Selectivity of ferric enterobactin binding and cooperativity of transport in gram-negative bacteria

Exploring the Antibacterial Activity and Cellular Fates of Enterobactin-Drug Conjugates That Target Gram-Negative Bacterial Pathogens

ConspectusSiderophores are secondary metabolites utilized by bacteria to acquire iron (Fe), an essential transition metal nutrient. Fe levels in the host environment are tightly regulated and can be further restricted to starve invading bacterial pathogens in a host-defense process known as nutritional immunity. To survive and colonize the Fe-limited host environment, bacteria produce siderophores and express cognate siderophore transport machinery. These active transport pathways present an opportunity for selective and efficient drug delivery into bacterial cells, motivating decades of research on synthetic siderophore-antibiotic conjugates (SACs) as a Trojan-horse strategy for the development of targeted antibiotics.Enterobactin (Ent) is a triscatecholate siderophore produced and utilized by many Gram-negative bacteria, including all Escherichia coli and Salmonella species. Within these species, pathogenic strains cause a variety of human diseases including urinary tract infections, gastroenteritis, and sepsis. Infections caused by these Gram-negative pathogens can be difficult to treat because of the impermeability of the outer membrane (OM). This impermeability can be overcome by utilizing siderophores as drug delivery vectors for targeting Gram-negative pathogens. Ent is a promising delivery vector because it undergoes active transport across the OM mediated by the Ent uptake machinery after scavenging Fe(III) from the extracellular environment. Despite the well-elucidated chemistry and biology of Ent, its use for SAC development was hampered by the lack of an appropriate functional group for cargo attachment. Our laboratory addressed this need by designing and synthesizing monofunctionalized Ent scaffolds. Over the past decade, we have used these scaffolds to explore Ent-based SACs with a variety of drug warheads, including β-lactam and fluoroquinolone antibiotics, and Pt(IV) prodrugs. Investigations of the antibacterial activities of these conjugates and their cellular fates have informed our design principles and revealed approaches to achieving enhanced antibacterial potency and pathogen-targeted activity. Collectively, our studies of Ent-drug conjugates have provided discoveries, understanding, and invaluable insights for future design and evaluation of SACs.In this Account, we present the story of our work on Ent-drug conjugates that began about ten years ago with the development of monofunctionalized Ent scaffolds and the design and synthesis of various conjugates based on these scaffolds. We describe the antibacterial activity profiles and uptake pathways of Ent-drug conjugates harboring traditional antibiotics and repurposed platinum anticancer agents as well as studies that address cellular targets and fates. Finally, we discuss other applications of monofunctionalized Ent scaffolds, including a siderophore-based immunization strategy. We intend for this Account to inspire further investigations into the fundamental understanding and translational applications of siderophores and siderophore-drug conjugates.

Specificity and mechanism of TonB-dependent ferric catecholate uptake by Fiu

We studied the Escherichia coli outer membrane protein Fiu, a presumed transporter of monomeric ferric catecholates, by introducing Cys residues in its surface loops and modifying them with fluorescein maleimide (FM). Fiu-FM bound iron complexes of the tricatecholate siderophore enterobactin (FeEnt) and glucosylated enterobactin (FeGEnt), their dicatecholate degradation product Fe(DHBS)2 (FeEnt*), the monocatecholates dihydroxybenzoic acid (FeDHBA) and dihydroxybenzoyl serine (FeDHBS), and the siderophore antibiotics cefiderocol (FDC) and MB-1. Unlike high-affinity ligand-gated porins (LGPs), Fiu-FM had only micromolar affinity for iron complexes. Its apparent KD values for FeDHBS, FeDHBA, FeEnt*, FeEnt, FeGEnt, FeFDC, and FeMB-1 were 0.1, 0.7, 0.7, 1.0, 0.3, 0.4, and 4 μM, respectively. Despite its broad binding abilities, the transport repertoires of E. coli Fiu, as well as those of Cir and FepA, were less broad. Fiu only transported FeEnt*. Cir transported FeEnt* and FeDHBS (weakly); FepA transported FeEnt, FeEnt*, and FeDHBA. Both Cir and FepA bound FeGEnt, albeit with lower affinity. Related transporters of Acinetobacter baumannii (PiuA, PirA, BauA) had similarly moderate affinity and broad specificity for di- or monomeric ferric catecholates. Both microbiological and radioisotopic experiments showed Fiu's exclusive transport of FeEnt*, rather than ferric monocatecholate compounds. Molecular docking and molecular dynamics simulations predicted three binding sites for FeEnt*in the external vestibule of Fiu, and a fourth site deeper in its interior. Alanine scanning mutagenesis in the outermost sites (1a, 1b, and 2) decreased FeEnt* binding affinity as much as 20-fold and reduced or eliminated FeEnt* uptake. Finally, the molecular dynamics simulations suggested a pathway of FeEnt* movement through Fiu that may generally describe the process of metal transport by TonB-dependent receptors.

Conjugation to Native and Nonnative Triscatecholate Siderophores Enhances Delivery and Antibacterial Activity of a β-Lactam to Gram-Negative Bacterial Pathogens

Developing new antibiotics and delivery strategies is of critical importance for treating infections caused by Gram-negative bacterial pathogens. Hijacking bacterial iron uptake machinery, such as that of the siderophore enterobactin (Ent), represents one promising approach toward these goals. Here, we report a novel Ent-inspired siderophore-antibiotic conjugate (SAC) employing an alternative siderophore moiety as the delivery vector and demonstrate the potency of our SACs harboring the β-lactam antibiotic ampicillin (Amp) against multiple pathogenic Gram-negative bacterial strains. We establish the ability of N,N',N''-(nitrilotris(ethane-2,1-diyl))tris(2,3-dihydroxybenzamide) (TRENCAM, hereafter TC), a synthetic mimic of Ent, to facilitate drug delivery across the outer membrane (OM) of Gram-negative pathogens. Conjugation of Amp to a new monofunctionalized TC scaffold affords TC-Amp, which displays markedly enhanced antibacterial activity against the gastrointestinal pathogen Salmonella enterica serovar Typhimurium (STm) compared with unmodified Amp. Bacterial uptake, antibiotic susceptibility, and microscopy studies with STm show that the TC moiety facilitates TC-Amp uptake by the OM receptors FepA and IroN and that the Amp warhead inhibits penicillin-binding proteins. Moreover, TC-Amp achieves targeted activity, selectively killing STm in the presence of a commensal lactobacillus. Remarkably, we uncover that TC-Amp and its Ent-based predecessor Ent-Amp achieve enhanced antibacterial activity against diverse Gram-negative ESKAPE pathogens that express Ent uptake machinery, including strains that possess intrinsic β-lactam resistance. TC-Amp and Ent-Amp exhibit potency comparable to that of the FDA-approved SAC cefiderocol against Gram-negative pathogens. These results demonstrate the effective application of native and appropriately designed nonnative siderophores as vectors for drug delivery across the OM of multiple Gram-negative bacterial pathogens.

Iron uptake pathway of Escherichia coli as an entry route for peptide nucleic acids conjugated with a siderophore mimic

Bacteria secrete various iron-chelators (siderophores), which scavenge Fe3+ from the environment, bind it with high affinity, and retrieve it inside the cell. After the Fe3+ uptake, bacteria extract the soluble iron(II) from the siderophore. Ferric siderophores are transported inside the cell via the TonB-dependent receptor system. Importantly, siderophore uptake paths have been also used by sideromycins, natural antibiotics. Our goal is to hijack the transport system for hydroxamate-type siderophores to deliver peptide nucleic acid oligomers into Escherichia coli cells. As siderophore mimics we designed and synthesized linear and cyclic Nδ-acetyl-Nδ-hydroxy-l-ornithine based peptides. Using circular dichroism spectroscopy, we found that iron(III) is coordinated by the linear trimer with hydroxamate groups but not by the cyclic peptide. The internal flexibility of the linear siderophore oxygen atoms and their interactions with Fe3+ were confirmed by all-atom molecular dynamics simulations. Using flow cytometry we found that the designed hydroxamate trimer transports PNA oligomers inside the E. coli cells. Growth recovery assays on various E. coli mutants suggest the pathway of this transport through the FhuE outer-membrane receptor, which is responsible for the uptake of the natural iron chelator, ferric-coprogen. This pathway also involves the FhuD periplasmic binding protein. Docking of the siderophores to the FhuE and FhuD receptor structures showed that binding of the hydroxamate trimer is energetically favorable corroborating the experimentally suggested uptake path. Therefore, this siderophore mimic, as well as its conjugate with PNA, is most probably internalized through the hydroxamate pathway.

Energization of Outer Membrane Transport by the ExbB ExbD Molecular Motor

The outer membranes (OM) of Gram-negative bacteria contain a class of proteins (TBDTs) that require energy for the import of nutrients and to serve as receptors for phages and protein toxins. Energy is derived from the proton motif force (pmf) of the cytoplasmic membrane (CM) through the action of three proteins, namely, TonB, ExbB, and ExbD, which are located in the CM and extend into the periplasm. The leaky phenotype of exbB exbD mutants is caused by partial complementation by homologous tolQ tolR. TonB, ExbB, and ExbD are genuine components of an energy transmission system from the CM into the OM. Mutant analyses, cross-linking experiments, and most recently X-ray and cryo-EM determinations were undertaken to arrive at a model that describes the energy transfer from the CM into the OM. These results are discussed in this paper. ExbB forms a pentamer with a pore inside, in which an ExbD dimer resides. This complex harvests the energy of the pmf and transmits it to TonB. TonB interacts with the TBDT at the TonB box, which triggers a conformational change in the TBDT that releases bound nutrients and opens the pore, through which nutrients pass into the periplasm. The structurally altered TBDT also changes the interactions of its periplasmic signaling domain with anti-sigma factors, with the consequence being that the sigma factors initiate transcription.

Innate Bacteriostatic Mechanisms Defend the Urinary Tract

Urinary tract infection (UTI) is the most common type of urogenital disease. UTI affects the urethra, bladder, ureter, and kidney. A total of 13.3% of women, 2.3% of men, and 3.4% of children in the United States will require treatment for UTI. Traditionally, bladder (cystitis) and kidney (pyelonephritis) infections are considered independently. However, both infections induce host defenses that are either shared or coordinated across the urinary tract. Here, we review the chemical and biophysical mechanisms of bacteriostasis, which limit the duration and severity of the illness. Urinary bacteria attempt to overcome each of these defenses, complicating description of the natural history of UTI.

Iron Acquisition Systems of Gram-negative Bacterial Pathogens Define TonB-Dependent Pathways to Novel Antibiotics

Iron is an indispensable metabolic cofactor in both pro- and eukaryotes, which engenders a natural competition for the metal between bacterial pathogens and their human or animal hosts. Bacteria secrete siderophores that extract Fe3+ from tissues, fluids, cells, and proteins; the ligand gated porins of the Gram-negative bacterial outer membrane actively acquire the resulting ferric siderophores, as well as other iron-containing molecules like heme. Conversely, eukaryotic hosts combat bacterial iron scavenging by sequestering Fe3+ in binding proteins and ferritin. The variety of iron uptake systems in Gram-negative bacterial pathogens illustrates a range of chemical and biochemical mechanisms that facilitate microbial pathogenesis. This document attempts to summarize and understand these processes, to guide discovery of immunological or chemical interventions that may thwart infectious disease.

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.

Formation of a tris(catecholato) iron(III) complex with a nature-inspired cyclic peptoid ligand

Siderophore-mimicking macrocyclic peptoids were synthesized. Peptoid 3 with intramolecular hydrogen bonds showed an optimally arranged primary coordination sphere leading to a stable catecholate-iron complex. The tris(catecholato) structure of 3-Fe(iii) was determined with UV-vis, fluorescence, and EPR spectroscopies and DFT calculations. The iron binding affinity was comparable to that of deferoxamine, with enhanced stability upon air exposure.

The Transcriptomic and Bioinformatic Characterizations of Iron Acquisition and Heme Utilization in Avibacterium paragallinarum in Response to Iron-Starvation

Avibacterium paragallinarum is the pathogen of infectious coryza, which is a highly contagious respiratory disease of chickens that brings a potentially serious threat to poultry husbandry. Iron is an important nutrient for bacteria and can be obtained from surroundings such as siderophores and hemophores. To date, the mechanisms of iron acquisition and heme utilization as well as detailed regulation in A. paragallinarum have been poorly understood. In this study, we investigated the transcriptomic profiles in detail and the changes of transcriptomes induced by iron restriction in A. paragallinarum using RNA-seq. Compared with the iron-sufficiency control group, many more differentially expressed genes (DEGs) and cellular functions as well as signaling pathways were verified in the iron-restriction group. Among these DEGs, the majority of genes showed decreased expression and some were found to be uniquely present in the iron-restriction group. With an in-depth study of bioinformatic analyses, we demonstrated the crucial roles of the Hut protein and DUF domain-containing proteins, which were preferentially activated in bacteria following iron restriction and contributed to the iron acquisition and heme utilization. Consequently, RT-qPCR results further verified the iron-related DEGs and were consistent with the RNA-seq data. In addition, several novel sRNAs were present in A. paragallinarum and had potential regulatory roles in iron homeostasis, especially in the regulation of Fic protein to ensure stable expression. This is the first report of the molecular mechanism of iron acquisition and heme utilization in A. paragallinarum from the perspective of transcriptomic profiles. The study will contribute to a better understanding of the transcriptomic response of A. paragallinarum to iron starvation and also provide novel insight into the development of new antigens for potential vaccines against infectious coryza by focusing on these iron-related genes.

Universal fluorescent sensors of high-affinity iron transport, applied to ESKAPE pathogens

Sensitive assays of biochemical specificity, affinity, and capacity are valuable both for basic research and drug discovery. We created fluorescent sensors that monitor high-affinity binding reactions and used them to study iron acquisition by ESKAPE bacteria, which are frequently responsible for antibiotic-resistant infections. By introducing site-directed Cys residues in bacterial iron transporters and modifying them with maleimide fluorophores, we generated living cells or purified proteins that bind but do not transport target compounds. These constructs sensitively detected ligand concentrations in solution, enabling accurate, real-time spectroscopic analysis of membrane transport by other cells. We assessed the efficacy of these "fluorescent decoy" (FD) sensors by characterizing active iron transport in the ESKAPE bacteria. The FD sensors monitored uptake of both ferric siderophores and hemin by the pathogens. An FD sensor for a particular ligand was universally effective in observing the uptake of that compound by all organisms we tested. We adapted the FD sensors to microtiter format, where they allow high-throughput screens for chemicals that block iron uptake, without genetic manipulations of the virulent target organisms. Hence, screening assays with FD sensors facilitate studies of mechanistic biochemistry, as well as discovery of chemicals that inhibit prokaryotic membrane transport. With appropriate design, FD sensors are potentially applicable to any pro- or eukaryotic high-affinity ligand transport process.

Interactions mediated by a public good transiently increase cooperativity in growing Pseudomonas putida metapopulations

Bacterial communities have rich social lives. A well-established interaction involves the exchange of a public good in Pseudomonas populations, where the iron-scavenging compound pyoverdine, synthesized by some cells, is shared with the rest. Pyoverdine thus mediates interactions between producers and non-producers and can constitute a public good. This interaction is often used to test game theoretical predictions on the "social dilemma" of producers. Such an approach, however, underestimates the impact of specific properties of the public good, for example consequences of its accumulation in the environment. Here, we experimentally quantify costs and benefits of pyoverdine production in a specific environment, and build a model of population dynamics that explicitly accounts for the changing significance of accumulating pyoverdine as chemical mediator of social interactions. The model predicts that, in an ensemble of growing populations (metapopulation) with different initial producer fractions (and consequently pyoverdine contents), the global producer fraction initially increases. Because the benefit of pyoverdine declines at saturating concentrations, the increase need only be transient. Confirmed by experiments on metapopulations, our results show how a changing benefit of a public good can shape social interactions in a bacterial population.

Determination of the Molecular Structures of Ferric Enterobactin and Ferric Enantioenterobactin Using Racemic Crystallography

Enterobactin is a secondary metabolite produced by Enterobacteriaceae for acquiring iron, an essential metal nutrient. The biosynthesis and utilization of enterobactin permits many Gram-negative bacteria to thrive in environments where low soluble iron concentrations would otherwise preclude survival. Despite extensive work carried out on this celebrated molecule since its discovery over 40 years ago, the ferric enterobactin complex has eluded crystallographic structural characterization. We report the successful growth of single crystals containing ferric enterobactin using racemic crystallization, a method that involves cocrystallization of a chiral molecule with its mirror image. The structures of ferric enterobactin and ferric enantioenterobactin obtained in this work provide a definitive assignment of the stereochemistry at the metal center and reveal secondary coordination sphere interactions. The structures were employed in computational investigations of the interactions of these complexes with two enterobactin-binding proteins, which illuminate the influence of metal-centered chirality on these interactions. This work highlights the utility of small-molecule racemic crystallography for obtaining elusive structures of coordination complexes.

Fluorescence High-Throughput Screening for Inhibitors of TonB Action

Gram-negative bacteria acquire ferric siderophores through TonB-dependent outer membrane transporters (TBDT). By fluorescence spectroscopic hgh-throughput screening (FLHTS), we identified inhibitors of TonB-dependent ferric enterobactin (FeEnt) uptake through Escherichia coli FepA (EcoFepA). Among 165 inhibitors found in a primary screen of 17,441 compounds, we evaluated 20 in secondary tests: TonB-dependent ferric siderophore uptake and colicin killing and proton motive force-dependent lactose transport. Six of 20 primary hits inhibited TonB-dependent activity in all tests. Comparison of their effects on [⁵⁹Fe]Ent and [¹⁴C]lactose accumulation suggested several as proton ionophores, but two chemicals, ebselen and ST0082990, are likely not proton ionophores and may inhibit TonB-ExbBD. The facility of FLHTS against E. coli led us to adapt it to Acinetobacter baumannii We identified its FepA ortholog (AbaFepA), deleted and cloned its structural gene, genetically engineered 8 Cys substitutions in its surface loops, labeled them with fluorescein, and made fluorescence spectroscopic observations of FeEnt uptake in A. baumannii Several Cys substitutions in AbaFepA (S279C, T562C, and S665C) were readily fluoresceinated and then suitable as sensors of FeEnt transport. As in E. coli, the test monitored TonB-dependent FeEnt uptake by AbaFepA. In microtiter format with A. baumannii, FLHTS produced Z' factors 0.6 to 0.8. These data validated the FLHTS strategy against even distantly related Gram-negative bacterial pathogens. Overall, it discovered agents that block TonB-dependent transport and showed the potential to find compounds that act against Gram-negative CRE (carbapenem-resistant Enterobacteriaceae)/ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) pathogens. Our results suggest that hundreds of such chemicals may exist in larger compound libraries.IMPORTANCE Antibiotic resistance in Gram-negative bacteria has spurred efforts to find novel compounds against new targets. The CRE/ESKAPE pathogens are resistant bacteria that include Acinetobacter baumannii, a common cause of ventilator-associated pneumonia and sepsis. We performed fluorescence high-throughput screening (FLHTS) against Escherichia coli to find inhibitors of TonB-dependent iron transport, tested them against A. baumannii, and then adapted the FLHTS technology to allow direct screening against A. baumannii This methodology is expandable to other drug-resistant Gram-negative pathogens. Compounds that block TonB action may interfere with iron acquisition from eukaryotic hosts and thereby constitute bacteriostatic antibiotics that prevent microbial colonization of human and animals. The FLHTS method may identify both species-specific and broad-spectrum agents against Gram-negative bacteria.

Bacterial Siderophores Hijack Neutrophil Functions

Neutrophils are the primary immune cells that respond to inflammation and combat microbial transgression. To thrive, the bacteria residing in their mammalian host have to withstand the antibactericidal responses of neutrophils. We report that enterobactin (Ent), a catecholate siderophore expressed by Escherichia coli, inhibited PMA-induced generation of reactive oxygen species (ROS) and neutrophil extracellular traps (NETs) in mouse and human neutrophils. Ent also impaired the degranulation of primary granules and inhibited phagocytosis and bactericidal activity of neutrophils, without affecting their migration and chemotaxis. Molecular analysis revealed that Ent can chelate intracellular labile iron that is required for neutrophil oxidative responses. Other siderophores (pyoverdine, ferrichrome, deferoxamine) likewise inhibited ROS and NETs in neutrophils, thus indicating that the chelation of iron may largely explain their inhibitory effects. To counter iron theft by Ent, neutrophils rely on the siderophore-binding protein lipocalin 2 (Lcn2) in a "tug-of-war" for iron. The inhibition of neutrophil ROS and NETs by Ent was augmented in Lcn2-deficient neutrophils compared with wild-type neutrophils but was rescued by the exogenous addition of recombinant Lcn2. Taken together, our findings illustrate the novel concept that microbial siderophore's iron-scavenging property may serve as an antiradical defense system that neutralizes the immune functions of neutrophils.

TonB-Dependent Heme/Hemoglobin Utilization by Caulobacter crescentus HutA

Siderophore nutrition tests with Caulobacter crescentus strain NA1000 revealed that it utilized a variety of ferric hydroxamate siderophores, including asperchromes, ferrichromes, ferrichrome A, malonichrome, and ferric aerobactin, as well as hemin and hemoglobin. C. crescentus did not transport ferrioxamine B or ferric catecholates. Because it did not use ferric enterobactin, the catecholate aposiderophore was an effective agent for iron deprivation. We determined the kinetics and thermodynamics of [⁵⁹Fe]apoferrichrome and ⁵⁹Fe-citrate binding and transport by NA1000. Its affinity and uptake rate for ferrichrome (equilibrium dissociation constant [K_d ], 1 nM; Michaelis-Menten constant [K_M ], 0.1 nM; V_max, 19 pMol/10⁹ cells/min) were similar to those of Escherichia coli FhuA. Transport properties for ⁵⁹Fe-citrate were similar to those of E. coli FecA (K_M , 5.3 nM; V_max, 29 pMol/10⁹ cells/min). Bioinformatic analyses implicated Fur-regulated loci 00028, 00138, 02277, and 03023 as TonB-dependent transporters (TBDT) that participate in iron acquisition. We resolved TBDT with elevated expression under high- or low-iron conditions by SDS-PAGE of sodium sarcosinate cell envelope extracts, excised bands of interest, and analyzed them by mass spectrometry. These data identified five TBDT: three were overexpressed during iron deficiency (00028, 02277, and 03023), and 2 were overexpressed during iron repletion (00210 and 01196). CLUSTALW analyses revealed homology of putative TBDT 02277 to Escherichia coli FepA and BtuB. A Δ02277 mutant did not transport hemin or hemoglobin in nutrition tests, leading us to designate the 02277 structural gene as hutA (for heme/hemoglobin utilization).IMPORTANCE The physiological roles of the 62 putative TBDT of C. crescentus are mostly unknown, as are their evolutionary relationships to TBDT of other bacteria. We biochemically studied the iron uptake systems of C. crescentus, identified potential iron transporters, and clarified the phylogenetic relationships among its numerous TBDT. Our findings identified the first outer membrane protein involved in iron acquisition by C. crescentus, its heme/hemoglobin transporter (HutA).

Mechanism of Action of ABC Importers: Conservation, Divergence, and Physiological Adaptations

The past decade has seen a remarkable surge in structural characterization of ATP binding cassette (ABC) transporters, which have spurred a more focused functional analysis of these elaborate molecular machines. As a result, it has become increasingly apparent that there is a substantial degree of mechanistic variation between ABC transporters that function as importers, which correlates with their physiological roles. Here, we summarize recent advances in ABC importers' structure-function studies and provide an explanation as to the origin of the different mechanisms of action.

Sociality in Escherichia coli: Enterochelin Is a Private Good at Low Cell Density and Can Be Shared at High Cell Density

Many bacteria produce secreted iron chelators called siderophores, which can be shared among cells with specific siderophore uptake systems regardless of whether the cell produces siderophores. Sharing secreted products allows freeloading, where individuals use resources without bearing the cost of production. Here we show that the Escherichia coli siderophore enterochelin is not evenly shared between producers and nonproducers. Wild-type Escherichia coli grows well in low-iron minimal medium, and an isogenic enterochelin synthesis mutant (ΔentF) grows very poorly. The enterochelin mutant grows well in low-iron medium supplemented with enterochelin. At high cell densities the ΔentF mutant can compete equally with the wild type in low-iron medium. At low cell densities the ΔentF mutant cannot compete. Furthermore, the growth rate of the wild type is unaffected by cell density. The wild type grows well in low-iron medium even at very low starting densities. Our experiments support a model where at least some enterochelin remains associated with the cells that produce it, and the cell-associated enterochelin enables iron acquisition even at very low cell density. Enterochelin that is not retained by producing cells at low density is lost to dilution. At high cell densities, cell-free enterochelin can accumulate and be shared by all cells in the group. Partial privatization is a solution to the problem of iron acquisition in low-iron, low-cell-density habitats. Cell-free enterochelin allows for iron scavenging at a distance at higher population densities. Our findings shed light on the conditions under which freeloaders might benefit from enterochelin uptake systems.

IMPORTANCE

Sociality in microbes has become a topic of great interest. One facet of sociality is the sharing of secreted products, such as the iron-scavenging siderophores. We present evidence that the Escherichia coli siderophore enterochelin is relatively inexpensive to produce and is partially privatized such that it can be efficiently shared only at high producer cell densities. At low cell densities, cell-free enterochelin is scarce and only enterochelin producers are able to grow in low-iron medium. Because freely shared products can be exploited by freeloaders, this partial privatization may help explain how enterochelin production is stabilized in E. coli and may provide insight into when enterochelin is available for freeloaders.

Enterobactin-mediated delivery of β-lactam antibiotics enhances antibacterial activity against pathogenic Escherichia coli

The design, synthesis, and characterization of enterobactin-antibiotic conjugates, hereafter Ent-Amp/Amx, where the β-lactam antibiotics ampicillin (Amp) and amoxicillin (Amx) are linked to a monofunctionalized enterobactin scaffold via a stable poly(ethylene glycol) linker are reported. Under conditions of iron limitation, these siderophore-modified antibiotics provide enhanced antibacterial activity against Escherichia coli strains, including uropathogenic E. coli CFT073 and UTI89, enterohemorrhagic E. coli O157:H7, and enterotoxigenic E. coli O78:H11, compared to the parent β-lactams. Studies with E. coli K-12 derivatives defective in ferric enterobactin transport reveal that the enhanced antibacterial activity observed for this strain requires the outer membrane ferric enterobactin transporter FepA. A remarkable 1000-fold decrease in minimum inhibitory concentration (MIC) value is observed for uropathogenic E. coli CFT073 relative to Amp/Amx, and time-kill kinetic studies demonstrate that Ent-Amp/Amx kill this strain more rapidly at 10-fold lower concentrations than the parent antibiotics. Moreover, Ent-Amp and Ent-Amx selectively kill E. coli CFT073 co-cultured with other bacterial species such as Staphylococcus aureus, and Ent-Amp exhibits low cytotoxicity against human T84 intestinal cells in both the apo and iron-bound forms. These studies demonstrate that the native enterobactin platform provides a means to effectively deliver antibacterial cargo across the outer membrane permeability barrier of Gram-negative pathogens utilizing enterobactin for iron acquisition.

Characterization of a gene encoding the outer membrane receptor for ferric enterobactin in Aeromonas hydrophila ATCC 7966(T)

Aeromonas hydrophila ATCC 7966(T) produces a catecholate siderophore amonabactin in response to iron starvation. In this study, we determined that this strain utilizes exogenously supplied enterobactin (Ent) for growth under iron-limiting conditions. A homology search of the A. hydrophila ATCC 7966(T) genomic sequence revealed the existence of a candidate gene encoding a protein homologous to Vibrio parahaemolyticus IrgA that functions as the outer membrane receptor for ferric Ent. SDS-PAGE showed induction of IrgA under iron-limiting conditions. The growth of the double mutant of irgA and entA (one of the amonabactin biosynthetic genes) was restored when it was complemented with irgA in the presence of Ent. Moreover, a growth assay of three isogenic tonB mutants indicated that the tonB2 system exclusively provides energy for IrgA to transport ferric Ent. Finally, reverse transcriptase-quantitative PCR revealed that the transcription of irgA and the TonB2 system cluster genes is iron-regulated, consistently with the presence of a predicted Fur box in the promoter region.

Involvement of multiple distinct Bordetella receptor proteins in the utilization of iron liberated from transferrin by host catecholamine stress hormones

Bordetella bronchiseptica is a pathogen that can acquire iron using its native alcaligin siderophore system, but can also use the catechol xenosiderophore enterobactin via the BfeA outer membrane receptor. Transcription of bfeA is positively controlled by a regulator that requires induction by enterobactin. Catecholamine hormones also induce bfeA transcription and B. bronchiseptica can use the catecholamine noradrenaline for growth on transferrin. In this study, B. bronchiseptica was shown to use catecholamines to obtain iron from both transferrin and lactoferrin in the absence of siderophore. In the presence of siderophore, noradrenaline augmented transferrin utilization by B. bronchiseptica, as well as siderophore function in vitro. Genetic analysis identified BfrA, BfrD and BfrE as TonB-dependent outer membrane catecholamine receptors. The BfeA enterobactin receptor was found to not be involved directly in catecholamine utilization; however, the BfrA, BfrD and BfrE catecholamine receptors could serve as receptors for enterobactin and its degradation product 2,3-dihydroxybenzoic acid. Thus, there is a functional link between enterobactin-dependent and catecholamine-dependent transferrin utilization. This investigation characterizes a new B. bronchiseptica mechanism for iron uptake from transferrin that uses host stress hormones that not only deliver iron directly to catecholamine receptors, but also potentiate siderophore activity by acting as iron shuttles.

Adsorption of enterobactin to metal oxides and the role of siderophores in bacterial adhesion to metals

The potential contribution of chemical bonds formed between bacterial cells and metal surfaces during biofilm initiation has received little attention. Previous work has suggested that bacterial siderophores may play a role in bacterial adhesion to metals. It has now been shown using in situ ATR-IR spectroscopy that enterobactin, a catecholate siderophore secreted by Escherichia coli, forms covalent bonds with particle films of titanium dioxide, boehmite (AlOOH), and chromium oxide-hydroxide which model the surfaces of metals of significance in medical and industrial settings. Adsorption of enterobactin to the metal oxides occurred through the 2,3-dihydroxybenzoyl moieties, with the trilactone macrocycle having little involvement. Vibrational modes of the 2,3-dihydroxybenzoyl moiety of enterobactin, adsorbed to TiO(2), were assigned by comparing the observed IR spectra with those calculated by the density functional method. Comparison of the observed adsorbate IR spectrum with the calculated spectra of catecholate-type [H(2)NCOC(6)H(3)O(2)Ti(OH)(4)](2-) and salicylate-type [H(2)NCOC(6)H(3)O(2)HTi(OH)(4)](2-) surface complexes indicated that the catecholate type is dominant. Analysis of the spectra for enterobactin in solution and that adsorbed to TiO(2) revealed that the amide of the 2,3-dihydroxybenzoylserine group reorientates during coordination to surface Ti(IV) ions. Investigation into the pH dependence of enterobactin adsorption to TiO(2) surfaces showed that all 2,3-dihydroxybenzoyl groups are involved. Infrared absorption bands attributed to adsorbed enterobactin were also strongly evident for E. coli cells attached to TiO(2) particle films. These studies give evidence of enterobactin-metal bond formation and further suggest the generality of siderophore involvement in bacterial biofilm initiation on metal surfaces.

Identification and characterization of a new ferric enterobactin receptor, CfrB, in Campylobacter

The ferric enterobactin (FeEnt) receptor CfrA is present in the majority of Campylobacter jejuni isolates and is responsible for high-affinity iron acquisition. Our recent work and that of others strongly suggested the existence of another FeEnt uptake system in Campylobacter. Here we have identified and characterized a new FeEnt receptor (designated CfrB) using both in vitro and in vivo systems. CfrB, a homolog of C. jejuni NCTC 11168 Cj0444, shares approximately 34% of amino acid identity with CfrA. Alignment of complete CfrB sequences showed that the CfrB is highly conserved in Campylobacter. Immunoblotting analysis using CfrB-specific antiserum demonstrated that CfrB was dramatically induced under iron-restricted conditions and was produced in the majority of Campylobacter coli (41 out of 45) and in some C. jejuni (8 out of 32) primary strains from various sources and from geographically diverse areas. All of the CfrB-producing C. coli strains also produced CfrA, which was rarely observed in the tested C. jejuni strains. Isogenic cfrB, cfrA, and cfrA cfrB double mutants were constructed in 43 diverse Campylobacter strains. Growth promotion assays using these mutants demonstrated that CfrB has a major role in FeEnt iron acquisition in C. coli. Chicken colonization experiments indicated that inactivation of the cfrB gene alone greatly reduced and even abolished Campylobacter colonization of the intestines. A growth assay using CfrB-specific antiserum strongly suggested that specific CfrB antibodies could block the function of CfrB and diminish FeEnt-mediated growth promotion under iron-restricted conditions. Together, this work reveals the complexity of FeEnt systems in the two closely related Campylobacter species and demonstrates the important role of the new FeEnt receptor CfrB in Campylobacter iron acquisition and in vivo colonization.

Abolition of biofilm formation in urinary tract Escherichia coli and Klebsiella isolates by metal interference through competition for fur

Bacterial biofilms are associated with a large number of persistent and chronic infections. Biofilm-dwelling bacteria are particularly resistant to antibiotics and immune defenses, which makes it hard if not impossible to eradicate biofilm-associated infections. In the urinary tract, free iron is strictly limited but is critical for bacterial growth. Biofilm-associated Escherichia coli cells are particularly desperate for iron. An attractive way of inhibiting biofilm formation is to fool the bacterial regulatory system for iron uptake. Here, we demonstrate that biofilm formation can be impaired by the addition of divalent metal ions, such as Zn(II) and Co(II), which inhibit iron uptake by virtue of their higher-than-iron affinity for the master controller protein of iron uptake, Fur. Reduced biofilm formation of urinary tract-infectious E. coli strains in the presence of Zn(II) was observed in microtiter plates and flow chambers as well as on urinary catheters. These results further support that iron uptake is indeed crucial for biofilm formation, and thereby, targeting these uptake systems might be an effective way to eradicate biofilms caused by infectious strains.

Enzymatic hydrolysis of trilactone siderophores: where chiral recognition occurs in enterobactin and bacillibactin iron transport

Bacillibactin and enterobactin are hexadentate catecholate siderophores produced by bacteria upon iron limitation to scavenge ferric ion and seem to be the ultimate siderophores of their two respective domains: Gram-positive and Gram-negative. Iron acquisition mediated by these trilactone-based ligands necessitates enzymatic hydrolysis of the scaffold for successful intracellular iron delivery. The esterases BesA and Fes hydrolyze bacillibactin and enterobactin, respectively, as well as the corresponding iron complexes. Bacillibactin binds iron through three 2,3-catecholamide moieties linked to a trithreonine scaffold via glycine spacers, whereas in enterobactin the iron-binding moieties are directly attached to a tri-l-serine backbone; although apparently minor, these structural differences result in markedly different iron coordination properties and iron transport behavior. Comparison of the solution thermodynamic and circular dichroism properties of bacillibactin, enterobactin and the synthetic analogs d-enterobactin, SERGlyCAM and d-SERGlyCAM has determined the role of each different feature in the siderophores' molecular structures in ferric complex stability and metal chirality. While opposite metal chiralities in the different complexes did not affect transport and incorporation in Bacillus subtilis, ferric complexes formed with the various siderophores did not systematically promote growth of the bacteria. The bacillibactin esterase BesA is less specific than the enterobactin esterase Fes; BesA can hydrolyze the trilactones of both siderophores, while only the tri-l-serine trilactone is a substrate of Fes. Both enzymes are stereospecific and cannot cleave tri-d-serine lactones. These data provide a complete picture of the microbial iron transport mediated by these two siderophores, from initial recognition and transport to intracellular iron release.

Retracted: A catechol type siderophore, bacillibactin: biosynthesis, regulation and transport in Bacillus subtilis

Retraction: The following article from the Journal of Basic Microbiology, "A catechol type siderophore, bacillibactin: biosynthesis, regulation and transport in Bacillus subtilis" by Waseem Raza, Hongsheng Wu, Muhammad Ali Abdullah Shah and Qirong Shen, published online on 11 September 2008 in Wiley InterScience (www.interscience.wiley.com), has been retracted by agreement between the authors, the journal Editor-in-Chief, Erika Kothe, and the publisher Wiley-VCH. The retraction has been agreed due to substantial overlap of the content of this article with previously published articles in other journals.The Journal of Basic Microbiology apologises to our readership.

Infrared multiphoton dissociation of the siderophore enterobactin and its Fe(III) complex. Influence of Fe(III) binding on dissociation kinetics and relative energetics

The dissociation pathways of the siderophore enterobactin and its complex with Fe(III) were examined using infrared multiphoton dissociation (IRMPD). Under experimental conditions (pH = 3.5), both compounds' electrospray spectra exhibited exclusively singly-charged anions. The compositions of the dissociation products were characterized by accurate mass measurements using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The primary dissociation channel for both species was determined to be the loss of one serine group from the precursor molecules. To further investigate the influence of Fe(III) binding on the intramolecular interactions, dissociation kinetics and relative energetics for the loss of this serine group were determined using the focused radiation for gaseous multiphoton energy-transfer (FRAGMENT) method. From the kinetic data, it was found that enterobactin was approximately seven times more reactive than its Fe(III) complex over the range of laser intensities investigated. The relative activation energies, however, exhibited similar values, approximately 7 kcal.mol(-1). These results suggest that at pH = 3.5, Fe(III) interacts with only two of the three serine groups. The results from the present work are believed to be valuable for the characterization of novel siderophores as well as their associated metabolites and synthetic analogues.

Genetic regulation of fluxes: iron homeostasis of Escherichia coli

Iron is an essential trace-element for most organisms. However, because high concentration of free intracellular iron is cytotoxic, cells have developed complex regulatory networks that keep free intracellular iron concentration at optimal range, allowing the incorporation of the metal into iron-using enzymes and minimizing damage to the cell. We built a mathematical model of the network that controls iron uptake and usage in the bacterium Escherichia coli to explore the dynamics of iron flow. We simulate the effect of sudden decrease or increase in the extracellular iron level on intracellular iron distribution. Based on the results of simulations we discuss the possible roles of the small RNA RyhB and the Fe–S cluster assembly systems in the optimal redistribution of iron flows. We suggest that Fe–S cluster assembly is crucial to prevent the accumulation of toxic levels of free intracellular iron when the environment suddenly becomes iron rich.

Siderophore-mediated iron transport in Bacillus subtilis and Corynebacterium glutamicum

Hexadentate bacillibactin is the siderophore of Bacillus subtilis and is structurally similar to the better known enterobactin of Gram-negative bacteria such as Escherichia coli. Although both are triscatecholamide trilactones, the structural differences of these two siderophores result in opposite metal chiralities, different affinity for ferric ion, and dissimilar iron transport behaviors. Bacillibactin was first reported as isolated from Corynebacterium glutamicum and called corynebactin. However, failure of iron-starved C. glutamicum to transport 55Fe bacillibactin and lack of required bacillibactin biosynthetic genes suggest that bacillibactin is not the siderophore produced by this organism. Iron transport mediated by siderophores in B. subtilis occurs through a transport process that is specific for the iron chelating moiety, with parallel pathways for catecholates and hydroxamates. For bacillibactin, enterobactin, and their analogs, neither chirality nor presence of an amino acid spacer affects the uptake and transport process, but alteration of the net charge and size of the molecule impedes the recognition.

Terephthalamide-Containing Analogues of TREN-Me-3,2-HOPO

A series of terephthalamide-containing analogues based on TREN-Me-3,2-HOPO have been prepared. These analogues contain one, two, or three bidentate 2,3-dihydroxyterephthalamide (TAM) units in place of the 3,2-hydroxypyridinone (HOPO) units on the parent hexadentate ligand. One representative ligand in the series, TRENHOPOTAM2, and its gallium complex have been structurally characterized by X-ray diffraction. TRENHOPOTAM2 crystallizes in the monoclinic space group P2(1)/c with cell parameters a = 16.0340(17) A, b = 17.0609(18) A, c = 16.0695(17) A, beta = 113.837(2) degrees, and Z = 4. Ga[TRENHOPOTAM2] also crystallizes in the monoclinic space group P2(1)/c, with cell parameters a = 16.3379(14) A, b = 15.2722(13) A, c = 19.4397(17) A, beta = 91.656(2) degrees, and Z = 4. The conformation of the TRENHOPOTAM2 ligand structure suggests that the ligand is predisposed for metal ion binding. The aqueous protonation and ferric ion coordination chemistry of all ligands in the series were examined using potentiometric and spectrophotometric methods, giving log formation constants of 34.6(2) (beta110) and 38.8(2) (beta111) for the ferric TRENHOPO2TAM complexes, 41.0(3) (beta110) and 45.4(3) (beta111) for the ferric TRENHOPOTAM2 complexes, and 45.2(2) (beta110) and 50.9(2) (beta111) for the ferric TRENTAM3 complexes. These thermodynamic data confirm that adding terephthalamide units to a hydroxypyridinone-containing ligand tends to increase the stability of the resulting iron complex. The ferric TRENTAM3 complex is one of the most stable iron complexes yet reported.

Cooperative uptake of microcin E492 by receptors FepA, Fiu, and Cir and inhibition by the siderophore enterochelin and its dimeric and trimeric hydrolysis products

Microcin E492 uptake by FepA, Fiu, and Cir is cooperative, with FepA being the main receptor. No TonB-mediated interaction with the ferric catecholate receptors is needed for microcin to exert action at the cytoplasmic membrane. Microcin E492 uptake by the receptors is inhibited by the dimer and trimer of dihydroxybenzoylserine.

Siderocalin (Lcn 2) Also Binds Carboxymycobactins, Potentially Defending against Mycobacterial Infections through Iron Sequestration

Siderocalin, a member of the lipocalin family of binding proteins, is found in neutrophil granules, uterine secretions, and at markedly elevated levels in serum and synovium during bacterial infection; it is also secreted from epithelial cells in response to inflammation or tumorigenesis. Identification of high-affinity ligands, bacterial catecholate-type siderophores (such as enterochelin), suggested a possible function for siderocalin: an antibacterial agent, complementing the general antimicrobial innate immune system iron-depletion strategy, sequestering iron as ferric siderophore complexes. Supporting this hypothesis, siderocalin is a potent bacteriostatic agent in vitro under iron-limiting conditions and, when knocked out, renders mice remarkably susceptible to bacterial infection. Here we show that siderocalin also binds soluble siderophores of mycobacteria, including M. tuberculosis: carboxymycobactins. Siderocalin employs a degenerate recognition mechanism to cross react with these dissimilar types of siderophores, broadening the potential utility of this innate immune defense.

The BfeR regulator mediates enterobactin-inducible expression of Bordetella enterobactin utilization genes

Utilization of the enterobactin siderophore by the respiratory pathogens Bordetella pertussis and Bordetella bronchiseptica is dependent on the BfeA outer membrane receptor. This study determined that production of BfeA was increased significantly in iron-starved bacteria upon supplementation of cultures with enterobactin. A 1.01-kb open reading frame, designated bfeR, encoding a predicted positive transcriptional regulator of the AraC family was identified upstream and divergently oriented from bfeA. In iron-depleted cultures containing enterobactin, a Bordetella bfeR mutant exhibited markedly decreased BfeA receptor production compared to that of the wild-type strain. Additionally, B. pertussis and B. bronchiseptica bfeR mutants exhibited impaired growth with ferric enterobactin as the sole source of iron, demonstrating that effective enterobactin utilization is bfeR dependent. Transcriptional analysis using bfeA-lacZ reporter fusions in wild-type strains demonstrated that bfeA transcription was stimulated in iron-depleted conditions in the presence of enterobactin, compared to modest expression levels in cultures lacking enterobactin. In contrast, bfeA transcription in B. pertussis and B. bronchiseptica bfeR mutants was completely unresponsive to the enterobactin inducer. bfeA transcriptional analyses of a bfeA mutant demonstrated that induction by enterobactin did not require BfeA receptor-mediated uptake of the siderophore. These studies establish that bfeR encodes an enterobactin-dependent positive regulator of bfeA transcription in these Bordetella species.

Recognition of ferric catecholates by FepA

Escherichia coli FepA transports certain catecholate ferric siderophores, but not others, nor any noncatecholate compounds. Direct binding and competition experiments demonstrated that this selectivity originates during the adsorption stage. The synthetic tricatecholate Fe-TRENCAM bound to FepA with 50- to 100-fold-lower affinity than Fe-enterobactin (FeEnt), despite an identical metal center, and Fe-corynebactin only bound at much higher concentrations. Neither Fe-agrobactin nor ferrichrome bound at all, even at concentrations 10(6)-fold above the Kd. Thus, FepA only adsorbs catecholate iron complexes, and it selects FeEnt among even its close homologs. We used alanine scanning mutagenesis to study the contributions of surface aromatic residues to FeEnt recognition. Although not apparent from crystallography, aromatic residues in L3, L5, L7, L8, and L10 affected FepA's interaction with FeEnt. Among 10 substitutions that eliminated aromatic residues, Kd increased as much as 20-fold (Y481A and Y638A) and Km increased as much as 400-fold (Y478), showing the importance of aromaticity around the pore entrance. Although many mutations equally reduced binding and transport, others caused greater deficiencies in the latter. Y638A and Y478A increased Km 10- and 200-fold more, respectively, than Kd. N-domain loop deletions created the same phenotype: Delta60-67 (in NL1) and Delta98-105 (in NL2) increased Kd 10- to 20-fold but raised Km 500- to 700-fold. W101A (in NL2) had little effect on Kd but increased Km 1,000-fold. These data suggested that the primary role of the N terminus is in ligand uptake. Fluorescence and radioisotopic experiments showed biphasic release of FeEnt from FepA. In spectroscopic determinations, k(off1) was 0.03/s and k(off2) was 0.003/s. However, FepAY272AF329A did not manifest the rapid dissociation phase, corroborating the role of aromatic residues in the initial binding of FeEnt. Thus, the beta-barrel loops contain the principal ligand recognition determinants, and the N-domain loops perform a role in ligand transport.

Proteus mirabilis genes that contribute to pathogenesis of urinary tract infection: identification of 25 signature-tagged mutants attenuated at least 100-fold

Proteus mirabilis, a common cause of urinary tract infections (UTI) in individuals with functional or structural abnormalities or with long-term catheterization, forms bladder and kidney stones as a consequence of urease-mediated urea hydrolysis. Known virulence factors, besides urease, are hemolysin, fimbriae, metalloproteases, and flagella. In this study we utilized the CBA mouse model of ascending UTI to evaluate the colonization of mutants of P. mirabilis HI4320 that were generated by signature-tagged mutagenesis. By performing primary screening of 2088 P. mirabilis transposon mutants, we identified 502 mutants that ranged from slightly attenuated to unrecoverable. Secondary screening of these mutants revealed that 114 transposon mutants were reproducibly attenuated. Cochallenge of 84 of these single mutants with the parent strain in the mouse model resulted in identification of 37 consistently out-competed P. mirabilis transposon mutants, 25 of which were out-competed >100-fold for colonization of the bladder and/or kidneys by the parent strain. We determined the sequence flanking the site of transposon insertion in 29 attenuated mutants and identified genes affecting motility, iron acquisition, transcriptional regulation, phosphate transport, urease activity, cell surface structure, and key metabolic pathways as requirements for P. mirabilis infection of the urinary tract. Two mutations localized to a approximately 42-kb plasmid present in the parent strain, suggesting that the plasmid is important for colonization. Isolation of disrupted genes encoding proteins with homologies to known bacterial virulence factors, especially the urease accessory protein UreF and the disulfide formation protein DsbA, showed that the CBA mouse model and mutant pools are a reliable source of attenuated mutants with mutations in virulence genes.

Enterobactin: an archetype for microbial iron transport

Bacteria have aggressive acquisition processes for iron, an essential nutrient. Siderophores are small iron chelators that facilitate cellular iron transport. The siderophore enterobactin is a triscatechol derivative of a cyclic triserine lactone. Studies of the chemistry, regulation, synthesis, recognition, and transport of enterobactin make it perhaps the best understood of the siderophore-mediated iron uptake systems, displaying a lot of function packed into this small molecule. However, recent surprises include the isolation of corynebactin, a closely related trithreonine triscatechol derivative lactone first found in Gram-positive bacteria, and the crystal structure of a ferric enterobactin complex of a protein identified as an antibacterial component of the human innate immune system.

Spectroscopic observations of ferric enterobactin transport

We characterized the uptake of ferric enterobactin (FeEnt), the native Escherichia coli ferric siderophore, through its cognate outer membrane receptor protein, FepA, using a site-directed fluorescence methodology. The experiments first defined locations in FepA that were accessible to covalent modification with fluorescein maleimide (FM) in vivo; among 10 sites that we tested by substituting single Cys residues, FM labeled W101C, S271C, F329C, and S397C, and all these exist within surface-exposed loops of the outer membrane protein. FeEnt normally adsorbed to the fluoresceinated S271C and S397C mutant FepA proteins in vivo, which we observed as quenching of fluorescence intensity, but the ferric siderophore did not bind to the FM-modified derivatives of W101C or F329C. These in vivo fluorescence determinations showed, for the first time, consistency with radioisotopic measurements of the affinity of the FeEnt-FepA interaction; K(d) was 0.2 nm by both methods. Analysis of the FepA mutants with AlexaFluor(680), a fluorescein derivative with red-shifted absorption and emission spectra that do not overlap the absorbance spectrum of FeEnt, refuted the possibility that the fluorescence quenching resulted from resonance energy transfer. These and other data instead indicated that the quenching originated from changes in the environment of the fluor as a result of loop conformational changes during ligand binding and transport. We used the fluorescence system to monitor FeEnt uptake by live bacteria and determined its dependence on ligand concentration, temperature, pH, and carbon sources and its susceptibility to inhibition by the metabolic poisons. Unlike cyanocobalamin transport through the outer membrane, FeEnt uptake was sensitive to inhibitors of electron transport and phosphorylation, in addition to its sensitivity to proton motive force depletion.

Mechanisms of colicin binding and transport through outer membrane porins

To kill Escherichia coli, toxic proteins, called colicins, pass through the permeability barrier created by the outer membrane (OM) of the bacterial cell envelope. We consider a variety of different colicins, including A, B, D, E1, E3, Ia, M and N, that penetrate through the porins OmpF, FepA, BtuB, Cir and FhuA, to subsequently interact with a few targets in the periplasm, including TolA, TolB, TolC and TonB. We review the mechanisms, demonstrated and postulated, by which such toxins enter bacterial cells, from the initial binding stage on the cell surface to the internalization reaction through the OM bilayer. Our discussions endeavor to answer two main questions: what is the origin of colicin-binding affinity and specificity, and after adsorption to OM porins, do colicin polypeptides translocate through porin channels, or enter by another, currently unknown pathway?

Expression of the putative siderophore receptor gene bfrZ is controlled by the extracytoplasmic-function sigma factor BupI in Bordetella bronchiseptica

A new gene from Bordetella bronchiseptica, bfrZ encoding a putative siderophore receptor, was identified in a Fur-repressor titration assay. A bfrZ null mutant was constructed by allelic exchange. The protein profile of this mutant is similar to that of the wild-type parent strain. The BfrZ(-)-BfrZ(+) isogenic pair was tested for utilization of 132 different siderophores as iron sources. None of these iron sources acted as a ligand for BfrZ. Translational bfrZ::phoA and transcriptional bfrZ::lacZ fusions were introduced into the B. bronchiseptica bfrZ locus. No alkaline phosphatase or beta-galactosidase activity was detected. Sequence analysis of the bfrZ upstream region revealed the presence of two tightly linked genes, bupI and bupR. Both of these genes are located downstream from a Fur-binding sequence. BupI is homologous to Escherichia coli FecI and Pseudomonas putida PupI and belongs to the family of extracytoplasmic-function sigma factors involved in transcription of genes with extracytoplasmic functions. BupR is homologous to the FecR and PupR antisigma factors and is predicted to be localized in the inner membrane. Similar to the surface signaling receptors FecA and PupB, BfrZ bears an N-terminal extension. We found that bfrZ is not transcribed when bupI and bupR are expressed at the same level. However, overexpression of bupI from a multicopy plasmid triggers bfrZ transcription, and under these conditions BfrZ was detected in membrane fractions. By analogy with the FecI-FecR-FecA and PupI-PupR-PupB systems, our data suggest that bfrZ expression is inducible by binding of the cognate ligand to BfrZ and transduction of a signal through the envelope.

Exchangeability of N termini in the ligand-gated porins of Escherichia coli

The ferric siderophore transporters of the Gram-negative bacterial outer membrane manifest a unique architecture: Their N termini fold into a globular domain that lodges within, and physically obstructs, a transmembrane porin beta-barrel formed by their C termini. We exchanged and deleted the N termini of two such siderophore receptors, FepA and FhuA, which recognize and transport ferric enterobactin and ferrichrome, respectively. The resultant chimeric proteins and empty beta-barrels avidly bound appropriate ligands, including iron complexes, protein toxins, and viruses. Thus, the ability to recognize and discriminate these molecules fully originates in the transmembrane beta-barrel domain. Both the hybrid and the deletion proteins also transported the ferric siderophore that they bound. The FepA constructs showed less transport activity than wild type receptor protein, but the FhuA constructs functioned with turnover numbers that were equivalent to wild type. The mutant proteins displayed the full range of transport functionalities, despite their aberrant or missing N termini, confirming (Braun, M., Killmann, H., and Braun, V. (1999) Mol. Microbiol. 33, 1037-1049) that the globular domain within the pore is dispensable to the siderophore internalization reaction, and when present, acts without specificity during solute uptake. These and other data suggest a transport process in which siderophore receptors undergo multiple conformational states that ultimately expel the N terminus from the channel concomitant with solute internalization.

Binding of ferric enterobactin by the Escherichia coli periplasmic protein FepB

The periplasmic protein FepB of Escherichia coli is a component of the ferric enterobactin transport system. We overexpressed and purified the binding protein 23-fold from periplasmic extracts by ammonium sulfate precipitation and chromatographic methods, with a yield of 20%, to a final specific activity of 15,500 pmol of ferric enterobactin bound/mg. Periplasmic fluid from cells overexpressing the binding protein adsorbed catecholate ferric siderophores with high affinity: in a gel filtration chromatography assay the K(d) of the ferric enterobactin-FepB binding reaction was approximately 135 nM. Intrinsic fluorescence measurements of binding by the purified protein, which were more accurate, showed higher affinity for both ferric enterobactin (K(d) = 30 nM) and ferric enantioenterobactin (K(d) = 15 nM), the left-handed stereoisomer of the natural E. coli siderophore. Purified FepB also adsorbed the apo-siderophore, enterobactin, with comparable affinity (K(d) = 60 nM) but did not bind ferric agrobactin. Polyclonal rabbit antisera and mouse monoclonal antibodies raised against nearly homogeneous preparations of FepB specifically recognized it in solid-phase immunoassays. These sera enabled the measurement of the FepB concentration in vivo when expressed from the chromosome (4,000 copies/cell) or from multicopy plasmids (>100,000 copies/cell). Overexpression of the binding protein did not enhance the overall affinity or rate of ferric enterobactin transport, supporting the conclusion that the rate-limiting step of ferric siderophore uptake through the cell envelope is passage through the outer membrane.

Microbial iron transport via a siderophore shuttle: a membrane ion transport paradigm

A mechanism of ion transport across membranes is reported. Microbial transport of Fe(3+) generally delivers iron, a growth-limiting nutrient, to cells via highly specific siderophore-mediated transport systems. In contrast, iron transport in the fresh water bacterium Aeromonas hydrophila is found to occur by means of an indiscriminant siderophore transport system composed of a single multifunctional receptor. It is shown that (i) the siderophore and Fe(3+) enter the bacterium together, (ii) a ligand exchange step occurs in the course of the transport, and (iii) a redox process is not involved in iron exchange. To the best of our knowledge, there have been no other reports of a ligand exchange mechanism in bacterial iron transport. The ligand exchange step occurs at the cell surface and involves the exchange of iron from a ferric siderophore to an iron-free siderophore already bound to the receptor. This ligand exchange mechanism is also found in Escherichia coli and seems likely to be widely distributed among microorganisms.

Aromatic components of two ferric enterobactin binding sites in Escherichia coli FepA

Ferric enterobactin is a catecholate siderophore that binds with high affinity (Kd approximately 10-10 M) to the Escherichia coli outer membrane protein FepA. We studied the involvement of aromatic amino acids in its uptake by determining the binding affinities, kinetics and transport properties of site-directed mutants. We replaced seven aromatic residues (Y260, Y272, Y285, Y289, W297, Y309 and F329) in the central part of FepA primary structure with alanine, individually and in double combinations, and determined the ability of the mutant proteins to interact with ferric enterobactin and the protein toxins colicins B and D. All the constructs showed normal expression and localization. Among single mutants, Y260A and F329A were most detrimental, reducing the affinity between FepA and ferric enterobactin 100- and 10-fold respectively. Double substitutions involving Y260, Y272 and F329 impaired (100- to 2500-fold) adsorption of the iron chelate more strongly. For Y260A and Y272A, the drop in adsorption affinity caused commensurate decreases in transport efficiency, suggesting that the target residues primarily act in ligand binding. F329A, like R316A, showed greater impairment of transport than binding, intimating mechanistic involvement during ligand internalization. Furthermore, immunochemical studies localized F329 in the FepA ligand binding site. The mutagenesis results suggested the existence of dual ligand binding sites in the FepA vestibule, and measurements of the rate of ferric enterobactin adsorption to fluoresceinated FepA mutant proteins confirmed this conclusion. The initial, outermost site contains aromatic residues and probably functions through hydrophobic interactions, whereas the secondary site exists deeper in the vestibule, contains both charged and aromatic residues and probably acts through hydrophobic and electrostatic bonds.