presented. Overexpression of yqhD results in increased resistance to the reactive oxygen species-generating compounds hydrogen peroxide, paraquat, chromate, and potassium tellurite. Increased tolerance was also observed for the lipid peroxidation-derived aldehydes butanaldehyde, propanaldehyde, acrolein, and malondialdehyde and the membrane-peroxidizing compound tert-butylhydroperoxide. Expression of yqhD was also associated with changes in the concentration of intracellular peroxides and cytoplasmic protein carbonyl content and with a reduction in intracellular acrolein levels. When compared with the wild type strain, an yqhD mutant exhibited a sensitive phenotype to all these compounds and also augmented levels of thiobarbituric acid-reactive substances, which may indicate an increased level of lipid peroxidation. Purified YqhD catalyzes the in vitro reduction of acetaldehyde, malondialdehyde, propanaldehyde, butanaldehyde, and acrolein in a NADPH-dependent reaction. Finally, yqhD transcription was induced in cells that had been exposed to conditions favoring lipid peroxidation. Taken together these results indicate that this enzyme may have a physiological function by protecting the cell against the toxic effect of aldehydes derived from lipid oxidation. We speculate that in Escherichia coli YqhD is part of a glutathione-independent, NADPH-dependent response mechanism to lipid peroxidation.

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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 CdCl2 and K2TeO3. 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.

Multiple applications of nanotechnology, especially those involving highly fluorescent nanoparticles (NPs) or quantum dots (QDs) have stimulated the research to develop simple, rapid and environmentally friendly protocols for synthesizing NPs exhibiting novel properties and increased biocompatibility.In this study, a simple protocol for the chemical synthesis of glutathione (GSH)-capped CdTe QDs (CdTe-GSH) resembling conditions found in biological systems is described. Using only CdCl2, K2TeO3 and GSH, highly fluorescent QDs were obtained under pH, temperature, buffer and oxygen conditions that allow microorganisms growth. These CdTe-GSH NPs displayed similar size, chemical composition, absorbance and fluorescence spectra and quantum yields as QDs synthesized using more complicated and expensive methods.CdTe QDs were not freely incorporated into eukaryotic cells thus favoring their biocompatibility and potential applications in biomedicine. In addition, NPs entry was facilitated by lipofectamine, resulting in intracellular fluorescence and a slight increase in cell death by necrosis. Toxicity of the as prepared CdTe QDs was lower than that observed with QDs produced by other chemical methods, probably as consequence of decreased levels of Cd+2 and higher amounts of GSH.We present here the simplest, fast and economical method for CdTe QDs synthesis described to date. Also, this biomimetic protocol favors NPs biocompatibility and helps to establish the basis for the development of new, “greener” methods to synthesize cadmium-containing QDs.

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Tellurium, a metalloid belonging to group 16 of the periodic table, displays very interesting physical and chemical properties and lately has attracted significant attention for its use in nanotechnology. In this context, the use of microorganisms for synthesizing nanostructures emerges as an eco-friendly and exciting approach compared to their chemical synthesis. To generate Tecontaining nanostructures, bacteria enzymatically reduce tellurite to elemental tellurium. In this work, using a classic biochemical approach, we looked for a novel tellurite reductase from the Antarctic bacterium Pseudomonas sp. strain BNF22 and used it to generate tellurium-containing nanostructures. A new tellurite reductase was identified as glutathione reductase, which was subsequently overproduced in Escherichia coli. The characterization of this enzyme showed that it is an NADPH-dependent tellurite reductase, with optimum reducing activity at 30°C and pH 9.0. Finally, the enzyme was able to generate Te-containing nanostructures, about 68 nm in size, which exhibit interesting antibacterial properties against E. coli, with no apparent cytotoxicity against eukaryotic cells.