Bioaccumulation and distribution of cadmium by Burkholderia cepacia GYP1 under oligotrophic condition and mechanism analysis at proteome level

Revisiting biochemical pathways for lead and cadmium tolerance by domain bacteria, eukarya, and their joint action in bioremediation

With the advent rise is in urbanization and industrialization, heavy metals (HMs) such as lead (Pb) and cadmium (Cd) contamination have increased considerably. It is among the most recalcitrant pollutants majorly affecting the biotic and abiotic components of the ecosystem like human well-being, animals, soil health, crop productivity, and diversity of prokaryotes (bacteria) and eukaryotes (plants, fungi, and algae). At higher concentrations, these metals are toxic for their growth and pose a significant environmental threat, necessitating innovative and sustainable remediation strategies. Bacteria exhibit diverse mechanisms to cope with HM exposure, including biosorption, chelation, and efflux mechanism, while fungi contribute through mycorrhizal associations and hyphal networks. Algae, especially microalgae, demonstrate effective biosorption and bioaccumulation capacities. Plants, as phytoremediators, hyperaccumulate metals, providing a nature-based approach for soil reclamation. Integration of these biological agents in combination presents opportunities for enhanced remediation efficiency. This comprehensive review aims to provide insights into joint action of prokaryotic and eukaryotic interactions in the management of HM stress in the environment.

Rhodopseudomonas palustris shapes bacterial community, reduces Cd bioavailability in Cd contaminated flooding paddy soil, and improves rice performance

Photosynthetic bacteria (PSB) are suitable to live and remediate cadmium (Cd) in the slightly oxygenated or anaerobic flooding paddy field. However, there is currently limited study on the inhibition of Cd accumulation in rice by PSB, and the relevant mechanisms has yet to be elucidated. In the current study, we firstly used Rhodopseudomonas palustris SC06 (a typical PSB) as research target and combined physiology, biochemistry, microbiome and metabolome to evaluate the mechanisms of remeding Cd pollution in paddy field and inhibiting Cd accumulation in rice. Microbiome analysis results revealed that intensive inoculation with R. palustris SC06 successfully survived and multiplied in flooding paddy soil, and significantly increased the relatively abundance of anaerobic bacteria including Desulfobacterota, Anaerolineaceae, Geobacteraceae, and Gemmatimonadaceae by 46.40 %, 45.00 %, 50.12 %, and 21.30 %, respectively. Simultaneously, the structure of microbial community was regulated to maintain relative stability in the rhizosphere soil of rice under Cd stress. In turn, these bacteria communities reduced bioavailable Cd and enhanced residual Cd in soil, and induced the upregulation of sugar and organic acids in the rice roots, which further inhibited Cd uptake in rice seedlings, and dramatically improved the photosynthetic efficiency in the leaves and the activities of antioxidative enzymes in the roots. Finally, Cd content of the roots, stems, leaves, and grains significantly decreased by 38.14 %, 69.10 %, 83.40 %, and 37.24 % comparing with the control, respectively. This study provides a new strategy for the remediation of Cd-contaminated flooding paddy fields and the safe production of rice.

Proteomic insights into the response of Halomonas sp. MNB13 to excess Mn(Ⅱ) and the role of H2S in Mn(Ⅱ) resistance

Halomonas spp. are moderately halophilic bacteria with the ability to tolerate various heavy metals. However, the role of basic cellular metabolism, particularly amino acid metabolism, has not been investigated in Halomonas spp. under excess Mn(Ⅱ). The strain Halomonas sp. MNB13 was isolated from a deep-sea ferromanganese nodule and can tolerate 80 mM Mn(Ⅱ). To comprehensively explore the mechanisms underlying its resistance to excess Mn(Ⅱ), we conducted a comparative proteome analysis. The data revealed that both 10 mM and 50 mM Mn(Ⅱ) significantly up-regulated the expression of proteins involved in Mn(Ⅱ) transport (MntE), oxidative stress response (alkyl hydroperoxide reductase and the Suf system), and amino acid metabolism (arginine, cysteine, methionine, and phenylalanine). We further investigated the role of cysteine metabolism in Mn(Ⅱ) resistance by examining the function of its downstream product, H2S. Consistent with the up-regulation of cysteine desulfurase, we detected an elevated level of H2S in Halomonas sp. MNB13 cells under Mn(Ⅱ) stress, along with increased intracellular levels of H2O2 and O2•-. Upon exogenous addition of H2S, we observed a significant restoration of the growth of Halomonas sp. MNB13. Moreover, we identified decreased intracellular levels of H2O2 and O2•- in MNB13 cells, which coincided with a decreased formation of Mn-oxides during cultivation. In contrast, in cultures containing NaHS, the residual Mn(Ⅱ) levels were higher than in cultures without NaHS. Therefore, H2S improves Mn(Ⅱ) tolerance by eliminating intracellular reactive oxygen species rather than decreasing Mn(Ⅱ) concentration in solution. Our findings indicate that cysteine metabolism, particularly the intermediate H2S, plays a pivotal role in Mn(Ⅱ) resistance by mitigating the damage caused by reactive oxygen species. These findings provide new insights into the amino acid mechanisms associated with Mn(Ⅱ) resistance in bacteria.

Responses of a polycyclic aromatic hydrocarbon-degrading bacterium, Paraburkholderia fungorum JT-M8, to Cd (II) under P-limited oligotrophic conditions

For the bioremediation of mixed-contamination sites, studies on polycyclic aromatic hydrocarbon (PAH) degradation or Cd (II) tolerance in bacteria are commonly implemented in nutrient-rich media. In contrast, in the field, inocula usually encounter harsh oligotrophic habitats. In this study, the environmental strain Paraburkholderia fungorum JT-M8 was used to explore the overlooked Cd (II) defense mechanism during PAH dissipation under P-limited oligotrophic condition. The results showed that the growth and PAH degradation ability of JT-M8 under Cd (II) stress were correlated with phosphate contents and exhibited self-regulating properties. Phosphates mainly affected the Cd (II) content in solution, while the cellular distribution of Cd (II) depended on Cd (II) levels; Cd (II) was mainly located in the cytoplasm when exposed to less Cd (II), and vice versa. The unique Cd (II) detoxification pathways could be classified into three aspects: (i) Cd (II) ionic equilibrium and dose-response effects regulated by environmental matrices (phosphate contents); (ii) bacterial physiological self-regulation, e.g., cell surface-binding, protein secretion and active transport systems; and (iii) specific adaptive responses (flagellum aggregation). This study emphasizes the importance of considering culture conditions when assessing the metal tolerance and provides new insight into the bacterial detoxification process of complex PAH-Cd (II) pollutants.

Heavy metal tolerance, and metal biosorption by exopolysaccharides produced by bacterial strains isolated from marine hydrothermal vents

The present study highlights heavy metal tolerance, EPS production, and biosorption capacity of four hydrothermal vent bacterial strains, namely Exiguobacterium aquaticum, Mammaliicoccus sciuri, Micrococcus luteus, and Jeotgalicoccus huakuii against As, Cd, Cr, Cu, Co, Pb and Ni. The biosorption assay showed high removal efficiency of As (83%) by E. aquaticum, Cd (95%) by M. sciuri, Cu (94%) by M. luteus, and Ni (89%) by J. huakuii and their produced EPS with these metals in aqueous solution were 84%, 85%, 98%, and 91%, respectively. The maximum EPS yield was attained by optimized medium composition consisting of 1% Xylose, and 1% NaCl at pH 7. In metal-amended conditions, the four bacterial strains showed induced EPS production in the initial concentrations. SEM with EDX and CLSM images showed that the growth and EPS production of bacterial strains were affected by metal ion concentrations. A phenol sulphuric acid method and BCA assay were used to identify both the carbohydrate and total protein content of four extracted EPS. A DPPH assay revealed that EPS influences free radical scavenging and has a highly enhanced synergistic effect with its antioxidant activity. FT-IR analysis of four extracted EPS showed the shifting of peaks in the functional groups of EPS before and after adsorption of metal ions. At pH 5 and after 60 min contact time metal removal efficiency and adsorption capacity increased as calculated for As, Cd, Cu, and Ni by four extracted EPS: (86%, 20 mg/g), (74%, 19 mg/g), (94%, 60 mg/g) and (89%, 32 mg/g) and (89%, 16 mg/g), (85%, 16 mg/g), (96%, 22 mg/g) and (91%, 16 mg/g), respectively. The Langmuir compared to the Freundlich model was found to better represent the adsorption by EPS providing maximum adsorption capacities for As (34.65 mg/g), Cd (52.88 mg/g), Cu (24.91 mg/g), and Ni (58.38 mg/g).

Biocomposite based on graphene oxide immobilized Pseudomonas psychrotolerans for the recovery of Y(III) in acid mine drainage

Rare earth elements (REEs) recovery is a critical issue concerning both resource recovery and wastewater utilization. In this study, a new bio-composite was fabricated using graphene oxide immobilized Pseudomonas psychrotolerans (PP@GO), which was isolated from the soil of REEs mine. Results showed that 99.6% Y(III) was removed in 48 h and various characterization confirmed that S-S, -NH2, HPO42-, -OH and -COOH from extracellular polymeric substances (EPS) secreted by microorganisms formed complexation with Y(III). As well, the Y(III) adsorption best followed Freundlich isotherm and non-linear pseudo-second-order kinetic model having R2 of 0.985 and 0.996, respectively, demonstrating that the adsorption was governed by multilayered chemisorption. Additionally, the effectiveness of PP@GO was not limited to Y(III), where 27.9% of this substance was removed in acid mine drainage (AMD), also exhibited great adsorption for other REEs, such as Er (45.0%) and Ho (43.8%). Furthermore, the adsorption efficiency of Y(III) remained high (70.0%) after a 5th cycle, emphasizing the consistent stability of PP@GO. Finally, REEs adsorbed could be greatly desorbed by KNO3, like Sm (80.1%) and La (80.0%), which revealed that PP@GO has great potential to recover REEs in AMD. Overall, this study offers a promising strategy for the green and sustainable REEs recovery and wastewater treatment.

Cadmium-tolerant Bacillus cereus 2-7 alleviates the phytotoxicity of cadmium exposure in banana plantlets

Bananas are the world's important fruit and staple crop in the developing countries. Cadmium (Cd) contamination in soils results in the decrease of crop yield and food safety. Bioremediation is an environmental-friendly and effective measure using Cd-tolerant plant growth promoting rhizobacteria (PGPR). In our study, a Cd-resistant PGPR Bacillus cereus 2-7 was isolated and identified from a discarded gold mine. It could produce multiple plant growth promoting biomolecules such as siderophores, indole-3-acetic acid (IAA), 1-aminocyclopropane-1-carboxylate (ACC)-deaminase and phosphatase. The extracellular accumulation was a main manner of Cd removal. Surplus Cd induced the expression of Cd resistance/transport genes of B. cereus 2-7 to maintain the intracellular Cd homeostasis. The pot experiment showed that Cd contents decreased by 50.31 % in soil, 45.43 % in roots, 56.42 % in stems and 79.69 % in leaves after the strain 2-7 inoculation for 40 d. Bacterial inoculation alleviated the Cd-induced oxidative stress to banana plantlets, supporting by the increase of chlorophyll contents, plant height and total protein contents. The Cd remediation mechanism revealed that B. cereus 2-7 could remodel the rhizosphere bacterial community structure and improve soil enzyme activities to enhance the immobilization of Cd. Our study provides a Cd-bioremediation strategy using Cd-resistant PGPR in tropical and subtropical area.

S(-II) reactivates Cd2+-stressed Shewanella oneidensis via promoting low-molecular-weight thiols synthesis and activating antioxidant defense

Efficient remedies for living organisms including bacteria to counteract cadmium (Cd²⁺) toxicity are still highly needed. Plant toxicity studies have showed that exogenous S(-II) (including hydrogen sulfide and its ionic forms, i.e., H₂S, HS^-, and S^2-) application can effectively alleviate adverse effects of Cd stress, but whether S(-II) could mitigate bacterial Cd toxicity remains unclear. In this study, S(-II) was applied exogenously to Cd-stressed Shewanella oneidensis MR-1 and the results showed that S(-II) can significantly reactivate impaired physiological processes including growth arrest and enzymatic ferric (Fe(III) reduction inhibition. The efficacy of S(-II) treatment is negatively correlated with the concentration and time length of Cd exposure. Energy-dispersive X-ray (EDX) analysis suggested the presence of cadmium sulfide inside cells treated with S(-II). Both compared proteomic analysis and RT-qPCR showed that enzymes associated with sulfate transport, sulfur assimilation, methionine, and glutathione biosynthesis were up-regulated in both mRNA and protein levels after the treatment, indicating S(-II) may induce the biosynthesis of functional low-molecular-weight (LMW) thiols to counteract Cd toxicity. Meanwhile, the antioxidant enzymes were positively modulated by S(-II) and thus the activity of intracellular reactive oxygen species was attenuated. The study demonstrated that exogenous S(-II) can effectively alleviate Cd stress for S. oneidensis likely through inducing intracellular trapping mechanisms and modulating cellular redox status. It suggested that S(-II) may be a highly effective remedy for bacteria such as S. oneidensis under Cd-polluted environments.

Characterization of a cadmium-resistant functional bacteria (Burkholderia sp. SRB-1) and mechanism analysis at physiochemical and genetic level

In this study, the capacity of cadmium (Cd)-resistant plant growth-promoting bacteria (PGPB) Burkholderia sp. SRB-1 (SRB-1) and its mechanisms were explored through morphological characterizations, biochemical response, plant growth-promoting traits, and functional gene expression patterns. The results showed that SRB-1 was an excellent Cd-resistant bacteria (MIC was 420 mg L^-1), and its maximum Cd removal rate reached 72.25%. Biosorption was the main removal method of Cd for SRB-1, preventing intracellular Cd accumulation and maintaining cellular metabolism. Various functional groups on the cell wall were involved in Cd binding, which deposited as CdS and CdCO₃ on the cell surface according to XPS analysis and might be critical for reducing Cd physiochemical toxicity. Furthermore, metals exporting (zntA, czcA, czcB, czcC), detoxification (dsbA, cysM), and antioxidation (katE, katG, SOD1) related genes were annotated in the SRB-1 genome. The results of Cd distribution and antioxidative enzyme activity in SRB-1 also illustrated that Cd²⁺ efflux and antioxidative response were the main intracellular Cd-resistant mechanisms. These conclusions were further verified by qRT-PCR analysis. Overall, the strategies of extracellular biosorption, cation efflux, and intracellular detoxification jointly build the Cd-resistant system, which invested Burkholderia sp. SRB-1 with potential for bioremediation in heavily Cd-contaminated environmental sites.

Ability of Cupriavidus necator H16 to resist, bioremove, and accumulate some hazardous metal ions in water

Bacterial biomasses are suitable and inexpensive biosorbents for the removal of metal ions. The Gram-negative betaproteobacterium Cupriavidus necator H16 is found in soil and freshwater environments. In this study, C. necator H16 was used to remove chromium (Cr), arsenic (As), aluminum (Al), and cadmium (Cd) ions from water. Minimum inhibition concentration (MIC) values of C. necator to Cr, As, Al, and Cd were found as 76, 69, 341, and 275 mg/L, respectively. The highest rates of Cr, As, Al, and Cd bioremoval were 45, 60, 54, and 78%, respectively. pH levels between 6.0 and 8.0 and an average temperature of 30 °C were optimum for the most efficient bioremoval. Scanning electron microscopy (SEM) images of Cd-treated cells showed that the morphology of the cells was significantly impaired compared to the control. Shifts in the Fourier transform infrared spectroscopy analysis (FTIR) spectra of the Cd-treated cell walls also confirmed the presence of active groups. As a result, it can be said that C. necator H16 has a moderate bioremoval efficiency for Cr, As, and Al and a high bioremoval efficiency for Cd.

Insight into the roles of soluble, loosely bound and tightly bound extracellular polymeric substances produced by Enterobacter sp. in the Cd2+ and Pb2+ biosorption process: Characterization and mechanism

Extracellular polymeric substances (EPSs) are macromolecular polymers formed by metabolic secretion, and they have great potential for removing heavy metal (HM) ions from the aquatic phase. In this study, the contributions of soluble EPSs (S-EPSs), loosely bound EPSs (LB-EPSs) and tightly bound EPSs (TB-EPSs) secreted by Enterobacter sp. to Cd²⁺ and Pb²⁺ adsorption were analyzed. The results indicated that in a solution containing both Cd²⁺ and Pb²⁺, pH= 6.0 was best suited for the adsorption process, and adsorption equilibrium was reached in approximately 120 min. Moreover, the mechanism for adsorption of Cd²⁺ and Pb²⁺ by the different layers of EPSs involved spontaneous chemical processes. However, Cd²⁺ adsorption by the three layers of the EPSs was an exothermic process (∆H⁰ <0), but Pb²⁺ adsorption by the three layers of the EPSs was an endothermic process (∆H⁰ >0). The variations in zeta potentials indicated that ion exchange occurred during Cd²⁺ and Pb²⁺ adsorption. FT-IR, XPS and 3D-EEM analyses indicated that the functional groups of the EPSs involved in adsorption were mainly the CO, C-O and C-O-C groups of the polysaccharides; furthermore, fulvic acid-like substances, humic-like substances and tyrosine-like proteins played important roles in the adsorption of Cd²⁺ and Pb²⁺ by the different EPS layers.

Core Species Derived from Multispecies Interactions Facilitate the Immobilization of Cadmium

Microbial consortia have opened new avenues for heavy-metal remediation. However, the limited understanding of the overall effect of interspecific interactions on remediation efficacy hinders its application. Here, the effects of multispecies growth and biofilm formation on Cd immobilization were explored from direct and multiple interactions through random combinations of two or three rhizosphere bacteria. In monocultures, Cd stress resulted in an average decrease in planktonic biomass of 26%, but through cooperation, the decrease was attenuated in dual (21%) and triple cultures (13%), possibly involving an increase in surface polysaccharides. More than 65% of the co-cultures exhibited induction of biofilm formation under Cd stress, which further enhanced the role of biofilms in Cd immobilization. Notably, excellent biofilm-forming ability or extensive social induction makes Pseudomonas putida and Brevundimonas diminuta stand out in multispecies biofilm formation and Cd immobilization. These two core species significantly increase the colonization of soil microorganisms on rice roots compared to the control, resulting in a 40% decrease in Cd uptake by rice. Our study enhances the understanding of bacterial interactions under Cd stress and provides a novel strategy for adjusting beneficial soil consortia for heavy-metal remediation.

Recent advancements in cadmium-microbe interactive relations and their application for environmental remediation: a mechanistic overview

The toxic and persistent nature of cadmium (Cd) in the environment has become a matter of concern with its drastic increase in the concentrations over past few decades. Among the various techniques, the microbial remediation has been accepted as an effective decontamination tool for environmental applications, which is sustainable over a period of time. The Cd decontamination potential of the microbes depends on various internal and external factors that play a crucial role in selection of the microbes for application in a particular environment. Thus, it is important to understand the role of these factors for optimal application of the microbes. This study provides an insight into the mechanisms involved between the microbes and the environmental Cd. The study also briefly reviews the mathematical models that have been used to predict the remediation potential of the microbes and the kinetics involved during the process. A critical analysis of the recent advancements in the techniques for use of bacteria, fungi, and algal cells to remove Cd has been also presented in the manuscript.

Oleic Acid Facilitates Cd Excretion by Increasing the Abundance of Burkholderia in Cd-Exposed Mice

As a global pollutant, cadmium (Cd) can easily enter the body through food chains, threatening human health. Most Cd is initially absorbed in the gut, with the gut microbiota playing a pivotal role in reducing Cd absorption and accumulation. This study assessed the effects of three fatty acids on Cd accumulation and toxicity in Cd-exposed mice. The results showed that oleic acid (OA) was the most effective in facilitating Cd excretion in mice among these fatty acids. The use of OA led to reduced Cd accumulation in the organs and increased Cd content in the feces. The metagenomic analysis of the gut microbiota showed that the genus Burkholderia was the most significantly restored by OA in Cd-exposed mice. Burkholderia cepacia, as the type species for the genus Burkholderia, also exhibited strong Cd tolerance after treatment with OA. Furthermore, the electron microscopy analysis showed that most of the Cd was adsorbed on the surface of B. cepacia, where the extracellular polymeric substances (EPSs) secreted by B. cepacia play a key role, displaying a strong capacity for Cd adsorption. The peak at 2355 cm^-1 and the total sulfhydryl group content of EPSs showed significant increases following co-treatment with Cd and OA. The results demonstrated the potential roles that gut Burkholderia may play in OA-mediated Cd excretion in mice.

Tolerance and Cadmium (Cd) Immobilization by Native Bacteria Isolated in Cocoa Soils with Increased Metal Content

Twelve cadmium native bacteria previously isolated in soils of cocoa farms located in the western Colombian Andes (Santander), and tolerant to 2500 µM CdCl2 (120 mg Cd/L), were chosen in order to test their tolerance and Cd immobilization using liquid culture medium (Nutritive broth) at different concentrations of heavy metals. Furthermore, in the greenhouse experiments, the strains Exiguobacterium sp. (11-4A), Klebsiella variicola sp. (18-4B), and Enterobacter sp. (29-4B) were applied in combined treatments using CCN51 cacao genotype seeds grown in soil with different concentrations of Cd. All bacterial strains’ cell morphologies were deformed in TEM pictures, which also identified six strain interactions with biosorption and four strain capacities for bioaccumulation; FT-IR suggested that the amide, carbonyl, hydroxyl, ethyl, and phosphate groups on the bacteria biomass were the main Cd binding sites. In the pot experiments, the concentration of Cd was distributed throughout the cacao plant, but certain degrees of immobilization of Cd can occur in soil to prevent an increase in this level in roots with the presence of Klebsiella sp.

The dynamic behavior and mechanism of uranium (VI) biomineralization in Enterobacter sp. X57

The important role of microbes in the biomineralization and migration behavior of uranium in the field of environmental chemistry has been well emphasized in previous work. However, limited work on mineralization processes of indigenous microorganism has prevented us from a deeper understanding of the process and mechanisms of uranium biomineralization. In this work, the dynamic process and mechanism of uranium biomineralization in Enterobacter sp. X57, a novel uranium-tolerant microorganism separated from uranium contaminated soil, were systematically investigated. Enterobacter sp. X57 can induce intracellular mineralization of U (VI) to Uramphite (NH₄UO₂PO₄·3H₂O) under neutral conditions by alkaline phosphatase. In this biomineralization process, soluble U (VI) first bonded with the amino and phosphate groups on the plasma membrane, providing initial nucleation site for the formation of U (VI) biominerals. Then the impairment of cell barrier function and the enhancement of alkaline phosphatase metabolism occurred with the accumulation of uranium in cells, creating a possible pathway for soluble U (VI) to diffuse into the cell and be further mineralized into U (VI)-phosphate minerals. All the results revealed that the intracellular biomineralization of uranium by Enterobacter sp. X57 was a combined result of biosorption, intracellular accumulation and phosphatase metabolism. These findings may contribute to a better understanding of uranium biomineralization behavior and mechanism of microorganisms, as well as possible in-situ bioremediation strategies for uranium by indigenous microorganisms.

Effective immobilization of heavy metals via reactive barrier by rhizosphere bacteria and their biofilms

As the portal of plants, rhizosphere microorganisms play an essential role in controlling the species, transformation, and bioavailability of heavy metals, yet the potential passivation mechanism is still unclear. In this study, two heavy metal resistant and growth-promoting rhizosphere bacteria were screened, and their mechanisms in dealing with external stress and immobilizing heavy metal were explored. The results showed that heavy metals inhibited the ability of Pseudomonas sp. H13 and Brevundomonas sp. H16 to promote plant growth, but stimulated the production of extracellular polysaccharides and inorganic labile sulfide, and enhanced biofilm formation, thereby significantly improved the removal efficiency of Cu²⁺, Zn²⁺, Cd²⁺, and Pb²⁺. Compared with H16, the biofilm of H13 disintegrated rapidly in the later stage, so more metal ions were adsorbed on the planktonic cells. The C-OH and PO groups related to polysaccharides play a crucial role in heavy metal adsorption, and the immobilization mechanism of the planktonic cell is mainly ion exchange and group complex, but for H16, intracellular enrichment cannot be ignored. Functional group complexes played a dominant role in biofilm, and the immobilized heavy metals were more difficult to release into the environment. This study highlighted the potential application prospects of biofilm bacteria in heavy metal remediation and explained the reactive barrier of rhizosphere bacteria to heavy metals.

Sb(III) resistance mechanism and oxidation characteristics of Klebsiella aerogenes X

Resistant bacteria are potential natural materials for the bioremediation of soil metalloid pollution. A strain isolated from farmland soil chronically exposed to Sb was identified as K. aerogenes X with high antimonite [Sb(III)] tolerance and oxidation ability. The resistance mechanism of K. aerogenes X and its extracellular polymeric substances (EPS), antioxidant enzymes, and oxidation characteristics in Sb(III) stress were investigated in this study by stress incubation experiments and FTIR. The biotoxicity of Sb was limited by the binding of the organic compounds in EPS, and the anionic functional groups (e.g., amino, carboxyl and hydroxyl groups, etc.) present in the cell envelope were the components primarily responsible for the metalloid-binding capability of K. aerogenes X. The K. aerogenes X can oxidize Sb(III), and its metabolites induce changes in reactive oxygen species (ROS), catalase (CAT), total superoxide dismutase (SOD) and glutathione s-transferase (GSH-S) activity, indicating that the resistance mechanisms of K. aerogenes X are mediated by oxidative stress, EPS restriction and cell damage. Oxidation of Sb(III) is driven by interactions in intracellular oxidation, cell electron transport, extracellular metabolism including proteins and low molecular weight components (LMWs). LMWs (molecular weight <3 kDa) are the main driving factor of Sb(III) oxidation. In addition, Sb resistance genes arsA, arsB, arsC, arsD and acr3 and potential oxidation gene arsH were identified in K. aerogenes X. Owing to its natural origin, high tolerance and oxidation ability, K. aerogenes X could serve as a potential bioremediation material for the mitigation of Sb(III) in contaminated areas.

Bacterial Biosorbents, an Efficient Heavy Metals Green Clean-Up Strategy: Prospects, Challenges, and Opportunities

Rapid industrialization has led to the pollution of soil and water by various types of contaminants. Heavy metals (HMs) are considered the most reactive toxic contaminants, even at low concentrations, which cause health problems through accumulation in the food chain and water. Remediation using conventional methods, including physical and chemical techniques, is a costly treatment process and generates toxic by-products, which may negatively affect the surrounding environment. Therefore, biosorption has attracted significant research interest in the recent decades. In contrast to existing methods, bacterial biomass offers a potential alternative for recovering toxic/persistent HMs from the environment through different mechanisms for metal ion uptake. This review provides an outlook of the advantages and disadvantages of the current bioremediation technologies and describes bacterial groups, especially extremophiles with biosorbent potential for heavy metal removal with relevant examples and perspectives.

Cadmium-tolerant bacteria: current trends and applications in agriculture

Cadmium (Cd) is considered a toxic heavy metal; nevertheless, its toxicity fluctuates for different organisms. Cadmium-tolerant bacteria (CdtB) are diverse and non-phylogenetically related. Because of their ecological importance these bacteria become particularly relevant when pollution occurs and where human health is impacted. The aim of this review is to show the significance, culturable diversity, metabolic detoxification mechanisms of CdtB and their current uses in several bioremediation processes applied to agricultural soils. Further discussion addressed the technological devices and the possible advantages of genetically modified CdtB for diagnostic purposes in the future.

Deciphering Cadmium (Cd) Tolerance in Newly Isolated Bacterial Strain, Ochrobactrum intermedium BB12, and Its Role in Alleviation of Cd Stress in Spinach Plant (Spinacia oleracea L.)

A cadmium (Cd)-tolerant bacterium Ochrobactrum intermedium BB12 was isolated from sewage waste collected from the municipal sewage dumping site of Bhopal, India. The bacterium showed multiple heavy metal tolerance ability and had the highest minimum inhibitory concentration of 150 mg L-1 of Cd. Growth kinetics, biosorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier transform infrared (FTIR) spectroscopy studies on BB12 in the presence of Cd suggested biosorption as primary mode of interaction. SEM and TEM studies revealed surface deposition of Cd. FTIR spectra indicated nitrogen atom in exopolysaccharides secreted by BB12 to be the main site for Cd attachment. The potential of BB12 to alleviate the impact of Cd toxicity in spinach plants (Spinacia oleracea L.) var. F1-MULAYAM grown in the soil containing Cd at 25, 50, and 75 mg kg-1 was evaluated. Without bacterial inoculation, plants showed delayed germination, decrease in the chlorophyll content, and stunted growth at 50 and 75 mg kg-1 Cd content. Bacterial inoculation, however, resulted in the early germination, increased chlorophyll, and increase in shoot (28.33%) and root fresh weight (72.60%) at 50 mg kg-1 of Cd concentration after 75 days of sowing. Due to bacterial inoculation, elevated proline accumulation and lowered down superoxide dismutase (SOD) enzyme activity was observed in the Cd-stressed plants. The isolate BB12 was capable of alleviating Cd from the soil by biosorption as evident from significant reduction in the uptake/translocation and bioaccumulation of Cd in bacteria itself and in the plant parts of treated spinach. Potential PGP prospects and heavy metal bioremediation capability of BB12 can make the environmental application of the organism a promising approach to reduce Cd toxicity in the crops grown in metal-contaminated soils.

Role of RND Efflux Pumps in Drug Resistance of Cystic Fibrosis Pathogens

Drug resistance represents a great concern among people with cystic fibrosis (CF), due to the recurrent and prolonged antibiotic therapy they should often undergo. Among Multi Drug Resistance (MDR) determinants, Resistance-Nodulation-cell Division (RND) efflux pumps have been reported as the main contributors, due to their ability to extrude a wide variety of molecules out of the bacterial cell. In this review, we summarize the principal RND efflux pump families described in CF pathogens, focusing on the main Gram-negative bacterial species (Pseudomonas aeruginosa, Burkholderia cenocepacia, Achromobacter xylosoxidans, Stenotrophomonas maltophilia) for which a predominant role of RND pumps has been associated to MDR phenotypes.

Potential of cadmium resistant Burkholderia contaminans strain ZCC in promoting growth of soy beans in the presence of cadmium

Bioremediation of Cd contaminated environments can be assisted by plant-growth-promoting bacteria (PGPB) enabling plant growth in these sites. Here a gram-negative Burkholderia contaminans ZCC was isolated from mining soil at a copper-gold mine. When exposed to Cd(II), ZCC displayed high Cd resistance and the minimal inhibitory concentration was 7 mM in LB medium. Complete genome analysis uncovered B. contaminans ZCC contained 3 chromosomes and 2 plasmids. One of these plasmids was shown to contain a multitude of heavy metal resistance determinants including genes encoding a putative Cd-translocating PIB-type ATPase and an RND-type related to the Czc-system. These additional heavy metal resistance determinants are likely responsible for the increased resistance to Cd(II) and other heavy metals in comparison to other strains of B. contaminans. B. contaminans ZCC also displayed PGPB traits such as 1-aminocyclopropane-1-carboxylate deaminase activity, siderophore production, organic and inorganic phosphate solubilization and indole acetic acid production. Moreover, the properties and Cd(II) binding characteristics of extracellular polymeric substances was investigated. ZCC was able to induce extracellular polymeric substances production in response to Cd and was shown to be chemically coordinated to Cd(II). It could promote the growth of soybean in the presence of elevated concentrations of Cd(II). This work will help to better understand processes important in bioremediation of Cd-contaminated environment.

Biosorption behavior and mechanism of cadmium from aqueous solutions by Synechocystis sp. PCC6803

Cyanobacteria are promising adsorbents that are widely used for heavy metal removal in aqueous solutions. However, the underlying adsorption mechanism of Synechocystis sp. PCC6803 is currently unclear. In this study, the adsorption behavior and mechanism of cadmium (Cd²⁺) were investigated. Batch biosorption experiments showed that the optimal adsorption conditions were pH 7.0, 30 °C, 15 min, and an initial ion concentration of 4.0 mg L⁻¹. The adsorption process fitted well with the pseudo-second order kinetic model, mainly based on chemisorption. Complexation of Cd²⁺ with carboxyl, hydroxyl, carbonyl, and amido groups was demonstrated by Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectrometry (EDX) analyses confirmed the presence of Cd²⁺ on the cyanobacterial cell surface and intracellularly. Cd²⁺ could lead to reactive oxygen species (ROS) accumulation and photosynthesis inhibition in cyanobacterial cells, and glutathione (GSH) played an important role in alleviating Cd²⁺ toxicity. Analyses of three-dimensional fluorescence spectroscopy (3D-EEM) and high performance anion exchange chromatography-pulsed amperometric detection (HPAEC-PAD) revealed the changes of the composition and content of EPS after Cd²⁺ adsorption, respectively. Real-time quantitative polymerase chain reaction (RT-qPCR) revealed the potential molecular regulatory mechanisms involved in Cd²⁺ biosorption. These results revealed the adsorption mechanism of Cd²⁺ by Synechocystis sp. PCC6803 and provided theoretical guidance for insight into the biosorption mechanisms of heavy metals by other strains.

The results of extracellular polymeric substances (EPS) extraction, physiological and biochemical determination and gene expression revealed the adsorption mechanism of Synechocystis sp. PCC6803 under cadmium stress.

Burkholderia sp. Y4 inhibits cadmium accumulation in rice by increasing essential nutrient uptake and preferentially absorbing cadmium

Microbial remediation of heavy metal-polluted soil is a commonly used method. Burkholderia sp. Y4, isolated from cadmium (Cd)-contaminated rice rhizosphere soil, was investigated for its direct and indirect effects on Cd accumulation in rice by SEM-EDS, FITR and sequencing analysis of the soil bacterial community. Burkholderia sp. Y4 inoculation reduced Cd accumulation in rice roots, rachises, and grains of the two rice varieties T705 and X24 and increased levels of essential elements, especially Fe and Mn, which competitively inhibited Cd transport through cationic channels. Living Burkholderia sp. Y4 cells, rather than non-living ones, could colonize the surface of rice roots and accumulated more Cd through direct biosorption associated with -CO and -NH/-CO bonds of amino acids and proteins. The results of soil microbial community showed that the colonization of externally added Burkholderia sp. Y4 could be maintained over some time to impact the total rhizospheric environment. Burkholderia sp. Y4 inoculation decreased the abundance of microbes involved in the iron cycle (Acidobacteria) as well as of those mediating the transformation of ammonium nitrogen to nitrate nitrogen (Nitrosomonadaceae and Nitrospira). So Burkholderia sp. Y4 inoculation may indirectly change the availability of micronutrients and Cd in rice rhizosphere soil through iron-nitrogen coupled cycles to increase essential nutrient uptake and inhibit Cd accumulation in rice by preferential Cd-biosorption. Therefore, Burkholderia sp. Y4 is potentially suitable for the bioremediation of Cd-contaminated paddy soil.

Efficiency of Pb, Zn, Cd, and Mn Removal from Karst Water by Eichhornia crassipes

This study experimentally investigated heavy metal removal and accumulation in the aquatic plant Eichhornia crassipes. Pb, Zn, Cd, and Mn concentrations, plant morphology, and plant functional groups were analyzed. Eichhornia crassipes achieved high removal efficiency of Pb and Mn from karst water (over 79.5%), with high proportion of Pb, Zn, and Cd absorption occurring in the first eight days. The highest removal efficiencies were obtained at initial Pb, Zn, Cd, and Mn concentrations of 1 mg/L, 2 mg/L, 0.02 mg/L, and 0.2 mg/L, respectively. Eichhornia crassipes exhibited a high bioconcentration factor (Mn = 199,567 > Pb = 19,605 > Cd = 3403 > Zn = 1913) and a low translocation factor (<1). The roots accumulated more Pb, Zn, Cd, and Mn than the stolons and leaves due to the stronger tolerance of roots. The voids, stomas, air chambers, and airways promoted this accumulation. Pb, Cd, Zn, and Mn likely exchanged with Mg, Na, and K through the cation exchange. C≡C, C=O, SO₄²⁻, O-H, C-H, and C-O played different roles during uptake, which led to different removal and accumulation effects.