"NiCo Buster": engineering E. coli for fast and efficient capture of cobalt and nickel

Cadmium biosorption and mechanism investigation using two cadmium-tolerant microorganisms isolated from rhizosphere soil of rice

Microbial remediation of cadmium-contaminated soil offers advantages like environmental friendliness, cost-effectiveness, and simple operation. However, the efficacy of this remediation process relies on obtaining dominant strains and a comprehensive understanding of their Cd adsorption mechanisms. This study identified two Cd-resistant bacteria, Burkholderia sp. 1-22 and Bacillus sp. 6-6, with significant growth-promoting effects from rice rhizosphere soil. The strains showed remarkable Cd resistance up to ∼200 mg/L and alleviated Cd toxicity by regulating pH and facilitating bacterial adsorption of Cd. FTIR analysis showed crucial surface functional groups, like carboxyl and amino groups, on bacteria played significant roles in Cd adsorption. The strains could induce CdCO3 formation via a microbially induced calcium precipitation (MICP) mechanism, confirmed by SEM-EDS, X-ray analysis, and elemental mapping. Pot experiments showed these strains significantly increased organic matter and enzyme activity (e.g., urease, sucrase, peroxidase) in the rhizosphere soil versus the control group. These changes are crucial for restricting Cd mobility. Furthermore, strains 6-6 and 1-22 significantly enhance plant root detoxification of Cd, alleviating toxicity. Notably, increased pH likely plays a vital role in enhancing Cd precipitation and adsorption by strains, converting free Cd into non-bioavailable forms.

Effect of EPS production on the performance of membrane-based biofilm reactors

This study explored the effect of extracellular polymeric substance (EPS) production on the performance of membrane-based biofilm reactors. Changing EPS production was induced by eliminating one of the main EPS polysaccharides, i.e., Pel. The studies were carried out using a pure culture of either Pseudomonas aeruginosa or an isogenic P. aeruginosa mutant that was unable to produce the Pel polysaccharide. The biofilm cell density for both strains was compared to confirm the Pel deletion mutant decreased overall EPS production in a bioreactor system. When the Pel-deficient mutant was grown as a biofilm, its cell density, i.e., ratio of cells/(cells + EPS), was 74 % higher than the wild type, showing EPS production was reduced by eliminating pel production. The growth kinetics were determined for both strains. The Pel-deficient mutant had a maximum specific growth rate (μ^) that was 14% higher than the wild type. Next, the effects of EPS reduction on reactor performance were assessed for a membrane aerated biofilm reactor (MABR) and a membrane bioreactor (MBR). For the MABR, the organic removal with the Pel-deficient mutant was around 8% higher than for the wild type. For the MBR, the time to reach the fouling threshold was 65 % greater for the Pel-deficient mutant than for the wild type. These results suggest that amount of EPS production can have significant effects on bacterial growth kinetics and bacterial cell density, which in turn can affect the performance of the membrane-based biofilm reactors. In both cases, lower EPS production correlated with more efficient treatment processes.

The removal of heavy metal pollution from wastewaters using thermophilic B. cereus SO-16 bacteria

In this study, bioaccumulation, remediation, tolerance, and effects of manganese ions (Mn(II)) and copper ions (Cu(II)) on antioxidant enzymes of thermophilic Bacillus cereus (B. cereus) SO-16 were investigated in detail. The findings of the study showed that Mn(II) was less toxic than Cu(II) to B. cereus SO-16. Moreover, B. cereus SO-16 was exhibited less tolerance to Mn(II) and Cu(II) ions in the liquid medium compared to the solid medium. The growth of bacteria was expressively effective for Mn(II) and Cu(II) concentrations of 2.5 mg/L at 24th h. The highest Mn(II) and Cu(II) bioaccumulation values after 48 h incubation of thermophilic B. cereus SO-16 were measured as 102.04 (24th h) and 87.96 (36th h) metal/dry bacteria weight. The change in morphology and functionality of B. cereus SO-16 after interaction with Mn(II) and Cu(II) was tested using various methods. The results indicated that B. cereus SO-16, a thermophilic bacterium, can be utilized in industrial wastewaters to recover and remediation of toxic metals.

Toward Plant Cyborgs: Hydrogels Incorporated onto Plant Tissues Enable Programmable Shape Control

Engineered living materials (ELMs) that incorporate living organisms and synthetic materials enable advanced functional properties. Here, we seek to create plant cyborgs by combining plants or plant tissues with stimuli-responsive polymeric materials. Plant tissues with integrated shape control may find applications in regenerative medicine, and the shape control of living plants enables another dimension of adaptability and response to environmental threats, which can be applied to next-generation precision farming. In this work, we develop chemistry to integrate stimuli-responsive poly(N-isopropylacrylamide) (PNIPAM) hydrogels with decellularized plant tissues assisted by 3D printing. We demonstrate programmable shape morphing in response to thermal cues and ultraviolet (UV) light. Specifically, by taking advantage of the extrusion-based 3D printing method, we deposit nanocomposite PNIPAM precursors onto silane-treated decellularized leaf surface with prescribed shapes and spatial control. When subjected to external stimuli, the strain mismatch generated between the swellable nanocomposite PNIPAM and nonswellable decellularized leaf enables folding and bending to occur. This strategy to integrate the plant tissues with stimuli-responsive hydrogels allows the control of leaf morphology, opening avenues for plant-based biosensors and soft actuators to enhance food security; such materials also may find applications in biomedicine as tissue-engineering scaffolds.

Preconcentrations of Cu (II) and Mn (II) by magnetic solid-phase extraction on Bacillus cereus loaded γ-Fe2O3 nanomaterials

For the simultaneous preconcentrations of Cu(II) and Mn(II), a novel preconcentration technique was developed and described. Bacillus cereus loaded magnetic ɣ-Fe₂O₃ nanoparticles were prepared and used as support materials on solid-phase extraction procedure. Important experimental parameters were investigated in details and pH 6.0, 3 mL min^-1 of flow rate, 5 mL of 1 mol L^-1 of HCl as eluent, 200 mg of biomass, and 200 mg of magnetic ɣ-Fe₂O₃ nanoparticles as support material was found as the best conditions. The preconcentrations factor were found to be 80 for Cu (II) and Mn(II). It was confirmed by the results that SPE columns could be used in 32 cycles. The LOD values calculated for Cu (II) and Mn (II) were 0.09 and 0.08 ng mL^-1, respectively. The RSD values found were less than 3.4%. The extraction recoveries were achieved as higher than 98%. The biosorption capacities of Cu (II), and Mn (II) were 26.0 mg g^-1, 30.3 mg g^-1 respectively. The approach devised for analyzing analyte concentrations in food samples proved to be successful.

Technologies for the cobalt-contaminated soil remediation: A review

The cobalt-contaminated soil has exposed potential toxicity to humans, plants, and animals. Industrial activities like ore smelting, alloy manufacture, and electric and electronic devices production have induced the increased cobalt content in soil resulting in higher ecosystem risk in diverse environmental media. However, knowledge gaps in cobalt transfer in soil and the limited understanding of remediation techniques make it challenging to estimate their potential application scenarios. Thus, keeping in view the above facts, this paper summarizes the natural and anthropogenic sources arousing the increase of cobalt in soil and reviews the cobalt species in soil and factors that influence the mobilization of cobalt. Moreover, the status of the remediation technologies is critically evaluated, including phytoremediation, immobilization, and separation technologies (soil washing and electroremediation) with a focus on the application and mechanism of phytoremediation and immobilization. Based on the actual application, further improvements and prospects of all techniques are proposed. This comprehensive review might serve to guide technique selection and inspire more scientific exploration on the remediation of cobalt-contaminated soil.

A review on biofiltration techniques: Recent advancements in the removal of volatile organic compounds and heavy metals in the treatment of polluted water

Good quality of water determines the healthy life of living beings on this earth. The cleanliness of water was interrupted by the pollutants emerging out of several human activities. Industrialization, urbanization, heavy population, and improper disposal of wastes are found to be the major reasons for the contamination of water. Globally, the inclusion of volatile organic compounds (VOCs) and heavy metals released by manufacturing industries, pharmaceuticals, and petrochemical processes have created environmental issues. The toxic nature of these pollutants has led researchers, scientists, and industries to exhibit concern towards the complete eradication of them. In this scenario, the development of wastewater treatment methodologies at low cost and in an eco-friendly way had gained importance at the international level. Recently, bio-based technologies were considered for environmental remedies. Biofiltration based works have shown a significant result for the removal of volatile organic compounds and heavy metals in the treatment of wastewater. This was done with several biological sources such as bacteria, fungi, algae, plants, yeasts, etc. The biofiltration technique is cost-effective, simple, biocompatible, sustainable, and eco-friendly compared to conventional techniques. This review article provides deep insight into biofiltration technologies engaged in the removal of volatile organic compounds and heavy metals in the wastewater treatment process.

Bioelectrochemical systems-based metal recovery: Resource, conservation and recycling of metallic industrial effluents

Metals represent a large proportion of industrial effluents, which due to their high hazardous nature and toxicity are responsible to create environmental pollution that can pose significant threat to the global flora and fauna. Strict ecological rules compromise sustainable recovery of metals from industrial effluents by replacing unsustainable and energy-consuming physical and chemical techniques. Innovative technologies based on the bioelectrochemical systems (BES) are a rapidly developing research field with proven encouraging outcomes for many industrial commodities, considering the worthy options for recovering metals from industrial effluents. BES technology platform has redox capabilities with small energy-intensive processes. The positive stigma of BES in metals recovery is addressed in this review by demonstrating the significance of BES over the current physical and chemical techniques. The mechanisms of action of BES towards metal recovery have been postulated with the schematic representation. Operational limitations in BES-based metal recovery such as biocathode and metal toxicity are deeply discussed based on the available literature results. Eventually, a progressive inspection towards a BES-based metal recovery platform with possibilities of integration with other modern technologies is foreseen to meet the real-time challenges of viable industrial commercialization.

Biologically engineered microbes for bioremediation of electronic waste: Wayposts, challenges and future directions

In the face of a burgeoning stream of e-waste globally, e-waste recycling becomes increasingly imperative, not only to mitigate the environmental and health risks it poses but also as an urban mining strategy for resource recovery of precious metals, rare Earth elements, and even plastics. As part of the continual efforts to develop greener alternatives to conventional approaches of e-waste recycling, biologically assisted degradation of e-waste offers a promising recourse by capitalising on certain microorganisms' innate ability to interact with metals or degrade plastics. By harnessing emerging genetic tools in synthetic biology, the evolution of novel or enhanced capabilities needed to advance bioremediation and resource recovery could be potentially accelerated by improving enzyme catalytic abilities, modifying substrate specificities, and increasing toxicity tolerance. Yet, the management of e-waste presents formidable challenges due to its massive volume, high component complexity, and associated toxicity. Several limitations will need to be addressed before nascent laboratory-scale achievements in bioremediation can be translated to viable industrial applications. Nonetheless, vested groups, involving both start-up and established companies, have taken visionary steps towards deploying microbes for commercial implementation in e-waste recycling.

Nickel bioaccumulation by a marine bacterium Brevibacterium sp. (X6) isolated from Shenzhen Bay, China

Nickel bioaccumulation capacity of a marine Brevibacterium sp., designated as X6, was evaluated to explore its potential application in the bioremediation of Ni²⁺ pollutants in marine environments. The minimum Ni²⁺ inhibitory concentration and maximum Ni²⁺ bioaccumulation of X6 were 1000 mg/L and 100.95 mg/g, respectively, higher than most reported strains. Among the co-existing metal ions in seawater, K⁺ caused a slight adverse impact on Ni²⁺ uptake, followed by Na⁺ and Ca²⁺, whereas Mg²⁺ drastically inhibited Ni²⁺ bioaccumulation. Other heavy metals such as Co²⁺, Zn²⁺ and Cd²⁺ moderately affected Ni²⁺ binding, but the adverse effect of Cu²⁺ was severe. The investigation of the mechanism of Ni²⁺ bioaccumulation revealed that 66.34% of the accumulated Ni²⁺ was bound to the cell surface. Carboxylic, hydroxyl, amino and thiol groups participated in Ni²⁺ binding, while carboxylic group contributed the most, while thiol group may be more involved in Ni²⁺ binding at low Ni²⁺ concentrations.

Transporter engineering for the development of cyanobacteria as cell factories: A text analytics guided survey

Cyanobacteria are attractive candidates for photoautotrophic production of platform chemicals due to their inherent ability to utilize carbon dioxide as the sole carbon source. Metabolic pathways can be engineered more readily in cyanobacteria compared to higher photosynthetic organisms. Although significant progress has been made in pathway engineering, intracellular accumulation of the product is a potential bottleneck in large-scale production. Likewise, substrate uptake is known to limit growth and product formation. These limitations can potentially be addressed by targeted and controlled expression of transporter proteins in the metabolically engineered strains. This review focuses on the transporters that have been explored in cyanobacteria. To highlight the progress on characterization and application of cyanobacterial transporters, we applied text analytics to extract relevant information from over 1000 publications. We have categorized the transporters based on their source, their function and the solute they transport. Further, the review provides insights into the potential of transporters in the metabolic engineering of cyanobacteria for improved product titer.

Using a Riboswitch Sensor to Detect Co2+/Ni2+ Transport in E. coli

Intracellular concentrations of essential mental ions must be tightly maintained to avoid metal deprivation and toxicity. However, their levels in cells are still difficult to monitor. In this report, the combination of a Co2+Ni2+-specific riboswitch and an engineered downstream mCherry fluorescent protein allowed a highly sensitive and selective whole-cell Co2+/Ni2+ detection process. The sensors were applied to examine the resistance system of Co2+/Ni2+ in E. coli, and the sensors were able to monitor the effects of genetic deletions. These results indicate that riboswitch-based sensors can be employed in the study of related cellular processes.

Synthetic biology approaches towards the recycling of metals from the environment

Metals are a finite resource and their demand for use within existing and new technologies means metal scarcity is increasingly a global challenge. Conversely, there are areas containing such high levels of metal pollution that they are hazardous to life, and there is loss of material at every stage of the lifecycle of metals and their products. While traditional resource extraction methods are becoming less cost effective, due to a lowering quality of ore, industrial practices have begun turning to newer technologies to tap into metal resources currently locked up in contaminated land or lost in the extraction and manufacturing processes. One such technology uses biology for the remediation of metals, simultaneously extracting resources, decontaminating land, and reducing waste. Using biology for the identification and recovery of metals is considered a much 'greener' alternative to that of chemical methods, and this approach is about to undergo a renaissance thanks to synthetic biology. Synthetic biology couples molecular genetics with traditional engineering principles, incorporating a modular and standardised practice into the assembly of genetic parts. This has allowed the use of non-model organisms in place of the normal laboratory strains, as well as the adaption of environmentally sourced genetic material to standardised parts and practices. While synthetic biology is revolutionising the genetic capability of standard model organisms, there has been limited incursion into current practices for the biological recovery of metals from environmental sources. This mini-review will focus on some of the areas that have potential roles to play in these processes.

Efficient Overproduction of Active Nitrile Hydratase by Coupling Expression Induction and Enzyme Maturation via Programming a Controllable Cobalt-Responsive Gene Circuit

A robust and portable expression system is of great importance in enzyme production, metabolic engineering, and synthetic biology, which maximizes the performance of the engineered system. In this study, a tailor-made cobalt-induced expression system (CIES) was developed for low-cost and eco-friendly nitrile hydratase (NHase) production. First, the strong promoter P_veg from Bacillus subtilis, the Ni(II)/Co(II) responsive repressor RcnR, and its operator were reorganized to construct a CIES. In this system, the expression of reporter green fluorescent protein (GFP) was specifically triggered by Co(II) over a broad range of concentration. The performance of the cobalt-induced system was evolved to version 2.0 (CIES 2.0) for adaptation to different concentrations of Co(II) through programming a homeostasis system that rebalances cobalt efflux and influx with RcnA and NiCoT, respectively. Harnessing these synthetic platforms, the induced expression of NHase was coupled with enzyme maturation by Co(II) in a synchronizable manner without requiring additional inducers, which is a unique feature relative to other induced systems for production of NHase. The yield of NHase was 111.2 ± 17.9 U/ml using CIES and 114.9 ± 1.4 U/ml using CIES 2.0, which has a producing capability equivalent to that of commonly used isopropyl thiogalactoside (IPTG)-induced systems. In a scale-up system using a 5-L fermenter, the yielded enzymatic activity reached 542.2 ± 42.8 U/ml, suggesting that the designer platform for NHase is readily applied to the industry. The design of CIES in this study not only provided a low-cost and eco-friendly platform to overproduce NHase but also proposed a promising pipeline for development of synthetic platforms for expression of metalloenzymes.

Heavy Metal Removal by Bioaccumulation Using Genetically Engineered Microorganisms

Wastewater effluents from mines and metal refineries are often contaminated with heavy metal ions, so they pose hazards to human and environmental health. Conventional technologies to remove heavy metal ions are well-established, but the most popular methods have drawbacks: chemical precipitation generates sludge waste, and activated carbon and ion exchange resins are made from unsustainable non-renewable resources. Using microbial biomass as the platform for heavy metal ion removal is an alternative method. Specifically, bioaccumulation is a natural biological phenomenon where microorganisms use proteins to uptake and sequester metal ions in the intracellular space to utilize in cellular processes (e.g., enzyme catalysis, signaling, stabilizing charges on biomolecules). Recombinant expression of these import-storage systems in genetically engineered microorganisms allows for enhanced uptake and sequestration of heavy metal ions. This has been studied for over two decades for bioremediative applications, but successful translation to industrial-scale processes is virtually non-existent. Meanwhile, demands for metal resources are increasing while discovery rates to supply primary grade ores are not. This review re-thinks how bioaccumulation can be used and proposes that it can be developed for bioextractive applications-the removal and recovery of heavy metal ions for downstream purification and refining, rather than disposal. This review consolidates previously tested import-storage systems into a biochemical framework and highlights efforts to overcome obstacles that limit industrial feasibility, thereby identifying gaps in knowledge and potential avenues of research in bioaccumulation.

Engineered Living Materials: Prospects and Challenges for Using Biological Systems to Direct the Assembly of Smart Materials

Vast potential exists for the development of novel, engineered platforms that manipulate biology for the production of programmed advanced materials. Such systems would possess the autonomous, adaptive, and self-healing characteristics of living organisms, but would be engineered with the goal of assembling bulk materials with designer physicochemical or mechanical properties, across multiple length scales. Early efforts toward such engineered living materials (ELMs) are reviewed here, with an emphasis on engineered bacterial systems, living composite materials which integrate inorganic components, successful examples of large-scale implementation, and production methods. In addition, a conceptual exploration of the fundamental criteria of ELM technology and its future challenges is presented. Cradled within the rich intersection of synthetic biology and self-assembling materials, the development of ELM technologies allows the power of biology to be leveraged to grow complex structures and objects using a palette of bio-nanomaterials.

Manganese and cobalt recovery by surface display of metal binding peptide on various loops of OmpC in Escherichia coli

In a cell-surface display (CSD) system, successful display of a protein or peptide is highly dependent on the anchoring motif and the position of the display in that anchoring motif. In this study, a recombinant bacterial CSD system for manganese (Mn) and cobalt (Co) recovery was developed by employing OmpC as an anchoring motif on three different external loops. A portion of Cap43 protein (TRSRSHTSEG)3 was employed as a manganese and cobalt binding peptide (MCBP), which was fused with OmpC at three different external loops. The fusions were made at the loop 2 [fusion protein-2 (FP2)], loop 6 (FP6), and loop 8 (FP8) of OmpC, respectively. The efficacy of the three recombinant strains in the recovery of Mn and Co was evaluated by varying the concentration of the respective metal. Molecular modeling studies showed that the short trimeric repeats of peptide probably form a secondary structure with OmpC, thereby giving rise to a difference in metal recovery among the three recombinant strains. Among the three recombinant strains, FP6 showed increased metal recovery with both Mn and Co, at 1235.14 (1 mM) and 379.68 (0.2 mM) µmol/g dry cell weight (DCW), respectively.

Pushing the limits of nickel detection to nanomolar range using a set of engineered bioluminescent Escherichia coli

The detection of nickel in water is of great importance due to its harmfulness for living organism. A way to detect Ni is the use of whole-cell biosensors. The aim of the present work was to build a light-emitting bacterial biosensor for the detection of Ni with high specificity and low detection limit properties. For that purpose, the regulatory circuit implemented relied on the RcnR Ni/Co metallo-regulator and its rcnA natural target promoter fused to the lux reporter genes. To convert RcnR to specifically detect Ni, several mutations were tested and the C35A retained. Deleting the Ni efflux pump rcnA and introducing genes encoding several Ni-uptake systems lowered the detection thresholds. When these constructs were assayed in several Escherichia coli strains, it appeared that the detection thresholds were highly variable. The TD2158 wild-type E. coli gave rise to a biosensor ten times more active and sensitive than its W3110 E. coli K12 equivalent. This biosensor was able to confidently detect Ni concentrations as little as 80 nM (4.7 μg l^-1), which makes its use compatible with the norms governing the drinking water quality.

Mapping the membrane proteome of anaerobic gut fungi identifies a wealth of carbohydrate binding proteins and transporters

BACKGROUND

Engineered cell factories that convert biomass into value-added compounds are emerging as a timely alternative to petroleum-based industries. Although often overlooked, integral membrane proteins such as solute transporters are pivotal for engineering efficient microbial chassis. Anaerobic gut fungi, adapted to degrade raw plant biomass in the intestines of herbivores, are a potential source of valuable transporters for biotechnology, yet very little is known about the membrane constituents of these non-conventional organisms. Here, we mined the transcriptome of three recently isolated strains of anaerobic fungi to identify membrane proteins responsible for sensing and transporting biomass hydrolysates within a competitive and rather extreme environment.

RESULTS

Using sequence analyses and homology, we identified membrane protein-coding sequences from assembled transcriptomes from three strains of anaerobic gut fungi: Neocallimastix californiae, Anaeromyces robustus, and Piromyces finnis. We identified nearly 2000 transporter components: about half of these are involved in the general secretory pathway and intracellular sorting of proteins; the rest are predicted to be small-solute transporters. Unexpectedly, we found a number of putative sugar binding proteins that are associated with prokaryotic uptake systems; and approximately 100 class C G-protein coupled receptors (GPCRs) with non-canonical putative sugar binding domains.

CONCLUSIONS

We report the first comprehensive characterization of the membrane protein machinery of biotechnologically relevant anaerobic gut fungi. Apart from identifying conserved machinery for protein sorting and secretion, we identify a large number of putative solute transporters that are of interest for biotechnological applications. Notably, our data suggests that the fungi display a plethora of carbohydrate binding domains at their surface, perhaps as a means to sense and sequester some of the sugars that their biomass degrading, extracellular enzymes produce.

Temporal control of Dickeya dadantii main virulence gene expression by growth phase-dependent alteration of regulatory nucleoprotein complexes

In bacteria, important genes are often controlled at the transcriptional level by several factors, forming a complex and intertwined web of interactions. Yet, transcriptional regulators are often studied separately and little information is available concerning their interactions. In this work, we dissect the regulation of the major virulence gene pelD in D. dadantii by taking into account the effects of individual binding sites for regulatory proteins FIS and CRP, and the impact of a newly discovered divergent promoter div. Using a combination of biochemistry and genetics approaches we provide an unprecedented level of detail on the multifactorial regulation of bacterial transcription. We show that the growth phase dependent regulation of pelD is under the control of changing composition of higher-order nucleoprotein complexes between FIS, CRP, div and pelD during the growth cycle that allow sequential expression of div and pelD in the early and late exponential growth phases, respectively. This work highlights the importance of "orphan" promoters in gene regulation and that the individual binding sites for a regulator can serve several purposes and have different effects on transcription, adding a new level of complexity to bacterial transcriptional regulation.

Genetic basis and importance of metal resistant genes in bacteria for bioremediation of contaminated environments with toxic metal pollutants

Metal pollution is one of the most persistent and complex environmental issues, causing threat to the ecosystem and human health. On exposure to several toxic metals such as arsenic, cadmium, chromium, copper, lead, and mercury, several bacteria has evolved with many metal-resistant genes as a means of their adaptation. These genes can be further exploited for bioremediation of the metal-contaminated environments. Many operon-clustered metal-resistant genes such as cadB, chrA, copAB, pbrA, merA, and NiCoT have been reported in bacterial systems for cadmium, chromium, copper, lead, mercury, and nickel resistance and detoxification, respectively. The field of environmental bioremediation has been ameliorated by exploiting diverse bacterial detoxification genes. Genetic engineering integrated with bioremediation assists in manipulation of bacterial genome which can enhance toxic metal detoxification that is not usually performed by normal bacteria. These techniques include genetic engineering with single genes or operons, pathway construction, and alternations of the sequences of existing genes. However, numerous facets of bacterial novel metal-resistant genes are yet to be explored for application in microbial bioremediation practices. This review describes the role of bacteria and their adaptive mechanisms for toxic metal detoxification and restoration of contaminated sites.

IscR Regulates Synthesis of Colonization Factor Antigen I Fimbriae in Response to Iron Starvation in Enterotoxigenic Escherichia coli

Iron availability functions as an environmental cue for enteropathogenic bacteria, signaling arrival within the human host. As enterotoxigenic Escherichia coli (ETEC) is a major cause of human diarrhea, the effect of iron on ETEC virulence factors was evaluated here. ETEC pathogenicity is directly linked to production of fimbrial colonization factors and secretion of heat-labile enterotoxin (LT) and/or heat-stable enterotoxin (ST). Efficient colonization of the small intestine further requires at least the flagellin binding adhesin EtpA. Under iron starvation, production of the CFA/I fimbriae was increased in the ETEC H10407 prototype strain. In contrast, LT secretion was inhibited. Furthermore, under iron starvation, gene expression of the cfa (CFA/I) and etp (EtpBAC) operons was induced, whereas transcription of toxin genes was either unchanged or repressed. Transcriptional reporter fusion experiments focusing on the cfa operon further showed that iron starvation stimulated cfaA promoter activity in ETEC, indicating that the impact of iron on CFA/I production was mediated by transcriptional regulation. Evaluation of cfaA promoter activity in heterologous E. coli single mutant knockout strains identified IscR as the regulator responsible for inducing cfa fimbrial gene expression in response to iron starvation, and this was confirmed in an ETEC ΔiscR strain. The global iron response regulator, Fur, was not implicated. IscR binding sites were identified in silico within the cfaA promoter and fixation confirmed by DNase I footprinting, indicating that IscR directly binds the promoter region to induce CFA/I.

IMPORTANCE

Pathogenic enterobacteria modulate expression of virulence genes in response to iron availability. Although the Fur transcription factor represents the global regulator of iron homeostasis in Escherichia coli, we show that several ETEC virulence factors are modulated by iron, with expression of the major fimbriae under the control of the iron-sulfur cluster regulator, IscR. Furthermore, we demonstrate that the apo form of IscR, lacking an Fe-S cluster, is able to directly fix the corresponding promoter region. These results provide further evidence implicating IscR in bacterial virulence and suggest that IscR may represent a more general regulator mediating the iron response in enteropathogens.