Cadmium toxicity in Escherichia coli: Cell morphology, Z-ring formation and intracellular oxidative balance

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.

Characterization on nicotine degradation and research on heavy metal resistance of a strain Pseudomonas sp. NBB

The remediation of polluted sites containing multiple contaminants like nicotine and heavy metals poses significant challenges, due to detrimental effects like cell death. In this study, we isolated a new strain Pseudomonas sp. NBB capable of efficiently degrading nicotine even in high level of heavy metals. It degraded nicotine through pyrrolidine pathway and displayed minimum inhibitory concentrations of 2 mM for barium, copper, and lead, and 5 mM for manganese. In the presence of 2 mM Ba2+ or Pb2+, 3 g L-1 nicotine could be completely degraded within 24 h. Moreover, under 0.5 mM Cu2+ or 5 mM Mn2+ stress, 24.13% and 72.56% of nicotine degradation were achieved in 60 h, respectively. Strain NBB tolerances metal stress by various strategies, including morphological changes, up-regulation of macromolecule transporters, cellular response to DNA damage, and down-regulation of ABC transporters. Notably, among the 153 up-regulated genes, cds_821 was identified as manganese exporter (MneA) after gene disruption and recovery experiments. This study presents a novel strain capable of efficiently degrading nicotine and displaying remarkable resistance to heavy metals. The findings of this research provide valuable insights into the potential application of nicotine bioremediation in heavy metal-contaminated areas.

The Effect of Heavy Metals on Conjugation Efficiency of an F-Plasmid in Escherichia coli

Conjugation, the process by which conjugative plasmids are transferred between bacteria, is regarded as a major contributor to the spread of antibiotic resistance, in both environmental and clinical settings. Heavy metals are known to co-select for antibiotic resistance, but the impact of the presence of these metals on conjugation itself is not clear. Here, we systematically investigate the impact that five heavy metals (arsenic, cadmium, copper, manganese, and zinc) have on the transfer of an IncF conjugative plasmid in Escherichia coli. Our results show that two of the metals, cadmium and manganese, have no significant impact, while arsenic and zinc both reduce conjugation efficiency by approximately 2-fold. Copper showed the largest impact, with an almost 100-fold decrease in conjugation efficiency. This was not mediated by any change in transcription from the major Py promoter responsible for transcription of the conjugation machinery genes. Further, we show that in order to have this severe impact on the transfer of the plasmid, copper sulfate needs to be present during the mating process, and we suggest explanations for this.

The cytotoxicity of endogenous CdS and Cd2+ ions during CdS NPs biosynthesis

In the present study, cadmium-based nanoparticles (NPs) were biosynthesized by incubating their precursor salts with E. coli CD-2. Transmission electron microscopy (TEM) revealed the morphology of the NPs and confirmed that the NPs were formed via an intracellular growth. Energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) determined the elemental composition of the NPs and identified the NPs as CdS. The contents of extracellular Cd²⁺, intracellular Cd²⁺ and intracellular CdS NPs were determined during the whole CdS biosynthetic process. The results demonstrated that the contents of Cd²⁺ and CdS NPs changed during the biosynthetic process. The colony-forming capability test showed that strain CD-2 could maintain its growth during CdS biosynthesis. Protein oxidation levels confirmed that the E. coli cells faced oxidative stress induced both by Cd²⁺ and CdS. Both Cd²⁺ and CdS NPs affected the cellular antioxidative system by upregulating related gene expression. However, different pathways might be involved to eliminate ROS induced by Cd²⁺ ions or CdS NPs, respectively. The expression levels of ef-tu, ftsZ, mutS and dnaK were enhanced together with CdS accumulation, indicating that the cells had to overexpress certain related genes to respond to the NPs-induced stress.

Bacterial and Fungal Communities Are Differentially Modified by Melatonin in Agricultural Soils Under Abiotic Stress

An extensive body of evidence from the last decade has indicated that melatonin enhances plant resistance to a range of biotic and abiotic stressors. This has led to an interest in the application of melatonin in agriculture to reduce negative physiological effects from environmental stresses that affect yield and crop quality. However, there are no reports regarding the effects of melatonin on soil microbial communities under abiotic stress, despite the importance of microbes for plant root health and function. Three agricultural soils associated with different land usage histories (pasture, canola or wheat) were placed under abiotic stress by cadmium (100 or 280 mg kg-1 soil) or salt (4 or 7 g kg-1 soil) and treated with melatonin (0.2 and 4 mg kg-1 soil). Automated Ribosomal Intergenic Spacer Analysis (ARISA) was used to generate Operational Taxonomic Units (OTU) for microbial community analysis in each soil. Significant differences in richness (α diversity) and community structures (β diversity) were observed between bacterial and fungal assemblages across all three soils, demonstrating the effect of melatonin on soil microbial communities under abiotic stress. The analysis also indicated that the microbial response to melatonin is governed by the type of soil and history. The effects of melatonin on soil microbes need to be regarded in potential future agricultural applications.

Physiological adaptation and spectral annotation of Arsenic and Cadmium heavy metal-resistant and susceptible strain Pseudomonas taiwanensis

In the present study, the 16S-rRNA sequencing of heavy metal-resistant and susceptible bacterial strains isolated from the industrial and agriculture soil showed resemblance with Pseudomonas taiwanensis. Based on the growth rate, two bacterial strains SJPS_KUD54 and KUD-MBBT4 exhibited 10 ppm tolerance to Arsenic and Cadmium. These two heavy metals caused, a significant increase in stress enzymes like superoxide dismutase, catalase and glutathione S-transferase activities in SJPS_KUD54 when compared to KUD-MBBT4. Following heavy metal treatment, the atomic-force-microscopy observations showed no change in the cell-wall of SJPS_KUD54, whereas the cell-wall of KUD-MBBT4 got ruptured. Moreover, the protein-profile of SJPS_KUD54 treated with heavy metals exhibited varied patterns in comparison with untreated control. In addition, the accumulation of hydroxyl, thiol and amides were found in the SJPS_KUD54 relative to its control. Furthermore, the resistant SJPS_KUD54 strain showed a remarkable bioaccumulation properties to both Arsenic and Cadmium. Thus, it is inferred that the growth rate, stress enzymes and functional-groups play a significant role in the physiological-adaption of SJPS_KUD54 during stress conditions, which is positively involved in the prevention or repair mechanism for reducing the risks caused by heavy metal stress.

Protective Role of Freshwater and Marine Rotifer Glutathione S-Transferase Sigma and Omega Isoforms Transformed into Heavy Metal-Exposed Escherichia coli

Glutathione S-transferases (GSTs) play an important role in phase II of detoxification to protect cells in response to oxidative stress generated by exogenous toxicants. Despite their important role in defense, studies on invertebrate GSTs have mainly focused on identification and characterization. Here, we isolated omega and sigma classes of GSTs from the freshwater rotifer Brachionus calyciflorus and the marine rotifer Brachionus koreanus and explored their antioxidant function in response to metal-induced oxidative stress. The recombinant Bc- and Bk-GSTs were successfully transformed and expressed in Escherichia coli. Their antioxidant potential was characterized by measuring kinetic properties and enzymatic activity in response to pH, temperature, and chemical inhibitor. In addition, a disk diffusion assay, reactive oxygen species assay, and morphological analysis revealed that GST transformed into E. coli significantly protected cells from oxidative stress induced by H₂O₂ and metals (Hg, Cd, Cu, and Zn). Stronger antioxidant activity was exhibited by GST-S compared to GST-O in both rotifers, suggesting that GST-S plays a prominent function as an antioxidant defense mechanism in Brachionus spp. Overall, our study clearly shows the antioxidant role of Bk- and Bc-GSTs in E. coli and provides a greater understanding of GST class-specific and interspecific detoxification in rotifer Brachionus spp.

Stresses that Raise Np4A Levels Induce Protective Nucleoside Tetraphosphate Capping of Bacterial RNA

Present in all realms of life, dinucleoside tetraphosphates (Np₄Ns) are generally considered signaling molecules. However, only a single pathway for Np₄N signaling has been delineated in eukaryotes, and no receptor that mediates the influence of Np₄Ns has ever been identified in bacteria. Here we show that, under disulfide stress conditions that elevate cellular Np₄N concentrations, diverse Escherichia coli mRNAs and sRNAs acquire a cognate Np₄ cap. Purified E. coli RNA polymerase and lysyl-tRNA synthetase are both capable of adding such 5' caps. Cap removal by either of two pyrophosphatases, ApaH or RppH, triggers rapid RNA degradation in E. coli. ApaH, the predominant decapping enzyme, functions as both a sensor and an effector of disulfide stress, which inactivates it. These findings suggest that the physiological changes attributed to elevated Np₄N concentrations in bacteria may result from widespread Np₄ capping, leading to altered RNA stability and consequent changes in gene expression.

Simultaneous mitigation of tissue cadmium and lead accumulation in rice via sulfate-reducing bacterium

The objectives of this study were to investigate the mechanism responsible for Cd and Pb immobilization by sulfate reduction to sulfide and effectiveness of decreasing Cd²⁺ and Pb²⁺ bioavailability in culture solution and paddy soils via sulfate-reducing bacterium (SRB1-1). The SRB1-1 strain, exhibiting high resistances to Cd²⁺ and Pb²⁺, was isolated from bulk soils in the metal(loid)-contaminated paddy field. During the culture of the SRB1-1 strain, the removal percentages of Cd²⁺ and Pb²⁺ from culture solution reached 99.5% and 76.0% in 72 h, respectively. The surface morphology and composition of metal precipitates formed by SRB1-1 strain were analyzed by transmission electron microscopy (TEM) and further confirmed to be CdS and PbS by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). When living SRB1-1 strain was applied in Cd and Pb-contaminated soils, the SRB1-1 strain could stably colonize using its resistance to rifampicin, and showed significantly impact on the bacterial community composition. Cd and Pb contents in rice grains were decreased by 29.5% and 26.2%, respectively, while Cd and Pb contents in the roots, culms, leaves, and husk were also decreased ranging from 19.1% to 43%, respectively. Due to growth in highly Cd and Pb contaminated soils, Cd content of the rice grains did not meet the standard for limit of Cd and Pb, but safe production of rice plants may be obtained in slightly or moderately metal(loid)-contaminated soils in the presence of the living SRB1-1 strain. These results indicated that the SRB1-1 strain could effectively reduce the Cd and Pb bioavailability in soils and uptake in rice plants. Our results highlighted the possibility to develop a new bacterial-assisted technique for reduced metal accumulation in rice grains, and also showed potential for effective synergistic bioremediation of SRB1-1 strain and rice plants in metal(loid)-contaminated soils.

Metal complexation by histidine-rich peptides confers protective roles against cadmium stress in Escherichia coli as revealed by proteomics analysis

The underlying mechanism and cellular responses of bacteria against toxic cadmium ions is still not fully understood. Herein, Escherichia coli TG1 expressing hexahistidine-green fluorescent protein (His6GFP) and cells expressing polyhistidine-fused to the outer membrane protein A (His-OmpA) were applied as models to investigate roles of cytoplasmic metal complexation and metal chelation at the surface membrane, respectively, upon exposure to cadmium stress. Two-dimensional gel electrophoresis (2-DE) and two-dimensional difference in gel electrophoresis (2D-DIGE) in conjunction with mass spectrometry-based protein identification had successfully revealed the low level expression of antioxidative enzymes and stress-responsive proteins such as manganese-superoxide dismutase (MnSOD; +1.65 fold), alkyl hydroperoxide reductase subunit C (AhpC; +1.03 fold) and DNA starvation/stationary phase protection protein (Dps; −1.02 fold) in cells expressing His6GFP in the presence of 0.2 mM cadmium ions. By contrarily, cadmium exposure led to the up-regulation of MnSOD of up to +7.20 and +3.08 fold in TG1-carrying pUC19 control plasmid and TG1 expressing native GFP, respectively, for defensive purposes against Cd-induced oxidative cell damage. Our findings strongly support the idea that complex formation between cadmium ions and His6GFP could prevent reactive oxygen species (ROS) caused by interaction between Cd²⁺ and electron transport chain. This coincided with the evidence that cells expressing His6GFP could maintain their growth pattern in a similar fashion as that of the control cells even in the presence of harmful cadmium. Interestingly, overexpression of either OmpA or His-OmpA in E. coli cells has also been proven to confer protection against cadmium toxicity as comparable to that observed in cells expressing His6GFP. Blockage of metal uptake as a consequence of anchored polyhistidine residues on surface membrane limited certain amount of cadmium ions in which some portion could pass through and exert their toxic effects to cells as observed by the increased expression of MnSOD of up to +9.91 and +3.31 fold in case of TG1 expressing only OmpA and His-OmpA, respectively. Plausible mechanisms of cellular responses and protein mapping in the presence of cadmium ions were discussed. Taken together, we propose that the intracellular complexation of cadmium ions by metal-binding regions provides more efficiency to cope with cadmium stress than the blockage of metal uptake at the surface membrane. Such findings provide insights into the molecular mechanism and cellular adaptation against cadmium toxicity in bacteria.

The peroxyl radical-induced oxidation of Escherichia coli FtsZ and its single tryptophan mutant (Y222W) modifies specific side-chains, generates protein cross-links and affects biological function

FtsZ (filamenting temperature-sensitive mutant Z) is a key protein in bacteria cell division. The wild-type Escherichia coli FtsZ sequence (FtsZwt) contains three tyrosine (Tyr, Y) and sixteen methionine (Met, M) residues. The Tyr at position 222 is a key residue for FtsZ polymerization. Mutation of this residue to tryptophan (Trp, W; mutant Y222W) inhibits GTPase activity resulting in an extended time in the polymerized state compared to FtsZwt. Protein oxidation has been highlighted as a determinant process for bacteria resistance and consequently oxidation of FtsZwt and the Y222W mutant, by peroxyl radicals (ROO•) generated from AAPH (2,2'-azobis(2-methylpropionamidine) dihydrochloride) was studied. The non-oxidized proteins showed differences in their polymerization behavior, with this favored by the presence of Trp at position 222. AAPH-treatment of the proteins inhibited polymerization. Protein integrity studies using SDS-PAGE revealed the presence of both monomers and oligomers (dimers, trimers and high mass material) on oxidation. Western blotting indicated the presence of significant levels of protein carbonyls. Amino acid analysis showed that Tyr, Trp (in the Y222W mutant), and Met were consumed by ROO•. Quantification of the number of moles of amino acid consumed per mole of ROO• shows that most of the initial oxidant can be accounted for at low radical fluxes, with Met being a major target. Western blotting provided evidence for di-tyrosine cross-links in the dimeric and trimeric proteins, confirming that oxidation of Tyr residues, at positions 339 and/or 371, are critical to ROO•-mediated crosslinking of both the FtsZwt and Y222W mutant protein. These findings are in agreement with di-tyrosine, N-formyl kynurenine, and kynurenine quantification assessed by UPLC, and with LC-MS data obtained for AAPH-treated protein samples.

Comparative toxicity of Cd, Mo, and W sulphide nanomaterials toward E. coli under UV irradiation

In this study, the phototoxicity of cadmium sulfide (CdS), molybdenum disulfide (MoS₂), and tungsten disulfide (WS₂) nanoparticles (NPs) toward Escherichia coli (E. coli) under UV irradiation (365 nm) was investigated. At the same mass concentration of NPs, the toxicity of three NPs decreased in the order of CdS > MoS₂ > WS₂. For example, the death rates of E. coli exposed to 50 mg/L CdS, MoS₂, and WS₂ were 96.7%, 38.5%, and 31.2%, respectively. Transmission electron microscope and laser scanning confocal microscope images of E. coli exposed to three NPs showed the damage of cell walls and release of intracellular components. The CdS-treated cell wall was more extensively damaged than those of MoS₂-treated and WS₂-treated bacteria. WS₂ and MoS₂ generated superoxide radical (O₂^-), singlet oxygen (¹O₂), and hydroxyl radical under UV irradiation, CdS produced only O₂^- and ¹O₂. CdS and WS₂ released ions under UV irradiation, while MoS₂ did not. Reactive oxygen species (ROS) generation and toxic ion release jointly resulted in the antibacterial activities of CdS and WS₂. ROS generation was the dominant toxic mechanism of MoS₂ toward the bacteria. This study highlighted the importance of considering the hazardous effect of sulfide NPs after their release into natural waters under light irradiation condition.

Improving prediction of metal uptake by Chinese cabbage (Brassica pekinensis L.) based on a soil-plant stepwise analysis

It is crucial to develop predictive soil-plant transfer (SPT) models to derive the threshold values of toxic metals in contaminated arable soils. The present study was designed to examine the heavy metal uptake pattern and to improve the prediction of metal uptake by Chinese cabbage grown in agricultural soils with multiple contamination by Cd, Cu, Ni, Pb, and Zn. Pot experiments were performed with 25 historically contaminated soils to determine metal accumulation in different parts of Chinese cabbage. Different soil bioavailable metal fractions were determined using different extractants (0.43M HNO3, 0.01M CaCl2, 0.005M DTPA, and 0.01M LWMOAs), soil moisture samplers, and diffusive gradients in thin films (DGT), and the fractions were compared with shoot metal uptake using both direct and stepwise multiple regression analysis. The stepwise approach significantly improved the prediction of metal uptake by cabbage over the direct approach. Strongly pH dependent or nonlinear relationships were found for the adsorption of root surfaces and in root-shoot uptake processes. Metals were linearly translocated from the root surface to the root. Therefore, the nonlinearity of uptake pattern is an important explanation for the inadequacy of the direct approach in some cases. The stepwise approach offers an alternative and robust method to study the pattern of metal uptake by Chinese cabbage (Brassica pekinensis L.).

Zinc to cadmium replacement in the prokaryotic zinc-finger domain

Given the similar chemical properties of zinc and cadmium, zinc finger domains have been often proposed as mediators of the toxic and carcinogenic effects exerted by this xenobiotic metal. The effects of zinc replacement by cadmium in different eukaryotic zinc fingers have been reported. In the present work, to evaluate the effects of such substitution in the prokaryotic zinc finger, we report a detailed study of its functional and structural consequences on the Ros DNA binding domain (Ros87). We show that this protein, which bears important structural differences with respect to the eukaryotic domains, appears to structurally tolerate the zinc to cadmium substitution and the presence of cadmium does not affect the DNA binding activity of the protein. Moreover, we show for the first time how zinc to cadmium replacement can also take place in a cellular context. Our findings both complement and extend previous results obtained for different eukaryotic zinc fingers, suggesting that metal substitution in zinc fingers may be of relevance to the toxicity and/or carcinogenicity mechanisms of this metal.

Response mechanisms of bacterial degraders to environmental contaminants on the level of cell walls and cytoplasmic membrane

Bacterial strains living in the environment must cope with the toxic compounds originating from humans production. Surface bacterial structures, cell wall and cytoplasmic membrane, surround each bacterial cell and create selective barriers between the cell interior and the outside world. They are a first site of contact between the cell and toxic compounds. Organic pollutants are able to penetrate into cytoplasmic membrane and affect membrane physiological functions. Bacteria had to evolve adaptation mechanisms to counteract the damage originated from toxic contaminants and to prevent their accumulation in cell. This review deals with various adaptation mechanisms of bacterial cell concerning primarily the changes in cytoplasmic membrane and cell wall. Cell adaptation maintains the membrane fluidity status and ratio between bilayer/nonbilayer phospholipids as well as the efflux of toxic compounds, protein repair mechanisms, and degradation of contaminants. Low energy consumption of cell adaptation is required to provide other physiological functions. Bacteria able to survive in toxic environment could help us to clean contaminated areas when they are used in bioremediation technologies.

Toxicity of cadmium sulfide (CdS) nanoparticles against Escherichia coli and HeLa cells

The present study endeavours to assess the toxic effect of synthesized CdS nanoparticles (NPs) on Escherichia coli and HeLa cells. The CdS NPs were characterized by DLS, XRD, TEM and AFM studies and the average size of NPs was revealed as ∼3 nm. On CdS NPs exposure bacterial cells changed morphological features to filamentous form and damage of the cell surface was found by AFM study. The expression of two conserved cell division components namely ftsZ and ftsQ in E. coli was decreased both at transcriptional and translational levels upon CdS NPs exposure. CdS NPs inhibited proper cell septum formation without affecting the nucleoid segregation. Viability of HeLa cells declined with increasing concentration of CdS NPs and the IC₅₀ value was found to be 4 μg/mL. NPs treated HeLa cells showed changed morphology with condensed and fragmented nuclei. Increased level of reactive oxygen species (ROS) was found both in E. coli and HeLa cells on CdS NPs exposure. The inverse correlation between declined cell viabilities and elevated ROS level suggested that oxidative stress seems to be the key event by which NPs induce toxicity both in E. coli and HeLa cells.