Microarray analysis of the Escherichia coli response to CdTe-GSH Quantum Dots: understanding the bacterial toxicity of semiconductor nanoparticles

Minicells as an Escherichia coli mechanism for the accumulation and disposal of fluorescent cadmium sulphide nanoparticles

BACKGROUND

Bacterial biosynthesis of fluorescent nanoparticles or quantum dots (QDs) has emerged as a unique mechanism for heavy metal tolerance. However, the physiological pathways governing the removal of QDs from bacterial cells remains elusive. This study investigates the role of minicells, previously identified as a means of eliminating damaged proteins and enhancing bacterial resistance to stress. Building on our prior work, which unveiled the formation of minicells during cadmium QDs biosynthesis in Escherichia coli, we hypothesize that minicells serve as a mechanism for the accumulation and detoxification of QDs in bacterial cells.

RESULTS

Intracellular biosynthesis of CdS QDs was performed in E. coli mutants ΔminC and ΔminCDE, known for their minicell-producing capabilities. Fluorescence microscopy analysis demonstrated that the generated minicells exhibited fluorescence emission, indicative of QD loading. Transmission electron microscopy (TEM) confirmed the presence of nanoparticles in minicells, while energy dispersive spectroscopy (EDS) revealed the coexistence of cadmium and sulfur. Cadmium quantification through flame atomic absorption spectrometry (FAAS) demonstrated that minicells accumulated a higher cadmium content compared to rod cells. Moreover, fluorescence intensity analysis suggested that minicells accumulated a greater quantity of fluorescent nanoparticles, underscoring their efficacy in QD removal. Biosynthesis dynamics in minicell-producing strains indicated that biosynthesized QDs maintained high fluorescence intensity even during prolonged biosynthesis times, suggesting continuous QD clearance in minicells.

CONCLUSIONS

These findings support a model wherein E. coli utilizes minicells for the accumulation and removal of nanoparticles, highlighting their physiological role in eliminating harmful elements and maintaining cellular fitness. Additionally, this biosynthesis system presents an opportunity for generating minicell-coated nanoparticles with enhanced biocompatibility for diverse applications.

Exposure to Cadmium Telluride Quantum Dots and Gene Expression Profile of Huh-7 Hepatocellular Carcinoma Cell Line

Nanoparticles have shown promising potential for efficient drug delivery, circumventing biological interferences like immunological and renal clearance and mechanical and enzymatic destruction. However, a handful of research papers have questioned the biomedical use of metal-based nanoparticles like cadmium telluride quantum dots (CdTe-QDs) for their cytotoxic, genotoxic, and carcinogenic potential. Herein, we examined the effects of CdTe-QD NPs on gene expression profile of hepatocellular carcinoma (Huh-7) cell line. Huh-7 cells were treated with CdTe-QD NPs (10 μg/ml for 6, 12, and 24 hours, and 25 μg/ml for 6 and 12 hours), and transcriptomic analysis was performed using microarray to evaluate the global gene expression profile. Differential expressed genes (DEGs) were observed for both the doses (10 and 25 μg/ml) of CdTe-QD NPs at different time points. Gene ontology (GO) analysis revealed that genes involved in molecular function of cell cycle, organizational injury and abnormalities, cell death and survival, gene expression, cancer, organismal survival, and cellular development were differentially expressed. Overall, we have demonstrated differential expression of several genes, involved in maintaining cell survival, metabolism, and genome integrity. These findings were confirmed by RT-qPCR study for some canonical pathway genes signifying possible implication in NP toxicity-mediated cell survival and inhibition of cell death.

Inactivation mechanism of E. coli in water by enhanced photocatalysis under visible light irradiation

Developing efficient and economical technologies for drinking water disinfection remains a challenge. We synthesized Ag/AgBr/LDH doped with various silver mass concentrations and explored its ability to inactivate E. coli under visible light irradiation (λ ≥ 400 nm). Our results indicated a total inactivation of E. coli (10⁷ CFU·mL^-1) within 80 min using 2 % Ag/AgBr/LDH in a laboratory-scale test. The method was evaluated for disinfecting three effluent samples collected from one drinking water treatment plant, covering representative water treatment processes. After five consecutive runs, the inactivation efficiency decreased slightly to 89 % in CFU·mL^-1, indicating that the photocatalysts had excellent stability and reusability. The mechanisms were analyzed by combining chemical and biological methods. It was verified that singlet oxygen (¹O₂), hydrogen peroxide (H₂O₂), and photo-generated electrons (e^-) were significant contributors to the inactivation process. Scanning electron microscopy images analysis showed the disruption of the membrane integrity of E. coli by photocatalytic oxidation. Internal component leakage and reduced enzyme activity were also observed in terms of K⁺ leakage, β-galactosidase activity, and antioxidant enzyme activity. The results by the transcriptomic analysis implied that Ag/AgBr/LDH regulating the oxidative stress response and cell membrane damage related genes was the main inactivation mechanism.

Interactions of semiconductor Cd-based quantum dots and Cd2+ with gut bacteria isolated from wild Salmo trutta fry

Background

With the rapid development of nanotechnology, more and more nanoproducts are being released into the environment where they may both pose ecological risks and be toxic to living organisms. The ecotoxicological impact of quantum dots (QDs), a class of nanoparticles (NPs), on aquatic organisms is becoming an emerging issue, this due to their nano-specific properties, to the physico-chemical transformation in the environment and to the possible release of toxic metals from their structure such as Cd.

Methods

In this work, (i) spectroscopic measurements of commercially available Cd-based QDs (CdSe/ZnS-COOH) were made at various pH values (5.0 and 7.0) to study their interactions (at a concentration of 4 nm) with various strains of Gram-positive and Gram-negative gut bacteria after short-term exposure and (ii) the antibacterial efficacy of QDs and Cd²⁺ (at a concentration 0.09-3.56 mM) against gut bacteria isolated from wild freshwater Salmo trutta fry was studied at different temperatures (15 °C and 25 °C) and pH values (5.0 and 7.0) by applying a well-established disc diffusion assay.

Results

Twenty-six gut bacterial isolates from wild Salmo trutta fry were identified as Aeromonas spp., A. popoffii, A. salmonicida, A. sobria, Carnobacterium maltaromaticum, Buttiauxella sp., Listeria sp., Microbacterium sp., Shewanella putrefaciens and Serratia sp. Cd-based (CdSe/ZnS-COOH) QDs at a concentration of 4 nm were found to be stable in aqueous media (with pH 7.0) or starting to form aggregates (at pH 5.0), thus, apparently, did not release heavy metals (HMs) into the media over 48 h in conditions of light or dark and did not show antibacterial efficacy on the gut bacteria isolated from wild Salmo trutta fry after short-term (9 h and 48 h) incubations. Cd²⁺ was found to produce significant dose-dependent toxic effects on bacterial growth, and the size of the inhibition zones on some of the tested strains significantly correlated with temperature. The most sensitive and the most resistant to Cd²⁺ were the Gram-positive bacteria, for which the minimum inhibitory concentration (MIC) values of Cd²⁺ were 0.09-0.27 mM and 3.11-3.29 mM respectively and varied significantly between the tested temperatures (15 °C and 25 °C). The MIC values of Cd²⁺ for the Gram-negative bacteria (18 out of 22 strains) ranged from 0.44 to 0.71 mM and did not differ significantly between the tested temperatures. Among the selected Gram-positive and Gram-negative strains, those with the higher sensitivity towards Cd²⁺ also revealed relatively stronger signals of QDs photoluminescence (PL) when transferred after incubation into fresh medium without QDs. In addition, the formation of endogenous metalloporphyrins observed spectroscopically in some bacterial strains indicates certain differences in metabolic activity that may play a protective role against potential oxidative damage.

Graphene-derived antibacterial nanocomposites for water disinfection: Current and future perspectives

Antimicrobial nanomaterials provide numerous opportunities for the synthesis of next-generation sustainable water disinfectants. Using the keywords graphene and water disinfection and graphene antibacterial activity, a detailed search of the Scopus database yielded 198 and 1433 studies on using graphene for water disinfection applications and graphene antibacterial activity in the last ten years, respectively. Graphene family nanomaterials (GFNs) have emerged as effective antibacterial agents. The current innovations in graphene-, graphene oxide (GO)-, reduced graphene oxide (rGO)-, and graphene quantum dot (GQD)-based nanocomposites for water disinfection, including their functionalization with semiconductor photocatalysts and metal and metal oxide nanoparticles, have been thoroughly discussed in this review. Furthermore, their novel application in the fabrication of 3D porous hydrogels, thin films, and membranes has been emphasized. The physicochemical and structural properties affecting their antibacterial efficiency, such as sheet size, layer number, shape, edges, smoothness/roughness, arrangement mode, aggregation, dispersibility, and surface functionalization have been highlighted. The various mechanisms involved in GFN antibacterial action have been reviewed, including the mechanisms of membrane stress, ROS-dependent and -independent oxidative stress, cell wrapping/trapping, charge transfer, and interaction with cellular components. For safe applications, the potential biosafety and biocompatibility of GFNs in aquatic environments are emphasized. Finally, the current limitations and future perspectives are discussed. This review may provide ideas for developing efficient and practical solutions using graphene-, GO-, rGO-, and GQD-based nanocomposites in water disinfection by rationally employing their unique properties.

Microbial biomanufacture of metal/metallic nanomaterials and metabolic engineering: design strategies, fundamental mechanisms, and future opportunities

Biomanufacturing metal/metallic nanomaterials with ordered micro/nanostructures and controllable functions is of great importance in both fundamental studies and practical applications due to their low toxicity, lower pollution production, and energy conservation. Microorganisms, as efficient biofactories, have a significant ability to biomineralize and bioreduce metal ions that can be obtained as nanocrystals of varying morphologies and sizes. The development of nanoparticle biosynthesis maximizes the safety and sustainability of the nanoparticle preparation. Significant efforts and progress have been made to develop new green and environmentally friendly methods for biocompatible metal/metallic nanomaterials. In this review, we mainly focus on the microbial biomanufacture of different metal/metallic nanomaterials due to their unique advantages of wide availability, environmental acceptability, low cost, and circular sustainability. Specifically, we summarize recent and important advances in the synthesis strategies and mechanisms for different types of metal/metallic nanomaterials using different microorganisms. Finally, we highlight the current challenges and future research directions in this growing multidisciplinary field of biomaterials science, nanoscience, and nanobiotechnology.

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.

Transcriptome Profile Alterations with Carbon Nanotubes, Quantum Dots, and Silver Nanoparticles: A Review

Next-generation sequencing (NGS) technology has revolutionized sequence-based research. In recent years, high-throughput sequencing has become the method of choice in studying the toxicity of chemical agents through observing and measuring changes in transcript levels. Engineered nanomaterial (ENM)-toxicity has become a major field of research and has adopted microarray and newer RNA-Seq methods. Recently, nanotechnology has become a promising tool in the diagnosis and treatment of several diseases in humans. However, due to their high stability, they are likely capable of remaining in the body and environment for long periods of time. Their mechanisms of toxicity and long-lasting effects on our health is still poorly understood. This review explores the effects of three ENMs including carbon nanotubes (CNTs), quantum dots (QDs), and Ag nanoparticles (AgNPs) by cross examining publications on transcriptomic changes induced by these nanomaterials.

Bacterial phototoxicity of biomimetic CdTe-GSH quantum dots

AIM

Fluorescent semiconductor nanoparticles or quantum dots (QDs) have excellent properties as photosensitizers in photodynamic therapy. This is mainly a consequence of their nanometric size and the generation of light-activated redox species. In previous works, we have reported the low-cost biomimetic synthesis of glutathione (GSH) capped QDs (CdTe-GSH QDs) with high biocompatibility. However, no studies have been performed to determine their phototoxic effect. The aim of this work was to characterize the light-induced toxicity of green (QDs₅₀₀ ) and red (QDs₆₀₀ ) QDs in Escherichia coli, and to study the molecular mechanism involved.

METHODS AND RESULTS

Photodegradation and reduction power of biomimetic QDs was determined to analyse their potential for radical generation. Escherichia coli cells were exposed to photoactivated QDs and viability was evaluated at different times. High toxicity was determined in E. coli cells exposed to photoactivated QDs, particularly QDs₅₀₀ . The molecular mechanism involved in QDs phototoxicity was studied by determining Cd²⁺ -release and intracellular reactive oxygen species (ROS). Cells exposed to photoactivated QDs₅₀₀ presented high levels of ROS. Cells exposed to photoactivated QDs₅₀₀ presented high levels of ROS. Finally, to understand this phenomenon and the importance of oxidative and cadmium-stress in QDs-mediated phototoxicity, experiments were performed in E. coli mutants in ROS and Cd²⁺ response genes. As expected, E. coli mutants in ROS response genes were more sensitive than the wt strain to photoactivated QDs, with a higher effect in green-QDs₅₀₀ . No increase in phototoxicity was observed in cadmium-related mutants.

CONCLUSION

Obtained results indicate that light exposure increases the toxicity of biomimetic QDs on E. coli cells. The mechanism of bacterial phototoxicity of biomimetic CdTe-GSH QDs is mostly associated with ROS generation.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results presented establish biomimetic CdTe-GSH QDs as a promising cost-effective alternative against microbial infections, particularly QDs₅₀₀ .

Benefits at the nanoscale: a review of nanoparticle-enabled processes favouring microbial growth and functionality

Nanoparticles are ubiquitous and co-occur with microbial life in every environment on Earth. Interactions between microbes and nanoparticles impact the biogeochemical cycles via accelerating various reaction rates and enabling biological processes at the smallest scales. Distinct from microbe-mineral interactions at large, microbe-nanoparticle interactions may involve higher levels of active recognition and utilization of the reactive, changeable, and thereby 'moldable' nano-sized inorganic phases by microbes, which has been given minimal attention in previous reviews. Here we have compiled the various cases of microbe-nanoparticle interactions with clear and potential benefits to the microbial cells and communities. Specifically, we discussed (i) the high bioavailabilities of nanoparticles due to increased specific surface areas and size-dependent solubility, with a focus on environmentally-relevant iron(III) (oxyhydr)oxides and pyrite, (ii) microbial utilization of nanoparticles as 'nano-tools' for electron transfer, chemotaxis, and storage units, and (iii) speculated benefits of precipitating 'moldable' nanoparticles in extracellular biomineralization. We further discussed emergent questions concerning cellular level responses to nanoparticle-associated cues, and the factors that affect nanoparticles' bioavailabilities beyond size-dependent effects. We end the review by proposing a framework towards more quantitative approaches and by highlighting promising techniques to guide future research in this exciting field.

Genetic Circuits To Detect Nanomaterial Triggered Toxicity through Engineered Heat Shock Response Mechanism

Biocompatibility assessment of nanomaterials has been of great interest due to their potential toxicity. However, conventional biocompatibility tests fall short of providing a fast toxicity report. We developed a whole cell based biosensor to track biocompatibility of nanomaterials with the aim of providing fast feedback to engineer them with lower toxicity levels. We engineered promoters of four heat shock response (HSR) proteins utilizing synthetic biology approaches. As an initial design, a reporter coding gene was cloned downstream of the selected promoter regions. Initial results indicated that native heat shock protein (HSP) promoter regions were not very promising to generate signals with low background signals. Introducing riboregulators to native promoters eliminated unwanted background signals almost entirely. Yet, this approach also led to a decrease in expected sensor signal upon stress treatment. Thus, a repression based genetic circuit, inspired by the HSR mechanism of Mycobacterium tuberculosis, was constructed. These genetic circuits could report the toxicity of quantum dot nanoparticles in 1 h. Our designed nanoparticle toxicity sensors can provide quick reports, which can lower the demand for additional experiments with more complex organisms.

Visible-light-driven photocatalytic disinfection mechanism of Pb-BiFeO3/rGO photocatalyst

While the visible-light-driven photocatalytic disinfection techniques for drinking water have recently attracted tremendous attentions, it is necessary to further improve the solar energy utilization efficiency. In this study, we synthesized Pb-BiFeO₃ photocatalysts doped with different amounts of reduced graphene oxide (Pb-BiFeO₃/rGO). The photocatalytic disinfection efficiencies toward gram-negative Escherichia coli (E. coli) and gram-positive Staphylococcus aureus (S. aureus) were evaluated under visible-light irradiation (λ ≥ 400 nm). The results indicated that Pb-BiFeO₃ with 0.5 wt% rGO (Pb-BiFeO₃/0.5% rGO) exhibited the highest disinfection efficiency. Complete inactivation was reached within 30 min and 90 min for E. coli and S. aureus, respectively. The transcriptomic analysis results indicated that Pb-BiFeO₃/0.5% rGO deregulates the genes in E. coli cells that are involved in the cell membrane damage and oxidative stress responses. This was validated by the cell leakage of nucleic acids or proteins, transmission electron microscopy images of the bacteria, and the disinfection efficiency decrease caused by the introduction of scavenger of hydroxyl radical (HO•). Metal ions (Pb²⁺, Bi²⁺, and Fe³⁺) released from the photocatalysts did not contribute to the disinfection process. For the first time, our results elucidated that the photocatalytic disinfection mechanism of Pb-BiFeO₃/rGO toward E. coli was mainly associated with oxidative stress due to HO• generation and the loss of membrane integrity from direct contact with the photocatalyst. After four consecutive cycles, the Pb-BiFeO₃/0.5% rGO photocatalyst exhibited a strong antibacterial efficiency. The excellent disinfection efficiency and stability of Pb-BiFeO₃/0.5% rGO suggests that this photocatalyst shows great potential for drinking water disinfection.

Isolating the Escherichia coli Transcriptomic Response to Superoxide Generation from Cadmium Chalcogenide Quantum Dots

Nanomaterials have been extensively used in the biomedical field and have recently garnered attention as potential antimicrobial agents. Cadmium telluride quantum dots (QDs) with a bandgap of 2.4 eV (CdTe-2.4) were previously shown to inhibit multidrug-resistant clinical isolates of bacterial pathogens via light-activated superoxide generation. Here we investigate the transcriptomic response of Escherichia coli to phototherapeutic CdTe-2.4 QDs both with and without illumination, as well as in comparison with the non-superoxide-generating cadmium selenide QDs (CdSe-2.4) as a negative control. Our analysis sought to separate the transcriptomic response of E. coli to the generation of superoxide by the CdTe-2.4 QDs from the presence of cadmium chalcogenide nanoparticles alone. We used comparisons between illuminated CdTe-2.4 conditions and all others to establish the superoxide generation response and used comparisons between all QD conditions and the no treatment condition to establish the cadmium chalcogenide QD response. In our analysis of the gene expression experiments, we found eight genes to be consistently differentially expressed as a response to superoxide generation, and these genes demonstrate a consistent association with the DNA damage response and deactivation of iron-sulfur clusters. Each of these responses is characteristic of a bacterial superoxide response. We found 18 genes associated with the presence of cadmium chalcogenide QDs but not the generation of superoxide by CdTe-2.4, including several that implicated metabolism of amino acids in the E. coli response. To explore each of these gene sets further, we performed both gene knockout and amino acid supplementation experiments. We identified the importance of leucyl-tRNA downregulation as a cadmium chalcogenide QD response and reinforced the relationship between CdTe-2.4 stress and iron-sulfur clusters through examination of the gene tusA. This study demonstrates the transcriptomic response of E. coli to CdTe-2.4 and CdSe-2.4 QDs and parses the different effects of superoxide versus material effects on the bacteria. Our findings may provide useful information toward the development of QD-based antibacterial therapy in the future.

Biological Synthesis of CdS/CdSe Core/Shell Nanoparticles and Its Application in Quantum Dot Sensitized Solar Cells

In the present work, we report the use of bacterial cells for the production of CdS/CdSe Core/Shell quantum dots (QDs), a complex nanostructure specially designed to improve their performance as photosensitizer in photovoltaic devices. The method requires the incorporation of L-cysteine, CdCl₂ and Na₂SeO₃ to Escherichia coli cultures and allows a tight control of QDs properties. The obtained CdS/CdSe QDs were photophysically and structurally characterized. When compared to CdS QDs, the classical shift in the UV-visible spectra of Core/Shell nanostructures was observed in CdS/CdSe QDs. The nanosize, structure, and composition of Core/Shell QDs were confirmed by TEM and EDS analysis. QDs presented a size of approximately 12 nm (CdS) and 17 nm (CdS/CdSe) as determined by dynamic light scattering (DLS), whereas the fourier transform infrared (FTIR) spectra allowed to distinguish the presence of different biomolecules bound to both types of nanoparticles. An increased photostability was observed in CdS/CdSe nanoparticles when compared to CdS QDs. Finally, biosynthesized CdS/CdSe Core/Shell QDs were used as photosensitizers for quantum dots sensitized solar cells (QDSSCs) and their photovoltaic parameters determined. As expected, the efficiency of solar cells sensitized with biological CdS/CdSe QDs increased almost 2.5 times when compared to cells sensitized with CdS QDs. This work is the first report of biological synthesis of CdS/CdSe Core/Shell QDs using bacterial cells and represents a significant contribution to the development of green and low-cost photovoltaic technologies.

Toxicological effects of CdSe nanocrystals on the marine diatom Phaeodactylum tricornutum: The first mass spectrometry-based proteomic approach

In the marine environment, benthic diatoms from estuarine and coastal sediments are among the first targets of nanoparticle pollution whose potential toxicity on marine organisms is still largely unknown. It is therefore relevant to improve our knowledge of interactions between these new pollutants and microalgae, the key players in the control of marine resources. In this study, the response of P. tricornutum to CdSe nanocrystals (CdSe NPs) of 5 nm (NP5) and 12 nm (NP12) in diameter was evaluated through microscopic, physiological, biochemical and proteomic approaches. NP5 and NP12 affected cell growth but oxygen production was only slightly decreased by NP5 after 1-d incubation time. In our experimental conditions, a high CdSe NP dissolution was observed during the first day of culture, leading to Cd bioaccumulation and oxidative stress, particularly with NP12. However, after a 7-day incubation time, proteomic analysis highlighted that P. tricornutum responded to CdSe NP toxicity by regulating numerous proteins involved in protection against oxidative stress, cellular redox homeostasis, Ca²⁺ regulation and signalling, S-nitrosylation and S-glutathionylation processes and cell damage repair. These proteome changes allowed algae cells to regulate their intracellular ROS level in contaminated cultures. P. tricornutum was also capable to control its intracellular Cd concentration at a sufficiently low level to preserve its growth. To our knowledge, this is the first work allowing the identification of proteins differentially expressed by P. tricornutum subjected to NPs and thus the understanding of some molecular pathways involved in its cellular response to nanoparticles.

SIGNIFICANCE

The microalgae play a key role in the control of marine resources. Moreover, they produce 50% of the atmospheric oxygen. CdSe NPs are extensively used in the industry of renewable energies and it is regrettably expected that these pollutants will sometime soon appear in the marine environment through surface runoff, urban effluents and rivers. Since estuarine and coastal sediments concentrate pollutants, benthic microalgae which live in superficial sediments will be among the first targets of nanoparticle pollution. Thus, it is relevant to improve our knowledge of interactions between diatoms and nanoparticles. Proteomics is a powerful tool for understanding the molecular mechanisms triggered by nanoparticle exposure, and our study is the first one to use this tool to identify proteins differentially expressed by P. tricornutum subjected to CdSe nanocrystals. This work is fundamental to improve our knowledge about the defence mechanisms developed by algae cells to counteract damage caused by CdSe NPs.

Biosorption behavior and proteomic analysis of Escherichia coli P4 under cadmium stress

Bacteria develop a variety of adaptations at transcriptomic, metabolomic and proteomic levels in order to survive potentially damaging environmental perturbations. Present study is exploring the fluctuations in proteome of E. coli P4 to knob Cd⁺²-induced cytotoxicity. An attempt was also made to integrate all these approaches to gain comprehensive insight of Cd⁺² stress response in E. coli P4. This study is exposing the altered behavior of various proteins and their underlying metabolic pathways which have previously not been reported with reference to Cd⁺² stress such as sulfoquinovose biosynthesis and degradation pathway. Some of the responses studied on all integrated levels followed same dynamics and strategies to conserve energy by down regulating carbohydrate metabolism (depicted by the repression of succinyl-CoA ligase) and growth stasis (down regulation of ftsZ). Moreover, proteomic analysis clearly revealed the affection of Cd⁺² stress on various proteins expression including Rrf, MdaB, DapA, GpmA,Cdd, FabI, DsbA, ZnuA and YihW found modulating key cellular metabolic pathways enabling E. coli P4 to withstand Cd⁺²-induced toxic effects. Furthermore, over-expression of Mn-SOD provided evidence that Cd⁺²exposure induces superoxide free radicals mediated oxidative stress rather than hydrogen peroxide (H₂O₂). EnvZ/OmpR -a two component cell envelope regulatory system was observed operating to homeostat the cell's internal environment. Cd⁺² bioremediation potential of E. coli P4 and its kinetic and thermodynamic basis were studied by applying different isotherm models which nominated E. coli P4 a good bioresource for green chemistry to eradicate environmental Cd⁺².

Flavoprotein-Mediated Tellurite Reduction: Structural Basis and Applications to the Synthesis of Tellurium-Containing Nanostructures

The tellurium oxyanion tellurite (TeO₃^2-) is extremely harmful for most organisms. It has been suggested that a potential bacterial tellurite resistance mechanism would consist of an enzymatic, NAD(P)H-dependent, reduction to the less toxic form elemental tellurium (Te⁰). To date, a number of enzymes such as catalase, type II NADH dehydrogenase and terminal oxidases from the electron transport chain, nitrate reductases, and dihydrolipoamide dehydrogenase (E3), among others, have been shown to display tellurite-reducing activity. This activity is generically referred to as tellurite reductase (TR). Bioinformatic data resting on some of the abovementioned enzymes enabled the identification of common structures involved in tellurite reduction including vicinal catalytic cysteine residues and the FAD/NAD(P)⁺-binding domain, which is characteristic of some flavoproteins. Along this line, thioredoxin reductase (TrxB), alkyl hydroperoxide reductase (AhpF), glutathione reductase (GorA), mercuric reductase (MerA), NADH: flavorubredoxin reductase (NorW), dihydrolipoamide dehydrogenase, and the putative oxidoreductase YkgC from Escherichia coli or environmental bacteria were purified and assessed for TR activity. All of them displayed in vitro TR activity at the expense of NADH or NADPH oxidation. In general, optimal reducing conditions occurred around pH 9–10 and 37°C. Enzymes exhibiting strong TR activity produced Te-containing nanostructures (TeNS). While GorA and AhpF generated TeNS of 75 nm average diameter, E3 and YkgC produced larger structures (>100 nm). Electron-dense structures were observed in cells over-expressing genes encoding TrxB, GorA, and YkgC.