The copper supply pathway to aSalmonellaCu,Zn-superoxide dismutase (SodCII) involves P1B-type ATPase copper efflux and periplasmic CueP

Inflammatory immunity and bacteriological perspectives: A new direction for copper treatment of sepsis

Copper is an essential trace element for all aerobic organisms because of its unique biological functions. In recent years, researchers have discovered that copper can induce cell death through various regulatory mechanisms, thereby inducing inflammation. Efforts have also been made to alter the chemical structure of copper to achieve either anticancer or anti-inflammatory effects. The copper ion can exhibit bactericidal effects by interfering with the integrity of the cell membrane and promoting oxidative stress. Sepsis is a systemic inflammatory response caused by infection. Some studies have revealed that copper is involved in the pathophysiological process of sepsis and is closely related to its prognosis. During the infection of sepsis, the body may enhance the antimicrobial effect by increasing the release of copper. However, to avoid copper poisoning, all organisms have evolved copper resistance genes. Therefore, further analysis of the complex relationship between copper and bacteria may provide new ideas and research directions for the treatment of sepsis.

A gold speciation that adds a second layer to synergistic gold-copper toxicity in Cupriavidus metallidurans

The metal-resistant bacterium Cupriavidus metallidurans occurs in metal-rich environments. In auriferous soils, the bacterium is challenged by a mixture of copper ions and gold complexes, which exert synergistic toxicity. The previously used, self-made Au(III) solution caused a synergistic toxicity of copper and gold that was based on the inhibition of the CupA-mediated efflux of cytoplasmic Cu(I) by Au(I) in this cellular compartment. In this publication, the response of the bacterium to gold and copper was investigated by using a commercially available Au(III) solution instead of the self-made solution. The new solution was five times more toxic than the previously used one. Increased toxicity was accompanied by greater accumulation of gold atoms by the cells. The contribution of copper resistance determinants to the commercially available Au(III) solution and synergistic gold-copper toxicity was studied using single- and multiple-deletion mutants. The commercially available Au(III) solution inhibited periplasmic Cu(I) homeostasis, which is required for the allocation of copper ions to copper-dependent proteins in this compartment. The presence of the gene for the periplasmic Cu(I) and Au(I) oxidase, CopA, decreased the cellular copper and gold content. Transcriptional reporter gene fusions showed that up-regulation of gig, encoding a minor contributor to copper resistance, was strictly glutathione dependent. Glutathione was also required to resist synergistic gold-copper toxicity. The new data indicated a second layer of synergistic copper-gold toxicity caused by the commercial Au(III) solution, inhibition of the periplasmic copper homeostasis in addition to the cytoplasmic one.IMPORTANCEWhen living in auriferous soils, Cupriavidus metallidurans is not only confronted with synergistic toxicity of copper ions and gold complexes but also by different gold species. A previously used gold solution made by using aqua regia resulted in the formation of periplasmic gold nanoparticles, and the cells were protected against gold toxicity by the periplasmic Cu(I) and Au(I) oxidase CopA. To understand the role of different gold species in the environment, another Au(III) solution was commercially acquired. This compound was more toxic due to a higher accumulation of gold atoms by the cells and inhibition of periplasmic Cu(I) homeostasis. Thus, the geo-biochemical conditions might influence Au(III) speciation. The resulting Au(III) species may subsequently interact in different ways with C. metallidurans and its copper homeostasis system in the cytoplasm and periplasm. This study reveals that the geochemical conditions may decide whether bacteria are able to form gold nanoparticles or not.

Scs system links copper and redox homeostasis in bacterial pathogens

The bacterial envelope is an essential compartment involved in metabolism and metabolites transport, virulence, and stress defense. Its roles become more evident when homeostasis is challenged during host-pathogen interactions. In particular, the presence of free radical groups and excess copper in the periplasm causes noxious reactions, such as sulfhydryl group oxidation leading to enzymatic inactivation and protein denaturation. In response to this, canonical and accessory oxidoreductase systems are induced, performing quality control of thiol groups, and therefore contributing to restoring homeostasis and preserving survival under these conditions. Here, we examine recent advances in the characterization of the Dsb-like, Salmonella-specific Scs system. This system includes the ScsC/ScsB pair of Cu+-binding proteins with thiol-oxidoreductase activity, an alternative ScsB-partner, the membrane-linked ScsD, and a likely associated protein, ScsA, with a role in peroxide resistance. We discuss the acquisition of the scsABCD locus and its integration into a global regulatory pathway directing envelope response to Cu stress during the evolution of pathogens that also harbor the canonical Dsb systems. The evidence suggests that the canonical Dsb systems cannot satisfy the extra demands that the host-pathogen interface imposes to preserve functional thiol groups. This resulted in the acquisition of the Scs system by Salmonella. We propose that the ScsABCD complex evolved to connect Cu and redox stress responses in this pathogen as well as in other bacterial pathogens.

The interplay between copper metabolism and microbes: in perspective of host copper-dependent ATPases ATP7A/B

Copper, a vital element in various physiological processes, is transported from the gastrointestinal tract to tissues and cells through diverse copper transporters. Among these transporters, ATP7A and ATP7B play significant roles in regulating systemic copper metabolism and exhibit precise regulation in their intracellular trafficking. These transporters undergo dynamic shuttling between the trans-Golgi network (TGN) and the plasma membrane via the endocytic recycling mechanism, which involves the retromer and other associated factors. Interestingly, the antimicrobial attribute of copper implies a potential connection between microbial infection and copper metabolism. Several microbes, including Salmonella enterica, Cryptococcus, Influenza A virus (IAV) and Zika virus (ZIKV) have been observed to impact the regulatory mechanisms of ATP7A/B, either directly or indirectly, as a means of survival. This review summarizes the key features and trafficking mechanisms of the copper transporters ATP7A/B, and examines the intricate interplay between microbes and copper metabolism. Ultimately, it highlights how microbes can perturb copper homeostasis through interactions with host factors, offering valuable insights into the mechanistic aspects of host-microbe interactions.

Metal manipulators and regulators in human pathogens: A comprehensive review on microbial redox copper metalloenzymes "multicopper oxidases and superoxide dismutases"

The chemistry of metal ions with human pathogens is essential for their survival, energy generation, redox signaling, and niche dominance. To regulate and manipulate the metal ions, various enzymes and metal chelators are present in pathogenic bacteria. Metalloenzymes incorporate transition metal such as iron, zinc, cobalt, and copper in their reaction centers to perform essential metabolic functions; however, iron and copper have gained more importance. Multicopper oxidases have the ability to perform redox reaction on phenolic substrates with the help of copper ions. They have been reported from Enterobacteriaceae, namely Salmonella enterica, Escherichia coli, and Yersinia enterocolitica, but their role in virulence is still poorly understood. Similarly, superoxide dismutases participate in reducing oxidative stress and allow the survival of pathogens. Their role in virulence and survival is well established in Salmonella typhimurium and Mycobacterium tuberculosis. Further, to ensure survival against stress, like metal starvation or metal toxicity, redox metalloenzymes and metal transportation systems of pathogens actively participate in metal homeostasis. Recently, the omics and protein structure biology studies have helped to predict new targets for regulation the colonization potential of the pathogenic strains. The current review is focused on the major roles of redox metalloenzymes, especially MCOs and SODs of human pathogenic bacteria.

RNA-Seq Analysis Discovers the Critical Role of Rel in ppGpp Synthesis, Pathogenicity, and the VBNC State of Clavibacter michiganensis

The viable but nonculturable (VBNC) state is a unique survival strategy of bacteria in response to stress conditions. It was confirmed that Clavibacter michiganensis, the causal agent of bacterial canker in tomato, could be induced into the VBNC state by exposure to CuSO₄ in an oligotrophic solution. RNA-sequencing analysis was used to monitor the mechanisms of the VBNC state during CuSO₄ induction in C. michiganensis. The results identified that numerous genes involved in stringent response, copper resistance, and stress resistance were upregulated, and some involved in cell division were downregulated significantly. The study investigated the importance of Rel, which is an essential enzyme in the synthesis of the molecular alarmone ppGpp, via the generation of a Δrel mutant and its complementation strain. Biological characterization revealed that deficiency of rel reduced the bacterial growth, production of exopolysaccharides, and pathogenicity as well as ppGpp production. The Δrel mutant increased the sensitivity to environmental stress, exhibiting reduced growth on minimal media and a propensity to enter the VBNC state in response to CuSO₄. These findings have important implications for the understanding of survival mechanism and management of C. michiganensis and other phytopathogenic bacteria.

Copper microenvironments in the human body define patterns of copper adaptation in pathogenic bacteria

Copper is an essential micronutrient for most organisms that is required as a cofactor for crucial copper-dependent enzymes encoded by both prokaryotes and eukaryotes. Evidence accumulated over several decades has shown that copper plays important roles in the function of the mammalian immune system. Copper accumulates at sites of infection, including the gastrointestinal and respiratory tracts and in blood and urine, and its antibacterial toxicity is directly leveraged by phagocytic cells to kill pathogens. Copper-deficient animals are more susceptible to infection, whereas those fed copper-rich diets are more resistant. As a result, copper resistance genes are important virulence factors for bacterial pathogens, enabling them to detoxify the copper insult while maintaining copper supply to their essential cuproenzymes. Here, we describe the accumulated evidence for the varied roles of copper in the mammalian response to infections, demonstrating that this metal has numerous direct and indirect effects on immune function. We further illustrate the multifaceted response of pathogenic bacteria to the elevated copper concentrations that they experience when invading the host, describing both conserved and species-specific adaptations to copper toxicity. Together, these observations demonstrate the roles of copper at the host–pathogen interface and illustrate why bacterial copper detoxification systems can be viable targets for the future development of novel antibiotic drug development programs.

Copper is required by both animals and bacteria in small quantities as a micronutrient. During infection, the mammalian immune system increases the local concentration of copper, which gives rise to copper toxicity in the pathogen. In turn, bacterial pathogens possess specialized systems to resist this copper toxicity. Copper also plays important, indirect roles in the function of the immune system. In this review, we explain the diverse roles of copper in the human body with a focus on its functions within the immune system. We also describe how bacterial pathogens respond to the copper toxicity that they experience within the host during infection, illustrating both conserved copper homeostasis and detoxification systems in bacteria and species-specific adaptations that have been shown to be important to pathogenicity. The key role of copper at the host–pathogen interface and the essential requirement for pathogenic bacteria to resist copper toxicity makes the protein components that confer resistance on pathogens potential targets for future development of novel antibiotic drugs.

Unique underlying principles shaping copper homeostasis networks

Abstract

Copper is essential in cells as a cofactor for key redox enzymes. Bacteria have acquired molecular components that sense, uptake, distribute, and expel copper ensuring that cuproenzymes are metallated and steady-state metal levels are maintained. Toward preventing deleterious reactions, proteins bind copper ions with high affinities and transfer the metal via ligand exchange, warranting that copper ions are always complexed. Consequently, the directional copper distribution within cell compartments and across cell membranes requires specific dynamic interactions and metal exchange between cognate holo-apo protein partners. These metal exchange reactions are determined by thermodynamic and kinetics parameters and influenced by mass action. Then, copper distribution can be conceptualized as a molecular system of singular interacting elements that maintain a physiological copper homeostasis. This review focuses on the impact of copper high-affinity binding and exchange reactions on the homeostatic mechanisms, the conceptual models to describe the cell as a homeostatic system, the various molecule functions that contribute to copper homeostasis, and the alternative system architectures responsible for copper homeostasis in model bacteria.

Graphical Abstract

Evolution of Copper Homeostasis and Virulence in Salmonella

Salmonella enterica sv. Typhimurium modulates the expression of factors essential for virulence, contributing to its survival against the surge of copper (Cu) in the Salmonella-containing vacuole. This bactericidal host innate immune component primarily targets the bacterial envelope, where most cuproproteins are localized. While in most enteric species periplasmic Cu homeostasis is maintained by the CusR/CusS-controlled CusCFBA efflux system encoded in the cus locus, we noticed that these genes were lost from the Salmonella-core genome. At the same time, Salmonella acquired cueP, coding for a periplasmic Cu chaperone. As cus, cueP was shown to be essential for bacterial survival in a copper-rich environment under anaerobiosis, suggesting that it can functionally substitute the CusCFBA system. In the present study, the whole Escherichia coli cus locus was reintroduced to the chromosome of the Salmonella wild-type or the ΔcueP strain. While the integrated cus locus did not affect Cu resistance under aerobic conditions, it increases Cu tolerance under anaerobiosis, irrespective of the presence or absence of cueP. In contrast to the Cus system, CueP expression is higher at high copper concentrations and persisted over time, suggesting separate functions. Finally, we observed that, regardless of the presence or absence of cus, a mutant deleted of cueP shows a deficiency in replication inside macrophages compared to the wild-type strain. Our results demonstrate that CueP and CusCFBA exert redundant functions for metal resistance, but not for intracellular survival, and therefore for the virulence of this pathogen.

The suppressor of copper sensitivity protein C from Caulobacter crescentus is a trimeric disulfide isomerase that binds copper(I) with subpicomolar affinity

The characterization of the suppressor of copper sensitivity protein C from C. crescentus is reported.

The introduction of disulfide bonds into periplasmic proteins is a critical process in many Gram-negative bacteria. The formation and regulation of protein disulfide bonds have been linked to the production of virulence factors. Understanding the different pathways involved in this process is important in the development of strategies to disarm pathogenic bacteria. The well characterized disulfide bond-forming (DSB) proteins play a key role by introducing or isomerizing disulfide bonds between cysteines in substrate proteins. Curiously, the suppressor of copper sensitivity C proteins (ScsCs), which are part of the bacterial copper-resistance response, share structural and functional similarities with DSB oxidase and isomerase proteins, including the presence of a catalytic thioredoxin domain. However, the oxidoreductase activity of ScsC varies with its oligomerization state, which depends on a poorly conserved N-terminal domain. Here, the structure and function of Caulobacter crescentus ScsC (CcScsC) have been characterized. It is shown that CcScsC binds copper in the copper(I) form with subpicomolar affinity and that its isomerase activity is comparable to that of Escherichia coli DsbC, the prototypical dimeric bacterial isomerase. It is also reported that CcScsC functionally complements trimeric Proteus mirabilis ScsC (PmScsC) in vivo, enabling the swarming of P. mirabilis in the presence of copper. Using mass photometry and small-angle X-ray scattering (SAXS) the protein is demonstrated to be trimeric in solution, like PmScsC, and not dimeric like EcDsbC. The crystal structure of CcScsC was also determined at a resolution of 2.6 Å, confirming the trimeric state and indicating that the trimerization results from interactions between the N-terminal α-helical domains of three CcScsC protomers. The SAXS data analysis suggested that the protomers are dynamic, like those of PmScsC, and are able to sample different conformations in solution.

The copper-linked Escherichia coli AZY operon: Structure, metal binding, and a possible physiological role in copper delivery

The Escherichia coli yobA-yebZ-yebY (AZY) operon encodes the proteins YobA, YebZ, and YebY. YobA and YebZ are homologs of the CopC periplasmic copper-binding protein and the CopD putative copper importer, respectively, whereas YebY belongs to the uncharacterized Domain of Unknown Function 2511 family. Despite numerous studies of E. coli copper homeostasis and the existence of the AZY operon in a range of bacteria, the operon's proteins and their functional roles have not been explored. In this study, we present the first biochemical and functional studies of the AZY proteins. Biochemical characterization and structural modeling indicate that YobA binds a single Cu2+ ion with high affinity. Bioinformatics analysis shows that YebY is widespread and encoded either in AZY operons or in other genetic contexts unrelated to copper homeostasis. We also determined the 1.8 Å resolution crystal structure of E. coli YebY, which closely resembles that of the lantibiotic self-resistance protein MlbQ. Two strictly conserved cysteine residues form a disulfide bond, consistent with the observed periplasmic localization of YebY. Upon treatment with reductants, YebY binds Cu+ and Cu2+ with low affinity, as demonstrated by metal-binding analysis and tryptophan fluorescence. Finally, genetic manipulations show that the AZY operon is not involved in copper tolerance or antioxidant defense. Instead, YebY and YobA are required for the activity of the copper-related NADH dehydrogenase II. These results are consistent with a potential role of the AZY operon in copper delivery to membrane proteins.

Copper Homeostatic Mechanisms and Their Role in the Virulence of Escherichia coli and Salmonella enterica

Copper is an essential micronutrient that also exerts toxic effects at high concentrations. This review summarizes the current state of knowledge on copper handling and homeostasis systems in Escherichia coli and Salmonella enterica. We describe the mechanisms by which transcriptional regulators, efflux pumps, detoxification enzymes, metallochaperones, and ancillary copper response systems orchestrate cellular response to copper stress. E. coli and S. enterica are important pathogens of humans and animals. We discuss the critical role of copper during killing of these pathogens by macrophages and in nutritional immunity at the bacterial-pathogen-host interface. In closing, we identify opportunities to advance our understanding of the biological roles of copper in these model enteric bacterial pathogens.

The biochemical fate of Ag+ ions in Staphylococcus aureus, Escherichia coli, and biological media

Silver is commonly included in a range of household and medical items to provide bactericidal action. Despite this, the chemical fate of the metal in both mammalian and bacterial systems remains poorly understood. Here, we applied a metallomics approach using X-ray absorption spectroscopy (XAS) and size-exclusion chromatography hyphenated with inductively coupled plasma mass spectrometry (SEC-ICP-MS) to advance our understanding of the biochemical fate of silver ions in bacterial culture and cells, and the chemistry associated with these interactions. When silver ions were added to lysogeny broth, silver was exclusively associated with moderately-sized species (~30 kDa) and bound by thiolate ligands. In two representative bacterial pathogens cultured in lysogeny broth including sub-lethal concentrations of ionic silver, silver was found in cells to be predominantly coordinated by thiolate species. The silver biomacromolecule-binding profile in Staphylococcus aureus and Escherichia coli was complex, with silver bound by a range of species spanning from 20 kDa to >1220 kDa. In bacterial cells, silver was nonuniformly colocalised with copper-bound proteins, suggesting that cellular copper processing may, in part, confuse silver for nutrient copper. Notably, in the treated cells, silver was not detected bound to low molecular weight compounds such as glutathione or bacillithiol.

A dual role of cysteine residues in the maturation of prokaryotic Cu/Zn-superoxide dismutase

Bacterial Cu/Zn-superoxide dismutase (SodC) is an enzyme catalyzing the disproportionation of superoxide radicals, to which the binding of copper and zinc ions and the formation of an intramolecular disulfide bond are essential. We previously showed that Escherichia coli SodC (SodC) was prone to spontaneous degradation in vivo in an immature form prior to the introduction of the disulfide bond. The post-translational maintenance involving the metal binding and the disulfide formation would thus control the stability as well as the enzymatic function of SodC; however, a mechanism of the SodC maturation remains obscure. Here, we show that the disulfide-reduced SodC can secure a copper ion as well as a zinc ion through the thiolate groups. Furthermore, the disulfide-reduced SodC was found to bind cuprous and cupric ions more tightly than SodC with the disulfide bond. The thiolate groups ligating the copper ion were then autooxidized to form the intramolecular disulfide bond, leading to the production of enzymatically active SodC. Based upon the experiments in vitro, therefore, we propose a mechanism for the activation of SodC, in which the conserved Cys residues play a dual role: the acquisition of a copper ion for the enzymatic activity and the formation of the disulfide bond for the structural stabilization.

General Unified Microbiome Profiling Pipeline (GUMPP) for Large Scale, Streamlined and Reproducible Analysis of Bacterial 16S rRNA Data to Predicted Microbial Metagenomes, Enzymatic Reactions and Metabolic Pathways

General Unified Microbiome Profiling Pipeline (GUMPP) was developed for large scale, streamlined and reproducible analysis of bacterial 16S rRNA data and prediction of microbial metagenomes, enzymatic reactions and metabolic pathways from amplicon data. GUMPP workflow introduces reproducible data analyses at each of the three levels of resolution (genus; operational taxonomic units (OTUs); amplicon sequence variants (ASVs)). The ability to support reproducible analyses enables production of datasets that ultimately identify the biochemical pathways characteristic of disease pathology. These datasets coupled to biostatistics and mathematical approaches of machine learning can play a significant role in extraction of truly significant and meaningful information from a wide set of 16S rRNA datasets. The adoption of GUMPP in the gut-microbiota related research enables focusing on the generation of novel biomarkers that can lead to the development of mechanistic hypotheses applicable to the development of novel therapies in personalized medicine.

Bacterial evolutionary precursors of eukaryotic copper-zinc superoxide dismutases

Internalization of a bacteria by an archaeal cell expedited eukaryotic evolution. An important feature of the species that diversified into the great variety of eukaryotic life visible today was the ability to combat oxidative stress with a copper–zinc superoxide dismutase (CuZnSOD) enzyme activated by a specific, high-affinity copper chaperone. Adoption of a single protein interface that facilitates homodimerization and heterodimerization was essential; however, its evolution has been difficult to rationalize given the structural differences between bacterial and eukaryotic enzymes. In contrast, no consistent strategy for the maturation of periplasmic bacterial CuZnSODs has emerged. Here, 34 CuZnSODs are described that closely resemble the eukaryotic form but originate predominantly from aquatic bacteria. Crystal structures of a Bacteroidetes bacterium CuZnSOD portray both prokaryotic and eukaryotic characteristics and propose a mechanism for self-catalyzed disulfide maturation. Unification of a bacterial but eukaryotic-like CuZnSOD along with a ferredoxin-fold MXCXXC copper-binding domain within a single polypeptide created the advanced copper delivery system for CuZnSODs exemplified by the human copper chaperone for superoxide dismutase-1. The development of this system facilitated evolution of large and compartmentalized cells following endosymbiotic eukaryogenesis.

Synthetic biology approaches to copper remediation: bioleaching, accumulation and recycling

One of the current aims of synthetic biology is the development of novel microorganisms that can mine economically important elements from the environment or remediate toxic waste compounds. Copper, in particular, is a high-priority target for bioremediation owing to its extensive use in the food, metal and electronic industries and its resulting common presence as an environmental pollutant. Even though microbe-aided copper biomining is a mature technology, its application to waste treatment and remediation of contaminated sites still requires further research and development. Crucially, any engineered copper-remediating chassis must survive in copper-rich environments and adapt to copper toxicity; they also require bespoke adaptations to specifically extract copper and safely accumulate it as a human-recoverable deposit to enable biorecycling. Here, we review current strategies in copper bioremediation, biomining and biorecycling, as well as strategies that extant bacteria use to enhance copper tolerance, accumulation and mineralization in the native environment. By describing the existing toolbox of copper homeostasis proteins from naturally occurring bacteria, we show how these modular systems can be exploited through synthetic biology to enhance the properties of engineered microbes for biotechnological copper recovery applications.

An overview of Salmonella enterica metal homeostasis pathways during infection

Nutritional immunity is a powerful strategy at the core of the battlefield between host survival and pathogen proliferation. A host can prevent pathogens from accessing biological metals such as Mg, Fe, Zn, Mn, Cu, Co or Ni, or actively intoxicate them with metal overload. While the importance of metal homeostasis for the enteric pathogen Salmonella enterica Typhimurium was demonstrated many decades ago, inconsistent results across various mouse models, diverse Salmonella genotypes, and differing infection routes challenge aspects of our understanding of this phenomenon. With expanding access to CRISPR-Cas9 for host genome manipulation, it is now pertinent to re-visit past results in the context of specific mouse models, identify gaps and incongruities in current knowledge landscape of Salmonella homeostasis, and recommend a straight path forward towards a more universal understanding of this historic host-microbe relationship.

Copper Handling in the Salmonella Cell Envelope and Its Impact on Virulence

Copper (Cu) plays a key role at the host-pathogen interface as both an essential element and a toxic element. Intracellular strains of pathogenic Salmonella have acquired the periplasmic Cu chaperone, CueP, and the thiol oxidoreductases complex Scs, while losing the ancestral Cu-detoxification Cus system. Coregulation of these species-specific factors link Cu with redox stress and allows Salmonella to counteract Cu toxicity during infection.

Copper in infectious disease: Using both sides of the penny

The transition metal Cu is an essential micronutrient that serves as a co-factor for numerous enzymes involved in redox and oxygen chemistry. However, Cu is also a potentially toxic metal, especially to unicellular microbes that are in direct contact with their environment. Since 400 BCE, Cu toxicity has been leveraged for its antimicrobial properties and even today, Cu based materials are being explored as effective antimicrobials against human pathogens spanning bacteria, fungi, and viruses, including the SARS-CoV-2 agent of the 2019-2020 pandemic. Given that Cu has the double-edged property of being both highly toxic and an essential micronutrient, it plays an active and complicated role at the host-pathogen interface. Humans have evolved methods of incorporating Cu into innate and adaptive immune processes and both sides of the penny (Cu toxicity and Cu as a nutrient) are employed. Here we review the evolution of Cu in biology and its multi-faceted roles in infectious disease, from the viewpoints of the microbial pathogens as well as the animal hosts they infect.

Cu Homeostasis in Bacteria: The Ins and Outs

Copper (Cu) is an essential trace element for all living organisms and used as cofactor in key enzymes of important biological processes, such as aerobic respiration or superoxide dismutation. However, due to its toxicity, cells have developed elaborate mechanisms for Cu homeostasis, which balance Cu supply for cuproprotein biogenesis with the need to remove excess Cu. This review summarizes our current knowledge on bacterial Cu homeostasis with a focus on Gram-negative bacteria and describes the multiple strategies that bacteria use for uptake, storage and export of Cu. We furthermore describe general mechanistic principles that aid the bacterial response to toxic Cu concentrations and illustrate dedicated Cu relay systems that facilitate Cu delivery for cuproenzyme biogenesis. Progress in understanding how bacteria avoid Cu poisoning while maintaining a certain Cu quota for cell proliferation is of particular importance for microbial pathogens because Cu is utilized by the host immune system for attenuating pathogen survival in host cells.

Copper primes adaptation of uropathogenic Escherichia coli to superoxide stress by activating superoxide dismutases

Copper and superoxide are used by the phagocytes to kill bacteria. Copper is a host effector encountered by uropathogenic Escherichia coli (UPEC) during urinary tract infection in a non-human primate model, and in humans. UPEC is exposed to higher levels of copper in the gut prior to entering the urinary tract. Effects of pre-exposure to copper on bacterial killing by superoxide has not been reported. We hypothesized that copper-replete E. coli is more sensitive to killing by superoxide in vitro, and in activated macrophages. We utilized wild-type UPEC strain CFT073, and its isogenic mutants lacking copper efflux systems, superoxide dismutases (SODs), regulators of a superoxide dismutase, and complemented mutants to address this question. Surprisingly, our results reveal that copper protects UPEC against killing by superoxide in vitro. This copper-dependent protection was amplified in the mutants lacking copper efflux systems. Increased levels of copper and manganese were detected in UPEC exposed to sublethal concentration of copper. Copper activated the transcription of sodA in a SoxR- and SoxS-dependent manner resulting in enhanced levels of SodA activity. Importantly, pre-exposure to copper increased the survival of UPEC within RAW264.7 and bone marrow-derived murine macrophages. Loss of SodA, but not SodB or SodC, in UPEC obliterated copper-dependent protection from superoxide in vitro, and from killing within macrophages. Collectively, our results suggest a model in which sublethal levels of copper trigger the activation of SodA and SodC through independent mechanisms that converge to promote the survival of UPEC from killing by superoxide. A major implication of our findings is that bacteria colonizing copper-rich milieus are primed for efficient detoxification of superoxide.

Relationships Between Copper-Related Proteomes and Lifestyles in β Proteobacteria

Copper is an essential transition metal whose redox properties are used for a variety of enzymatic oxido-reductions and in electron transfer chains. It is also toxic to living beings, and therefore its cellular concentration must be strictly controlled. We have performed in silico analyses of the predicted proteomes of more than one hundred species of β proteobacteria to characterize their copper-related proteomes, including cuproproteins, i.e., proteins with active-site copper ions, copper chaperones, and copper-homeostasis systems. Copper-related proteomes represent between 0 and 1.48% of the total proteomes of β proteobacteria. The numbers of cuproproteins are globally proportional to the proteome sizes in all phylogenetic groups and strongly linked to aerobic respiration. In contrast, environmental bacteria have considerably larger proportions of copper-homeostasis systems than the other groups of bacteria, irrespective of their proteome sizes. Evolution toward commensalism, obligate, host-restricted pathogenesis or symbiosis is globally reflected in the loss of copper-homeostasis systems. In endosymbionts, defense systems and copper chaperones have disappeared, whereas residual cuproenzymes are electron transfer proteins for aerobic respiration. Lifestyle is thus a major determinant of the size and composition of the copper-related proteome, and it is particularly reflected in systems involved in copper homeostasis. Analyses of the copper-related proteomes of a number of species belonging to the Burkholderia, Bordetella, and Neisseria genera indicates that commensals are in the process of shedding their copper-homeostasis systems and chaperones to greater extents yet than pathogens.

The Scs disulfide reductase system cooperates with the metallochaperone CueP in Salmonella copper resistance

The human pathogen Salmonella enterica serovar Typhimurium (S Typhimurium) contains a complex disulfide bond (Dsb) catalytic machinery. This machinery encompasses multiple Dsb thiol-disulfide oxidoreductases that mediate oxidative protein folding and a less-characterized suppressor of copper sensitivity (scs) gene cluster, associated with increased tolerance to copper. To better understand the function of the Salmonella Scs system, here we characterized two of its key components, the membrane protein ScsB and the periplasmic protein ScsC. Our results revealed that these two proteins form a redox pair in which the electron transfer from the periplasmic domain of ScsB (n-ScsB) to ScsC is thermodynamically driven. We also demonstrate that the Scs reducing pathway remains separate from the Dsb oxidizing pathways and thereby avoids futile redox cycles. Additionally, we provide new insight into the molecular mechanism underlying Scs-mediated copper tolerance in Salmonella We show that both ScsB and ScsC can bind toxic copper(I) with femtomolar affinities and transfer it to the periplasmic copper metallochaperone CueP. Our results indicate that the Salmonella Scs machinery has evolved a dual mode of action, capable of transferring reducing power to the oxidizing periplasm and protecting against copper stress by cooperating with the cue regulon, a major copper resistance mechanism in Salmonella. Overall, these findings expand our understanding of the functional diversity of Dsb-like systems, ranging from those mediating oxidative folding of proteins required for infection to those contributing to defense mechanisms against oxidative stress and copper toxicity, critical traits for niche adaptation and survival.

A Mn-sensing riboswitch activates expression of a Mn2+/Ca2+ ATPase transporter in Streptococcus

Maintaining manganese (Mn) homeostasis is important for the virulence of numerous bacteria. In the human respiratory pathogen Streptococcus pneumoniae, the Mn-specific importer PsaBCA, exporter MntE, and transcriptional regulator PsaR establish Mn homeostasis. In other bacteria, Mn homeostasis is controlled by yybP-ykoY family riboswitches. Here, we characterize a yybP-ykoY family riboswitch upstream of the mgtA gene encoding a PII-type ATPase in S. pneumoniae, suggested previously to function in Ca2+ efflux. We show that the mgtA riboswitch aptamer domain adopts a canonical yybP-ykoY structure containing a three-way junction that is compacted in the presence of Ca2+ or Mn2+ at a physiological Mg2+ concentration. Although Ca2+ binds to the RNA aptamer with higher affinity than Mn2+, in vitro activation of transcription read-through of mgtA by Mn2+ is much greater than by Ca2+. Consistent with this result, mgtA mRNA and protein levels increase ≈5-fold during cellular Mn stress, but only in genetic backgrounds of S. pneumoniae and Bacillus subtilis that exhibit Mn2+ sensitivity, revealing that this riboswitch functions as a failsafe 'on' signal to prevent Mn2+ toxicity in the presence of high cellular Mn2+. In addition, our results suggest that the S. pneumoniae yybP-ykoY riboswitch functions to regulate Ca2+ efflux under these conditions.

Structural basis and mechanism for metallochaperone-assisted assembly of the CuA center in cytochrome oxidase

The mechanisms underlying the biogenesis of the structurally unique, binuclear Cu^1.5+•Cu^1.5+ redox center (Cu_A) on subunit II (CoxB) of cytochrome oxidases have been a long-standing mystery. Here, we reconstituted the CoxB•Cu_A center in vitro from apo-CoxB and the holo-forms of the copper transfer chaperones ScoI and PcuC. A previously unknown, highly stable ScoI•Cu²⁺•CoxB complex was shown to be rapidly formed as the first intermediate in the pathway. Moreover, our structural data revealed that PcuC has two copper-binding sites, one each for Cu¹⁺ and Cu²⁺, and that only PcuC•Cu¹⁺•Cu²⁺ can release CoxB•Cu²⁺ from the ScoI•Cu²⁺•CoxB complex. The CoxB•Cu_A center was then formed quantitatively by transfer of Cu¹⁺ from a second equivalent of PcuC•Cu¹⁺•Cu²⁺ to CoxB•Cu²⁺. This metalation pathway is consistent with all available in vivo data and identifies the sources of the Cu ions required for Cu_A center formation and the order of their delivery to CoxB.

SGI-4 in Monophasic Salmonella Typhimurium ST34 Is a Novel ICE That Enhances Resistance to Copper

A multi drug resistant Salmonella enterica 4,[5],12:i- of sequence type 34 (monophasic S. Typhimurium ST34) is a current pandemic clone associated with livestock, particularly pigs, and numerous outbreaks in the human population. A large genomic island, termed SGI-4, is present in the monophasic Typhimurium ST34 clade and absent from other S. Typhimurium strains. SGI-4 consists of 87 open reading frames including sil and pco genes previously implicated in resistance to copper (Cu) and silver, and multiple genes predicted to be involved in mobilization and transfer by conjugation. SGI-4 was excised from the chromosome, circularized, and transferred to recipient strains of S. Typhimurium at a frequency influenced by stress induced by mitomycin C, and oxygen tension. The presence of SGI-4 was associated with increased resistance to Cu, particularly but not exclusively under anaerobic conditions. The presence of silCBA genes, predicted to encode an RND family efflux pump that transports Cu from the periplasm to the external milieu, was sufficient to impart the observed enhanced resistance to Cu, above that commonly associated with S. Typhimurium isolates. The presence of these genes resulted in the absence of Cu-dependent induction of pco genes encoding multiple proteins linked to Cu resistance, also present on SGI-4, suggesting that the system effectively limits the Cu availability in the periplasm, but did not affect SodCI-dependent macrophage survival.

The copBL operon protects Staphylococcus aureus from copper toxicity: CopL is an extracellular membrane-associated copper-binding protein

As complications associated with antibiotic resistance have intensified, copper (Cu) is attracting attention as an antimicrobial agent. Recent studies have shown that copper surfaces decrease microbial burden, and host macrophages use Cu to increase bacterial killing. Not surprisingly, microbes have evolved mechanisms to tightly control intracellular Cu pools and protect against Cu toxicity. Here, we identified two genes (copB and copL) encoded within the Staphylococcus aureus arginine-catabolic mobile element (ACME) that we hypothesized function in Cu homeostasis. Supporting this hypothesis, mutational inactivation of copB or copL increased copper sensitivity. We found that copBL are co-transcribed and that their transcription is increased during copper stress and in a strain in which csoR, encoding a Cu-responsive transcriptional repressor, was mutated. Moreover, copB displayed genetic synergy with copA, suggesting that CopB functions in Cu export. We further observed that CopL functions independently of CopB or CopA in Cu toxicity protection and that CopL from the S. aureus clone USA300 is a membrane-bound and surface-exposed lipoprotein that binds up to four Cu+ ions. Solution NMR structures of the homologous Bacillus subtilis CopL, together with phylogenetic analysis and chemical-shift perturbation experiments, identified conserved residues potentially involved in Cu+ coordination. The solution NMR structure also revealed a novel Cu-binding architecture. Of note, a CopL variant with defective Cu+ binding did not protect against Cu toxicity in vivo Taken together, these findings indicate that the ACME-encoded CopB and CopL proteins are additional factors utilized by the highly successful S. aureus USA300 clone to suppress copper toxicity.

The Cu chaperone CopZ is required for Cu homeostasis in Rhodobacter capsulatus and influences cytochrome cbb3 oxidase assembly

Cu homeostasis depends on a tightly regulated network of proteins that transport or sequester Cu, preventing the accumulation of this toxic metal while sustaining Cu supply for cuproproteins. In Rhodobacter capsulatus, Cu-detoxification and Cu delivery for cytochrome c oxidase (cbb₃ -Cox) assembly depend on two distinct Cu-exporting P_1B -type ATPases. The low-affinity CopA is suggested to export excess Cu and the high-affinity CcoI feeds Cu into a periplasmic Cu relay system required for cbb₃ -Cox biogenesis. In most organisms, CopA-like ATPases receive Cu for export from small Cu chaperones like CopZ. However, whether these chaperones are also involved in Cu export via CcoI-like ATPases is unknown. Here we identified a CopZ-like chaperone in R. capsulatus, determined its cellular concentration and its Cu binding activity. Our data demonstrate that CopZ has a strong propensity to form redox-sensitive dimers via two conserved cysteine residues. A ΔcopZ strain, like a ΔcopA strain, is Cu-sensitive and accumulates intracellular Cu. In the absence of CopZ, cbb₃ -Cox activity is reduced, suggesting that CopZ not only supplies Cu to P_1B -type ATPases for detoxification but also for cuproprotein assembly via CcoI. This finding was further supported by the identification of a ~150 kDa CcoI-CopZ protein complex in native R. capsulatus membranes.

Handling of nutrient copper in the bacterial envelope

The insertion of copper into bacterial cuproenzymesin vivodoes not always require a copper-binding metallochaperone – why?

Exploiting the vulnerable active site of a copper-only superoxide dismutase to disrupt fungal pathogenesis

Copper-only superoxide dismutases (SODs) represent a new class of SOD enzymes that are exclusively extracellular and unique to fungi and oomycetes. These SODs are essential for virulence of fungal pathogens in pulmonary and disseminated infections, and we show here an additional role for copper-only SODs in promoting survival of fungal biofilms. The opportunistic fungal pathogen Candida albicans expresses three copper-only SODs, and deletion of one of them, SOD5, eradicated candidal biofilms on venous catheters in a rodent model. Fungal copper-only SODs harbor an irregular active site that, unlike their Cu,Zn-SOD counterparts, contains a copper co-factor unusually open to solvent and lacks zinc for stabilizing copper binding, making fungal copper-only SODs highly vulnerable to metal chelators. We found that unlike mammalian Cu,Zn-SOD1, C. albicans SOD5 indeed rapidly loses its copper to metal chelators such as EDTA, and binding constants for Cu(II) predict that copper-only SOD5 has a much lower affinity for copper than does Cu,Zn-SOD1. We screened compounds with a variety of indications and identified several metal-binding compounds, including the ionophore pyrithione zinc (PZ), that effectively inhibit C. albicans SOD5 but not mammalian Cu,Zn-SOD1. We observed that PZ both acts as an ionophore that promotes uptake of toxic metals and inhibits copper-only SODs. The pros and cons of a vulnerable active site for copper-only SODs and the possible exploitation of this vulnerability in antifungal drug design are discussed.

CpxR/CpxA Controls scsABCD Transcription To Counteract Copper and Oxidative Stress in Salmonella enterica Serovar Typhimurium

Periplasmic thiol/disulfide oxidoreductases participate in the formation and isomerization of disulfide bonds and contribute to the virulence of pathogenic microorganisms. Among the systems encoded in the Salmonella genome, the system encoded by the scsABCD locus was shown to be required to cope with Cu and H₂O₂ stress. Here we report that this locus forms an operon whose transcription is driven by a promoter upstream of scsA and depends on CpxR/CpxA and on Cu. Furthermore, genes homologous to scsB, scsC, and scsD are always detected immediately downstream of scsA and in the same genetic arrangement in all scsA-harboring enterobacterial species. Also, a CpxR-binding site is detected upstream of scsA in most of those species, providing evidence of evolutionarily conserved function and regulation. Each individual scs gene shows a different role in copper and/or H₂O₂ resistance, indicating hierarchical contributions of these factors in the defense against these intoxicants. A protective effect of Cu preincubation against H₂O₂ toxicity and the increased Cu-mediated activation of cpxP in the ΔscsABCD mutant suggest that the CpxR/CpxA-controlled transcription of the ScsABCD system contributes to prevent Cu toxicity and to restore the redox balance at the Salmonella envelope.IMPORTANCE Copper intoxication triggers both specific and nonspecific responses in Salmonella The scs locus, which codes for periplasmic thiol/disulfide-oxidoreductase/isomerase-like proteins, has been the focus of attention because it is necessary for copper resistance, oxidative stress responses, and virulence and because it is not present in nonpathogenic Escherichia coli Still, the conditions under which the scs locus is expressed and the roles of its individual components remain unknown. In this report, we examine the contribution of each Scs factor to survival under H₂O₂ and copper stress. We establish that the scs genes form a copper-activated operon controlled by the CpxR/CpxA signal transduction system, and we provide evidence of its conserved gene arrangement and regulation in other bacterial pathogens.

Chemical Warfare at the Microorganismal Level: A Closer Look at the Superoxide Dismutase Enzymes of Pathogens

Superoxide anion radical is generated as a natural byproduct of aerobic metabolism but is also produced as part of the oxidative burst of the innate immune response design to kill pathogens. In living systems, superoxide is largely managed through superoxide dismutases (SODs), families of metalloenzymes that use Fe, Mn, Ni, or Cu cofactors to catalyze the disproportionation of superoxide to oxygen and hydrogen peroxide. Given the bursts of superoxide faced by microbial pathogens, it comes as no surprise that SOD enzymes play important roles in microbial survival and virulence. Interestingly, microbial SOD enzymes not only detoxify host superoxide but also may participate in signaling pathways that involve reactive oxygen species derived from the microbe itself, particularly in the case of eukaryotic pathogens. In this Review, we will discuss the chemistry of superoxide radicals and the role of diverse SOD metalloenzymes in bacterial, fungal, and protozoan pathogens. We will highlight the unique features of microbial SOD enzymes that have evolved to accommodate the harsh lifestyle at the host-pathogen interface. Lastly, we will discuss key non-SOD superoxide scavengers that specific pathogens employ for defense against host superoxide.

Bacterial copper storage proteins

Copper is essential for most organisms as a cofactor for key enzymes involved in fundamental processes such as respiration and photosynthesis. However, copper also has toxic effects in cells, which is why eukaryotes and prokaryotes have evolved mechanisms for safe copper handling. A new family of bacterial proteins uses a Cys-rich four-helix bundle to safely store large quantities of Cu(I). The work leading to the discovery of these proteins, their properties and physiological functions, and how their presence potentially impacts the current views of bacterial copper handling and use are discussed in this review.

Contact killing and antimicrobial properties of copper

With the emergence of antibiotic resistance, the interest for antimicrobial agents has recently increased again in public health. Copper was recognized in 2008 by the United States Environmental Protection Agency (EPA) as the first metallic antimicrobial agent. This led to many investigations of the various properties of copper as an antibacterial, antifungal and antiviral agent. This review summarizes the latest findings about 'contact killing', the mechanism of action of copper nanoparticles and the different ways micro-organisms develop resistance to copper.

Cytoplasmic Copper Detoxification in Salmonella Can Contribute to SodC Metalation but Is Dispensable during Systemic Infection

Salmonella enterica serovar Typhimurium is a leading cause of foodborne disease worldwide. Severe infections result from the ability of S Typhimurium to survive within host immune cells, despite being exposed to various host antimicrobial factors. SodCI, a copper-zinc-cofactored superoxide dismutase, is required to defend against phagocytic superoxide. SodCII, an additional periplasmic superoxide dismutase, although produced during infection, does not function in the host. Previous studies suggested that CueP, a periplasmic copper binding protein, facilitates acquisition of copper by SodCII. CopA and GolT, both inner membrane ATPases that pump copper from the cytoplasm to the periplasm, are a source of copper for CueP. Using in vitro SOD assays, we found that SodCI can also utilize CueP to acquire copper. However, both SodCI and SodCII have a significant fraction of activity independent of CueP and cytoplasmic copper export. We utilized a series of mouse competition assays to address the in vivo role of CueP-mediated SodC activation. A copA golT cueP triple mutant was equally as competitive as the wild type, suggesting that sufficient SodCI is active to defend against phagocytic superoxide independent of CueP and cytoplasmic copper export. We also confirmed that a strain containing a modified SodCII, which is capable of complementing a sodCI deletion, was fully virulent in a copA golT cueP background competed against the wild type. These competitions also address the potential impact of cytoplasmic copper toxicity within the phagosome. Our data suggest that Salmonella does not encounter inhibitory concentrations of copper during systemic infection.IMPORTANCESalmonella is a leading cause of gastrointestinal disease worldwide. In severe cases, Salmonella can cause life-threatening systemic infections, particularly in very young children, the elderly, or people who are immunocompromised. To cause disease, Salmonella must survive the hostile environment inside host immune cells, a location in which most bacteria are killed. Our work examines how one particular metal, copper, is acquired by Salmonella to activate a protein important for survival within immune cells. At high levels, copper itself can inhibit Salmonella Using a strain of Salmonella that cannot detoxify intracellular copper, we also addressed the in vivo role of copper as an antimicrobial agent.

The biological chemistry of the transition metal "transportome" of Cupriavidus metallidurans

This review tries to illuminate how the bacterium Cupriavidus metallidurans CH34 is able to allocate essential transition metal cations to their target proteins although these metals have similar charge-to-surface ratios and chemical features, exert toxic effects, compete with each other, and occur in the bacterial environment over a huge range of concentrations and speciations. Central to this ability is the "transportome", the totality of all interacting metal import and export systems, which, as an emergent feature, transforms the environmental metal content and speciation into the cellular metal mélange. In a kinetic flow equilibrium resulting from controlled uptake and efflux reactions, the periplasmic and cytoplasmic metal content is adjusted in a way that minimizes toxic effects. A central core function of the transportome is to shape the metal ion composition using high-rate and low-specificity reactions to avoid time and/or energy-requiring metal discrimination reactions. This core is augmented by metal-specific channels that may even deliver metals all the way from outside of the cell to the cytoplasm. This review begins with a description of the basic chemical features of transition metal cations and the biochemical consequences of these attributes, and which transition metals are available to C. metallidurans. It then illustrates how the environment influences the metal content and speciation, and how the transportome adjusts this metal content. It concludes with an outlook on the fate of metals in the cytoplasm. By generalization, insights coming from C. metallidurans shed light on multiple transition metal homoeostatic mechanisms in all kinds of bacteria including pathogenic species, where the "battle" for metals is an important part of the host-pathogen interaction.

Host and Pathogen Copper-Transporting P-Type ATPases Function Antagonistically during Salmonella Infection

Copper is an essential yet potentially toxic trace element that is required by all aerobic organisms. A key regulator of copper homeostasis in mammalian cells is the copper-transporting P-type ATPase ATP7A, which mediates copper transport from the cytoplasm into the secretory pathway, as well as copper export across the plasma membrane. Previous studies have shown that ATP7A-dependent copper transport is required for killing phagocytosed Escherichia coli in a cultured macrophage cell line. In this investigation, we expanded on these studies by generating Atp7a^LysMcre mice, in which the Atp7a gene was specifically deleted in cells of the myeloid lineage, including macrophages. Primary macrophages isolated from Atp7a^LysMcre mice exhibit decreased copper transport into phagosomal compartments and a reduced ability to kill Salmonella enterica serovar Typhimurium compared to that of macrophages isolated from wild-type mice. The Atp7a^LysMcre mice were also more susceptible to systemic infection by S Typhimurium than wild-type mice. Deletion of the S Typhimurium copper exporters, CopA and GolT, was found to decrease infection in wild-type mice but not in the Atp7a^LysMcre mice. These studies suggest that ATP7A-dependent copper transport into the phagosome mediates host defense against S Typhimurium, which is counteracted by copper export from the bacteria via CopA and GolT. These findings reveal unique and opposing functions for copper transporters of the host and pathogen during infection.

Copper homeostasis networks in the bacterium Pseudomonas aeruginosa

Bacterial copper (Cu⁺) homeostasis enables both precise metallation of diverse cuproproteins and control of variable metal levels. To this end, protein networks mobilize Cu⁺ to cellular targets with remarkable specificity. However, the understanding of these processes is rather fragmented. Here, we use genome-wide transcriptomic analysis by RNA-Seq to characterize the response of Pseudomonas aeruginosa to external 0.5 mm CuSO₄, a condition that did not generate pleiotropic effects. Pre-steady-state (5-min) and steady-state (2-h) Cu⁺ fluxes resulted in distinct transcriptome landscapes. Cells quickly responded to Cu²⁺ stress by slowing down metabolism. This was restored once steady state was reached. Specific Cu⁺ homeostasis genes were strongly regulated in both conditions. Our system-wide analysis revealed induction of three Cu⁺ efflux systems (a P_1B-ATPase, a porin, and a resistance-nodulation-division (RND) system) and of a putative Cu⁺-binding periplasmic chaperone and the unusual presence of two cytoplasmic CopZ proteins. Both CopZ chaperones could bind Cu⁺ with high affinity. Importantly, novel transmembrane transporters probably mediating Cu⁺ influx were among those largely repressed upon Cu⁺ stress. Compartmental Cu⁺ levels appear independently controlled; the cytoplasmic Cu⁺ sensor CueR controls cytoplasmic chaperones and plasma membrane transporters, whereas CopR/S responds to periplasmic Cu⁺ Analysis of ΔcopR and ΔcueR mutant strains revealed a CopR regulon composed of genes involved in periplasmic Cu⁺ homeostasis and its putative DNA recognition sequence. In conclusion, our study establishes a system-wide model of a network of sensors/regulators, soluble chaperones, and influx/efflux transporters that control the Cu⁺ levels in P. aeruginosa compartments.

Copper import in Escherichia coli by the yersiniabactin metallophore system

Copper plays a dual role as nutrient and toxin during bacterial infections. While uropathogenic Escherichia coli (UPEC) strains can use the copper-binding metallophore yersiniabactin (Ybt) to resist copper toxicity, Ybt also converts bioavailable copper to Cu(II)-Ybt in low copper conditions. Although E. coli have long been considered to lack a copper import pathway, we observed Ybt-mediated copper import in UPEC using canonical Fe(III)-Ybt transport proteins. UPEC removed copper from Cu(II)-Ybt with subsequent re-export of metal-free Ybt to the extracellular space. Copper released through this process became available to an E. coli cuproenzyme (the amine oxidase TynA), linking this import pathway to a nutrient acquisition function. Ybt-expressing E. coli thus engage in nutritional passivation, a strategy of minimizing a metal ion's toxicity while preserving its nutritional availability. Copper acquisition through this process may contribute to the marked virulence defect of Ybt transport-deficient UPEC.

Metal Resistance and Its Association With Antibiotic Resistance

Antibiotic resistance is recognised as a major global threat to public health by the World Health Organization. Currently, several hundred thousand deaths yearly can be attributed to infections with antibiotic-resistant bacteria. The major driver for the development of antibiotic resistance is considered to be the use, misuse and overuse of antibiotics in humans and animals. Nonantibiotic compounds, such as antibacterial biocides and metals, may also contribute to the promotion of antibiotic resistance through co-selection. This may occur when resistance genes to both antibiotics and metals/biocides are co-located together in the same cell (co-resistance), or a single resistance mechanism (e.g. an efflux pump) confers resistance to both antibiotics and biocides/metals (cross-resistance), leading to co-selection of bacterial strains, or mobile genetic elements that they carry. Here, we review antimicrobial metal resistance in the context of the antibiotic resistance problem, discuss co-selection, and highlight critical knowledge gaps in our understanding.

Copper and Antibiotics: Discovery, Modes of Action, and Opportunities for Medicinal Applications

Copper is a ubiquitous element in the environment as well as living organisms, with its redox capabilities and complexation potential making it indispensable for many cellular functions. However, these same properties can be highly detrimental to prokaryotes and eukaryotes when not properly controlled, damaging many biomolecules including DNA, lipids, and proteins. To restrict free copper concentrations, all bacteria have developed mechanisms of resistance, sequestering and effluxing labile copper to minimize its deleterious effects. This weakness is actively exploited by phagocytes, which utilize a copper burst to destroy pathogens. Though administration of free copper is an unreasonable therapeutic antimicrobial itself, due to insufficient selectivity between host and pathogen, small-molecule ligands may provide an opportunity for therapeutic mimicry of the immune system. By modulating cellular entry, complex stability, resistance evasion, and target selectivity, ligand/metal coordination complexes can synergistically result in high levels of antibacterial activity. Several established therapeutic drugs, such as disulfiram and pyrithione, display remarkable copper-dependent inhibitory activity. These findings have led to development of new drug discovery techniques, using copper ions as the focal point. High-throughput screens for copper-dependent inhibitors against Mycobacterium tuberculosis and Staphylococcus aureus uncovered several new compounds, including a new class of inhibitors, the NNSNs. In this review, we highlight the microbial biology of copper, its antibacterial activities, and mechanisms to discover new inhibitors that synergize with copper.

Compartment and signal-specific codependence in the transcriptional control of Salmonella periplasmic copper homeostasis

Copper homeostasis is essential for bacterial pathogen fitness and infection, and has been the focus of a number of recent studies. In Salmonella, envelope protection against copper overload and macrophage survival depends on CueP, a major copper-binding protein in the periplasm. This protein is also required to deliver the metal ion to the Cu/Zn superoxide dismutase SodCII. The Salmonella-specific CueP-coding gene was originally identified as part of the Cue regulon under the transcriptional control of the cytoplasmic copper sensor CueR, but its expression differs from the rest of CueR-regulated genes. Here we show that cueP expression is controlled by the concerted action of CueR, which detects the presence of copper in the cytoplasm, and by CpxR/CpxA, which monitors envelope stress. Copper-activated CueR is necessary for the appropriate spatial arrangement of the -10 and -35 elements of the cueP promoter, and CpxR is essential to recruit the RNA polymerase. The integration of two ancestral sensory systems-CueR, which provides signal specificity, and CpxR/CpxA, which detects stress in the bacterial envelope-restricts the expression of this periplasmic copper resistance protein solely to cells encountering surplus copper that disturbs envelope homeostasis, emulating the role of the CusR/CusS regulatory system present in other enteric bacteria.

Bacterial Cu(+)-ATPases: models for molecular structure-function studies

The early discovery of the human Cu(+)-ATPases and their link to Menkes and Wilson's diseases brought attention to the unique role of these transporters in copper homeostasis. The characterization of bacterial Cu(+)-ATPases has significantly furthered our understanding of the structure, selectivity and transport mechanism of these enzymes, as well as their interplay with other elements of Cu(+) distribution networks. This review focuses on the structural-functional insights that have emerged from studies of bacterial Cu(+)-ATPases at the molecular level and how these observations have contributed to drawing up a comprehensive picture of cellular copper homeostasis.

Copper tolerance and virulence in bacteria

Copper (Cu) is an essential trace element for all aerobic organisms. It functions as a cofactor in enzymes that catalyze a wide variety of redox reactions due to its ability to cycle between two oxidation states, Cu(I) and Cu(II). This same redox property of copper has the potential to cause toxicity if copper homeostasis is not maintained. Studies suggest that the toxic properties of copper are harnessed by the innate immune system of the host to kill bacteria. To counter such defenses, bacteria rely on copper tolerance genes for virulence within the host. These discoveries suggest bacterial copper intoxication is a component of host nutritional immunity, thus expanding our knowledge of the roles of copper in biology. This review summarizes our current understanding of copper tolerance in bacteria, and the extent to which these pathways contribute to bacterial virulence within the host.

Mechanisms of Salmonella Typhi Host Restriction

Salmonella enterica serovar Typhi (S. Typhi) is the cause of typhoid fever, a life-threatening bacterial infection that is very common in the developing world. Recent spread of antimicrobial resistant isolates of S. Typhi makes typhoid fever, a global public health risk. Despite being a common disease, still very little is known about the molecular mechanisms underlying typhoid fever and S. Typhi pathogenesis. In contrast to other Salmonellae, S. Typhi can only infect humans. The molecular bases of this human restriction are mostly unknown. Recent studies identified a novel pathway that contributes to S. Typhi human restriction and is required for killing S. Typhi in macrophages of nonsusceptible species. The small Rab GTPase Rab32 and its guanine nucleotide exchange factor BLOC-3 are the critical components of this pathway. These proteins were already well known as important regulators of intracellular membrane transport. In particular, they are central for the transport of enzymes that synthetize melanin in pigment cells. The recent findings that Rab32 and BLOC-3 are required for S. Typhi host restriction point out to a novel mechanism restricting the growth of bacterial pathogen, dependent on the transport of still unknown molecule(s) to the S. Typhi vacuole. The identification of this novel antimicrobial pathway constitutes a critical starting point to study molecular mechanisms killing bacterial pathogens and possibly identify novel antimicrobial molecules.

Crystal Structure of DsbA from Corynebacterium diphtheriae and Its Functional Implications for CueP in Gram-Positive Bacteria

In Gram-negative bacteria in the periplasmic space, the dimeric thioredoxin-fold protein DsbC isomerizes and reduces incorrect disulfide bonds of unfolded proteins, while the monomeric thioredoxin-fold protein DsbA introduces disulfide bonds in folding proteins. In the Gram-negative bacteria Salmonella enterica serovar Typhimurium, the reduced form of CueP scavenges the production of hydroxyl radicals in the copper-mediated Fenton reaction, and DsbC is responsible for keeping CueP in the reduced, active form. Some DsbA proteins fulfill the functions of DsbCs, which are not present in Gram-positive bacteria. In this study, we identified a DsbA homologous protein (CdDsbA) in the Corynebacterium diphtheriae genome and determined its crystal structure in the reduced condition at 1.5 Å resolution. CdDsbA consists of a monomeric thioredoxin-like fold with an inserted helical domain and unique N-terminal extended region. We confirmed that CdDsbA has disulfide bond isomerase/reductase activity, and we present evidence that the N-terminal extended region is not required for this activity and folding of the core DsbA-like domain. Furthermore, we found that CdDsbA could reduce CueP from C. diphtheriae.

Generating a Metal-responsive Transcriptional Regulator to Test What Confers Metal Sensing in Cells

FrmR from Salmonella enterica serovar typhimurium (a CsoR/RcnR-like transcriptional de-repressor) is shown to repress the frmRA operator-promoter, and repression is alleviated by formaldehyde but not manganese, iron, cobalt, nickel, copper, or Zn(II) within cells. In contrast, repression by a mutant FrmRE64H (which gains an RcnR metal ligand) is alleviated by cobalt and Zn(II). Unexpectedly, FrmR was found to already bind Co(II), Zn(II), and Cu(I), and moreover metals, as well as formaldehyde, trigger an allosteric response that weakens DNA affinity. However, the sensory metal sites of the cells' endogenous metal sensors (RcnR, ZntR, Zur, and CueR) are all tighter than FrmR for their cognate metals. Furthermore, the endogenous metal sensors are shown to out-compete FrmR. The metal-sensing FrmRE64H mutant has tighter metal affinities than FrmR by approximately 1 order of magnitude. Gain of cobalt sensing by FrmRE64H remains enigmatic because the cobalt affinity of FrmRE64H is substantially weaker than that of the endogenous cobalt sensor. Cobalt sensing requires glutathione, which may assist cobalt access, conferring a kinetic advantage. For Zn(II), the metal affinity of FrmRE64H approaches the metal affinities of cognate Zn(II) sensors. Counter-intuitively, the allosteric coupling free energy for Zn(II) is smaller in metal-sensing FrmRE64H compared with nonsensing FrmR. By determining the copies of FrmR and FrmRE64H tetramers per cell, then estimating promoter occupancy as a function of intracellular Zn(II) concentration, we show how a modest tightening of Zn(II) affinity, plus weakened DNA affinity of the apoprotein, conspires to make the relative properties of FrmRE64H (compared with ZntR and Zur) sufficient to sense Zn(II) inside cells.