Inhibiting Iron Mobilization from Bacterioferritin in Pseudomonas aeruginosa Impairs Biofilm Formation Irrespective of Environmental Iron Availability

The Ferroxidase Centre of Escherichia coli Bacterioferritin Plays a Key Role in the Reductive Mobilisation of the Mineral Iron Core

Ferritins are multimeric cage-forming proteins that play a crucial role in cellular iron homeostasis. All H-chain-type ferritins harbour a diiron site, the ferroxidase centre, at the centre of a 4 α-helical bundle, but bacterioferritins are unique in also binding 12 hemes per 24 meric assembly. The ferroxidase centre is known to be required for the rapid oxidation of Fe2+ during deposition of an immobilised ferric mineral core within the protein's hollow interior. In contrast, the heme of bacterioferritin is required for the efficient reduction of the mineral core during iron release, but has little effect on the rate of either oxidation or mineralisation of iron. Thus, the current view is that these two cofactors function in iron uptake and release, respectively, with no functional overlap. However, rapid electron transfer between the heme and ferroxidase centre of bacterioferritin from Escherichia coli was recently demonstrated, suggesting that the two cofactors may be functionally connected. Here we report absorbance and (magnetic) circular dichroism spectroscopies, together with in vitro assays of iron-release kinetics, which demonstrate that the ferroxidase centre plays an important role in the reductive mobilisation of the bacterioferritin mineral core, which is dependent on the heme-ferroxidase centre electron transfer pathway.

Novel lactoferrin-conjugated gallium complex to treat Pseudomonas aeruginosa wound infection

Pseudomonas aeruginosa is one of the leading causes of opportunistic infections such as chronic wound infection that could lead to multiple organ failure and death. Gallium (Ga3+) ions are known to inhibit P. aeruginosa growth and biofilm formation but require carrier for localized controlled delivery. Lactoferrin (LTf), a two-lobed protein, can deliver Ga3+ at sites of infection. This study aimed to develop a Ga-LTf complex for the treatment of wound infection. The characterisation of the Ga-LTf complex was conducted using differential scanning calorimetry (DSC), Infra-Red (FTIR) and Inductive Coupled Plasma Optical Emission Spectrometry (ICP-OES). The antibacterial activity was assessed by agar disc diffusion, liquid broth and biofilm inhibition assays using the colony forming units (CFUs). The healing capacity and biocompatibility were evaluated using a P.aeruginosa infected wound in a rat model. DSC analyses showed thermal transition consistent with apo-lactoferrin; FTIR confirmed the complexation of gallium to lactoferrin. ICP-OES confirmed the controlled local delivery of Ga3+. Ga-LTf showed a 0.57 log10 CFUs reduction at 24 h compared with untreated control in planktonic liquid broth assay. Ga-LTf showed the highest antibiofilm activity with a 2.24 log10 CFUs reduction at 24 h. Furthermore, Ga-LTf complex is biocompatible without any adverse effect on brain, kidney, liver and spleen of rats tested in this study. Ga-LTf can be potentially promising novel therapeutic agent to treat pathogenic bacterial infections.

Mobilization of iron stored in bacterioferritin, a new target for perturbing iron homeostasis and developing antibacterial and antibiofilm molecules

Antibiotic resistance is a global public health threat. The care of chronic infections is complicated by bacterial biofilms. Biofilm embedded cells can be up to 1000-fold more tolerant to antibiotic treatment than planktonic cells. Antibiotic tolerance is a condition which does not involve mutation and enables bacteria to survive in the presence of antibiotics. The antibiotic tolerance of biofilm-cells often renders antibiotics ineffective, even against strains that do not carry resistance-impairing mutations. This review discusses bacterial iron homeostasis and the strategies being developed to target this bacterial vulnerability, with emphasis on a recently proposed approach which aims at targeting the iron storage protein bacterioferritin (Bfr) and its physiological partner, the ferredoxin Bfd. Bfr regulates cytosolic iron concentrations by oxidizing Fe2+ and storing Fe3+ in its internal cavity, and by forming a complex with Bfd to reduce Fe3+ in the internal cavity and release Fe2+ to the cytosol. Blocking the Bfr-Bfd complex in P. aeruginosa cells causes an irreversible accumulation of Fe3+ in BfrB and simultaneous cytosolic iron depletion, which leads to impaired biofilm maintenance and biofilm cell death. Recently discovered small molecule inhibitors of the Bfr-Bfd complex, which bind Bfr at the Bfd binding site, inhibit iron mobilization, and elicit biofilm cell death.

Unraveling the structure and function of bacterioferritin in Candidatus Kuenenia stuttgartiensis: Iron storage sites maintain cellular iron homeostasis

Anammox bacteria rely heavily on iron and have many iron storage sites. However, the biological significance of these iron storage sites has not been clearly defined. In this study, we explored the properties and location of iron storage sites to better understand their cellular function. To do this, the Candidatus Kuenenia stuttgartiensis iron storage protein, bacterioferritin (K.S Bfr), was successfully expressed and purified. In vitro, correctly assembled globulins were observed by transmission electron microscopy. The self-assembled K.S Bfr has active redox and can bind Fe²⁺ and mineralize it in the protein cavity. In vivo, engineered bacteria with K.S Bfr showed good adaptability to Fe²⁺, with a survival rate of 78.9% when exposed to 5 mM Fe²⁺, compared with only 66.0% for wild-type bacteria lacking K.S Bfr. A potential iron regulatory strategy similar to that of Anammox was identified in transcriptomic analysis of engineered bacteria. This system may be controlled by the iron uptake regulator Furto transport Fe²⁺ via FeoB and store excess Fe²⁺ in K.S Bfr to maintain cellular homeostasis. K.S Bfr has superior iron storage capacity both intracellularly and in vitro. The discovery of K.S Bfr reveals the storage location of iron-rich nanoparticles, increases our understanding of the adaptability of iron-dependent bacteria to Fe²⁺, and suggests possible iron regulation strategies in Anammox bacteria.

Optimization of Host Cell-Compatible, Antimicrobial Peptides Effective against Biofilms and Clinical Isolates of Drug-Resistant Bacteria

Here, we describe the continued synthetic molecular evolution of a lineage of host-compatible antimicrobial peptides (AMP) intended for the treatment of wounds infected with drug-resistant, biofilm-forming bacteria. The peptides tested are variants of an evolved AMP called d-amino acid CONsensus with Glycine Absent (d-CONGA), which has excellent antimicrobial activities in vitro and in vivo. In this newest generation of rational d-CONGA variants, we tested multiple sequence-structure-function hypotheses that had not been tested in previous generations. Many of the peptide variants have lower antibacterial activity against Gram-positive or Gram-negative pathogens, especially variants that have altered hydrophobicity, secondary structure potential, or spatial distribution of charged and hydrophobic residues. Thus, d-CONGA is generally well tuned for antimicrobial activity. However, we identified a variant, d-CONGA-Q7, with a polar glutamine inserted into the middle of the sequence, that has higher activity against both planktonic and biofilm-forming bacteria as well as lower cytotoxicity against human fibroblasts. Against clinical isolates of Klebsiella pneumoniae, innate resistance to d-CONGA was surprisingly common despite a lack of inducible resistance in Pseudomonas aeruginosa reported previously. Yet, these same isolates were susceptible to d-CONGA-Q7. d-CONGA-Q7 is much less vulnerable to AMP resistance in Gram-negative bacteria than its predecessor. Consistent with the spirit of synthetic molecular evolution, d-CONGA-Q7 achieved a critical gain-of-function and has a significantly better activity profile.

The action of phytochemicals in biofilm control

Covering: 2009 to 2021Antimicrobial resistance is now rising to dangerously high levels in all parts of the world, threatening the treatment of an ever-increasing range of infectious diseases. This has becoming a serious public health problem, especially due to the emergence of multidrug-resistance among clinically important bacterial species and their ability to form biofilms. In addition, current anti-infective therapies have low efficacy in the treatment of biofilm-related infections, leading to recurrence, chronicity, and increased morbidity and mortality. Therefore, it is necessary to search for innovative strategies/antibacterial agents capable of overcoming the limitations of conventional antibiotics. Natural compounds, in particular those obtained from plants, have been exhibiting promising properties in this field. Plant secondary metabolites (phytochemicals) can act as antibiofilm agents through different mechanisms of action from the available antibiotics (inhibition of quorum-sensing, motility, adhesion, and reactive oxygen species production, among others). The combination of different phytochemicals and antibiotics have revealed synergistic or additive effects in biofilm control. This review aims to bring together the most relevant reports on the antibiofilm properties of phytochemicals, as well as insights into their structure and mechanistic action against bacterial pathogens, spanning December 2008 to December 2021.

Pseudomonas aeruginosa Dps (PA0962) Functions in H2O2 Mediated Oxidative Stress Defense and Exhibits In Vitro DNA Cleaving Activity

We report the structural, biochemical, and functional characterization of the product of gene PA0962 from Pseudomonas aeruginosa PAO1. The protein, termed Pa Dps, adopts the Dps subunit fold and oligomerizes into a nearly spherical 12-mer quaternary structure at pH 6.0 or in the presence of divalent cations at neutral pH and above. The 12-Mer Pa Dps contains two di-iron centers at the interface of each subunit dimer, coordinated by conserved His, Glu, and Asp residues. In vitro, the di-iron centers catalyze the oxidation of Fe2+ utilizing H2O2 (not O2) as an oxidant, suggesting Pa Dps functions to aid P. aeruginosa to survive H2O2-mediated oxidative stress. In agreement, a P. aeruginosa Δdps mutant is significantly more susceptible to H2O2 than the parent strain. The Pa Dps structure harbors a novel network of Tyr residues at the interface of each subunit dimer between the two di-iron centers, which captures radicals generated during Fe2+ oxidation at the ferroxidase centers and forms di-tyrosine linkages, thus effectively trapping the radicals within the Dps shell. Surprisingly, incubating Pa Dps and DNA revealed unprecedented DNA cleaving activity that is independent of H2O2 or O2 but requires divalent cations and 12-mer Pa Dps.

Text-Mining to Identify Gene Sets Involved in Biocorrosion by Sulfate-Reducing Bacteria: A Semi-Automated Workflow

A significant amount of literature is available on biocorrosion, which makes manual extraction of crucial information such as genes and proteins a laborious task. Despite the fast growth of biology related corrosion studies, there is a limited number of gene collections relating to the corrosion process (biocorrosion). Text mining offers a potential solution by automatically extracting the essential information from unstructured text. We present a text mining workflow that extracts biocorrosion associated genes/proteins in sulfate-reducing bacteria (SRB) from literature databases (e.g., PubMed and PMC). This semi-automatic workflow is built with the Named Entity Recognition (NER) method and Convolutional Neural Network (CNN) model. With PubMed and PMCID as inputs, the workflow identified 227 genes belonging to several Desulfovibrio species. To validate their functions, Gene Ontology (GO) enrichment and biological network analysis was performed using UniprotKB and STRING-DB, respectively. The GO analysis showed that metal ion binding, sulfur binding, and electron transport were among the principal molecular functions. Furthermore, the biological network analysis generated three interlinked clusters containing genes involved in metal ion binding, cellular respiration, and electron transfer, which suggests the involvement of the extracted gene set in biocorrosion. Finally, the dataset was validated through manual curation, yielding a similar set of genes as our workflow; among these, hysB and hydA, and sat and dsrB were identified as the metal ion binding and sulfur metabolism genes, respectively. The identified genes were mapped with the pangenome of 63 SRB genomes that yielded the distribution of these genes across 63 SRB based on the amino acid sequence similarity and were further categorized as core and accessory gene families. SRB's role in biocorrosion involves the transfer of electrons from the metal surface via a hydrogen medium to the sulfate reduction pathway. Therefore, genes encoding hydrogenases and cytochromes might be participating in removing hydrogen from the metals through electron transfer. Moreover, the production of corrosive sulfide from the sulfur metabolism indirectly contributes to the localized pitting of the metals. After the corroboration of text mining results with SRB biocorrosion mechanisms, we suggest that the text mining framework could be utilized for genes/proteins extraction and significantly reduce the manual curation time.

Recombinant Actifensin and Defensin-d2 Induce Critical Changes in the Proteomes of Multidrug-Resistant Pseudomonas aeruginosa and Candida albicans

Drug-resistant strains of Pseudomonas aeruginosa and Candida albicans pose serious threats to human health because of their propensity to cause fatal infections. Defensin and defensin-like antimicrobial peptides (AMPs) are being explored as new lines of antimicrobials, due to their broad range of activity, low toxicity, and low pathogen resistance. Defensin-d2 and actifensin are AMPs from spinach and Actinomyces ruminicola, respectively, whose mechanisms of action are yet to be clearly elucidated. This study investigated the mechanisms of action of the recombinant AMPs through label-free quantitative proteomics. The data are available at PRIDE with accession number PXD034169. A total of 28 and 9 differentially expressed proteins (DEPs) were identified in the treated P. aeruginosa and C. albicans, respectively, with a 2-fold change threshold and P values of <0.05. Functional analysis revealed that the DEPs were involved in DNA replication and repair, translation, and membrane transport in P. aeruginosa, while they were related mainly to oxidative phosphorylation, RNA degradation, and energy metabolism in C. albicans. Protein-protein interactions showed that the DEPs formed linear or interdependent complexes with one another, indicative of functional interaction. Subcellular localization indicated that the majority of DEPs were cytoplasmic proteins in P. aeruginosa, while they were of nuclear or mitochondrial origin in C. albicans. These results show that recombinant defensin-d2 and actifensin can elicit complex multiple organism responses that cause cell death in P. aeruginosa and C. albicans. IMPORTANCE AMPs are considered essential alternatives to conventional antimicrobials because of their broad-spectrum efficacy and low potential for resistance by target cells. In this study, we established that the recombinant AMPs defensin-d2 and actifensin exert proteomic changes in P. aeruginosa and C. albicans within 1 h after treatment. We also found that the DEPs in peptide-treated P. aeruginosa are related to ion transport and homeostasis, molecular functions including nucleic and amino acid metabolism, and structural biogenesis and activity, while the DEPs in treated C. albicans are mainly involved in membrane synthesis and mitochondrial metabolism. Our results also highlight ATP synthase as a potential drug target for multidrug-resistant P. aeruginosa and C. albicans.

2-Hydroxymethyl-1-methyl-5-nitroimidazole, one siderophore inhibitor, occludes quorum sensing in Pseudomonas aeruginosa

Siderophore is necessary for the survival of microorganisms and is interregulated with quorum sensing (QS) systems. It is related to growth, proliferation, virulence, and other bacterial social activities as a virulence factor. Thus, we speculated that the QS system could be occluded by inhibiting siderophore production. 2-Hydroxymethyl-1-methyl-5-nitroimidazole (HMMN), one siderophore inhibitor of Pseudomonas aeruginosa PAO1 (P. aeruginosa PAO1), was obtained by using the Chromeazurol S (CAS) method. We found that HMMN inhibited siderophore production and influenced the biological effects of QS regulation, including biofilm formation and pyocyanin production. HMMN (150 μg/ml) inhibited the siderophore production of P. aeruginosa PAO1 by 69.37%. In addition, HMMN could inhibit pyocyanin production and biofilm formation and erase the formed biofilm of P. aeruginosa PAO1. HMMN (150 μg/ml) inhibited the biofilm formation of P. aeruginosa PAO1 by 28.24%. The erasure rate of the formed biofilm reached 17.03%. Furthermore, HMMN (150 μg/ml) inhibited P. aeruginosa PAO1 pyocyanin production by 36.06%. Meanwhile, positive-control hordenine (500.0 μg/ml) reduced the biofilm formation and pyocyanin production of P. aeruginosa PAO1 by 14.42% and 34.35%, respectively. The erasure rate of hordenine to the formed biofilm is 11.05% at 500 μg/ml. Quantitative real-time polymerase chain reaction (qRT-PCR) showed that HMMN downregulates not only siderophore-related genes but also QS-related genes, as well as hordenine. These results suggest that a siderophore inhibitor could be used as a QS inhibitor to occlude the QS system and reduce virulence.

Coordination of siderophore gene expression among clonal cells of the bacterium Pseudomonas aeruginosa

There has been great progress in understanding how bacterial groups coordinate social actions, such as biofilm formation and public-goods secretion. Less clear is whether the seemingly coordinated group-level responses actually mirror what individual cells do. Here, we use a microscopy approach to simultaneously quantify the investment of individual cells of the bacterium Pseudomonas aeruginosa into two public goods, the siderophores pyochelin and pyoverdine. Using gene expression as a proxy for investment, we initially observe no coordination but high heterogeneity and bimodality in siderophore investment across cells. With increasing cell density, gene expression becomes more homogenized across cells, accompanied by a moderate shift from pyochelin to pyoverdine expression. We find positive associations in the expression of pyochelin and pyoverdine genes across cells, with cell-to-cell variation correlating with cellular metabolic states. Our work suggests that siderophore-mediated signalling aligns behaviour of individuals over time and spurs a coordinated three-phase siderophore investment cycle.

Pseudomonas aeruginosa Bacterioferritin Is Assembled from FtnA and BfrB Subunits with the Relative Proportions Dependent on the Environmental Oxygen Availability

Ferritins are iron storage proteins assembled from 24 subunits into a spherical and hollow structure. The genomes of many bacteria harbor genes encoding two types of ferritin-like proteins, the bacterial ferritins (Ftn) and the bacterioferritins (Bfr), which bind heme. The genome of P. aeruginosa PAO1 (like the genomes of many bacteria) contains genes coding for two different types of ferritin-like molecules, ftnA (PA4235) and bfrB (PA3531). The reasons for requiring the presence of two distinct types of iron storage protein in bacterial cells have remained largely unexplained. Attempts to understand this issue in P. aeruginosa through the recombinant expression of the ftnA and bfrB genes in E. coli host cells, coupled to the biochemical and structural characterization of the recombinant 24-mer FtnA and 24-mer BfrB molecules, have shown that each of the recombinant molecules can form an Fe³⁺-mineral core. These observations led to the suggestion that 24-mer FtnA and 24-mer BfrB molecules coexist in P. aeruginosa cells where they share iron storage responsibilities. Herein, we demonstrate that P. aeruginosa utilizes a single heterooligomeric 24-mer Bfr assembled from FtnA and BfrB subunits. The relative content of the FtnA and BfrB subunits in Bfr depends on the O₂ availability during cell culture, such that Bfr isolated from aerobically cultured P. aeruginosa is assembled from a majority of BfrB subunits. In contrast, when the cells are cultured in O₂-limiting conditions, the proportion of FtnA subunits in the isolated Bfr increases significantly and can become the most abundant subunit. Despite the variability in the subunit composition of Bfr, the 24-mer assembly is consistently arranged from FtnA subunit dimers devoid of heme and BfrB subunit dimers each containing a heme molecule.

Osmotic stress induces long-term biofilm survival in Liberibacter crescens

Citrus greening, also known as Huanglongbing (HLB), is a devastating citrus plant disease caused predominantly by Liberibacter asiaticus. While nearly all Liberibacter species remain uncultured, here we used the culturable L. crescens BT-1 as a model to examine physiological changes in response to the variable osmotic conditions and nutrient availability encountered within the citrus host. Similarly, physiological responses to changes in growth temperature and dimethyl sulfoxide concentrations were also examined, due to their use in many of the currently employed therapies to control the spread of HLB. Sublethal heat stress was found to induce the expression of genes related to tryptophan biosynthesis, while repressing the expression of ribosomal proteins. Osmotic stress induces expression of transcriptional regulators involved in expression of extracellular structures, while repressing the biosynthesis of fatty acids and aromatic amino acids. The effects of osmotic stress were further evaluated by quantifying biofilm formation of L. crescens in presence of increasing sucrose concentrations at different stages of biofilm formation, where sucrose-induced osmotic stress delayed initial cell attachment while enhancing long-term biofilm viability. Our findings revealed that exposure to osmotic stress is a significant contributing factor to the long term survival of L. crescens and, possibly, to the pathogenicity of other Liberibacter species.

Integrating transcriptomics and metabolomics analysis on kojic acid combating Acinetobacter baumannii biofilm and its potential roles

As a major secondary metabolite derived from a dominant marine filamentous fungus A7, kojic acid might confer the strain a competitive advantage in natural colonization. The bioactivities of kojic acid against bacterial growth and biofilm formation were investigated against Acinetobacter baumannii (A. baumannii) ATCC 19606. Then, transcriptomics and metabolomics were integrated to characterize the underlying mechanisms. It turned out that kojic acid exhibited a significantly suppressive impact against biofilm but a weak bacteriostatic activity. Meanwhile, a variety of transcriptional and metabolomic profiles were altered within biofilm formation as a result of kojic acid exposure. The alterations highlighted the mechanisms underlying biofilm formation, comprising of quorum sensing, fimbria assembly, bacterial virulence and metabolic plasticity, which could somewhat be hampered by kojic acid. The present study comprehensively elucidated multifactorial schemes for kojic acid combating biofilm formation of A. baumannii, which might provide mechanistic insights into the development of therapeutic strategies against this notorious pathogen. Meanwhile, our observations might shed new light on the ecological roles of kojic acid, e.g., serving as chemical deterrents for host adaptation to marine niches, which, however, awaits further validation.

Bacterioferritin: a key iron storage modulator that affects strain growth and butenyl-spinosyn biosynthesis in Saccharopolyspora pogona

Background

Butenyl-spinosyn, produced by Saccharopolyspora pogona, is a promising biopesticide due to excellent insecticidal activity and broad pesticidal spectrum. Bacterioferritin (Bfr, encoded by bfr) regulates the storage and utilization of iron, which is essential for the growth and metabolism of microorganisms. However, the effect of Bfr on the growth and butenyl-spinosyn biosynthesis in S. pogona has not been explored.

Results

Here, we found that the storage of intracellular iron influenced butenyl-spinosyn biosynthesis and the stress resistance of S. pogona, which was regulated by Bfr. The overexpression of bfr increased the production of butenyl-spinosyn by 3.14-fold and enhanced the tolerance of S. pogona to iron toxicity and oxidative damage, while the knockout of bfr had the opposite effects. Based on the quantitative proteomics analysis and experimental verification, the inner mechanism of these phenomena was explored. Overexpression of bfr enhanced the iron storage capacity of the strain, which activated polyketide synthase genes and enhanced the supply of acyl-CoA precursors to improve butenyl-spinosyn biosynthesis. In addition, it induced the oxidative stress response to improve the stress resistance of S. pogona.

Conclusion

Our work reveals the role of Bfr in increasing the yield of butenyl-spinosyn and enhancing the stress resistance of S. pogona, and provides insights into its enhancement on secondary metabolism, which provides a reference for optimizing the production of secondary metabolites in actinomycetes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01651-x.

Can Targeting Iron Help in Combating Chronic Pseudomonas Infection? A Systematic Review

Cystic fibrosis is an autosomal recessive disorder caused by a mutation in genes for cystic fibrosis transmembrane conductance regulator (CFTR) protein. CFTR gene is responsible for the production of sweat, digestive fluids, and mucus, and any mutation in this would lead to the thickening of these secretions. Cystic fibrosis is a multi-organ disorder, but 80% of patients suffer from respiratory problems due to chronic infections most commonly caused by Pseudomonas aeruginosa (P. aeruginosa). Eradication of these infections has become a challenge as P. aeruginosa has developed resistance to multiple antibiotics. In several studies, iron has been shown to play an integral role in biofilm formation, which is the predominant resistance mechanism used by P. aeruginosa to combat antibiotics. The increased iron content in cystic fibrosis patients' sputum samples explains their increased susceptibility to Pseudomonas infections. Hence in this review article, we have used the research data available on therapeutic agents that target iron as an adjuvant treatment for chronic Pseudomonas infection. We systematically screened three databases using focused words and Medical Subject Headings (MeSH) terms for relevant articles. Further, we applied the inclusion and exclusion criteria and performed a thorough quality appraisal. Thirty shortlisted relevant studies were meticulously reviewed. In our opinion, novel therapeutic approaches targeting iron such as iron chelators, gallium, and cefiderocol have potent anti-biofilm properties. Future studies and clinical trials using these approaches in the management of chronic Pseudomonas infection might help in decreasing morbidity and mortality in patients with cystic fibrosis. Exploring these approaches might also help to combat other resistant organisms whose survival is dependent on iron.

Small Molecule Inhibitors of the Bacterioferritin (BfrB)-Ferredoxin (Bfd) Complex Kill Biofilm-Embedded Pseudomonas aeruginosa Cells

Bacteria depend on a well-regulated iron homeostasis to survive adverse environments. A key component of the iron homeostasis machinery is the compartmentalization of Fe3+ in bacterioferritin and its subsequent mobilization as Fe2+ to satisfy metabolic requirements. In Pseudomonas aeruginosa Fe3+ is compartmentalized in bacterioferritin (BfrB), and its mobilization to the cytosol requires binding of a ferredoxin (Bfd) to reduce the stored Fe3+ and release the soluble Fe2+. Blocking the BfrB-Bfd complex in P. aeruginosa by deletion of the bfd gene triggers an irreversible accumulation of Fe3+ in BfrB, concomitant cytosolic iron deficiency and significant impairment of biofilm development. Herein we report that small molecules developed to bind BfrB at the Bfd binding site block the BfrB-Bfd complex, inhibit the mobilization of iron from BfrB in P. aeruginosa cells, elicit a bacteriostatic effect on planktonic cells, and are bactericidal to cells embedded in mature biofilms.

Metallotherapeutics development in the age of iron-clad bacteria

Drug-resistant infections pose a significant risk to global health as pathogenic bacteria become increasingly difficult to treat. The rapid selection of resistant strains through poor antibiotic stewardship has reduced the number of viable treatments and increased morbidity of infections, especially among the immunocompromised. To circumvent such challenges, new strategies are required to stay ahead of emerging resistance trends, yet research and funding for antibiotic development lags other classes of therapeutics. Though the use of metals in therapeutics has been around for centuries, recent strategies have devoted a great deal of effort into the pathways through which bacteria acquire and utilize iron, which is critical for the establishment of infection. To target iron uptake systems, siderophore-drug conjugates have been developed that hijack siderophore-based iron uptake for delivery of antibiotics. While this strategy has produced several potential leads, the use of siderophores in infection is diminished over time when bacteria adapt to utilize heme as an iron source, leading to a need for the development of porphyrin mimetics as therapeutics. The use of such strategies as well as the inclusion of gallium, a redox-inert iron mimic, are herein reviewed.

Mobilization of Iron Stored in Bacterioferritin Is Required for Metabolic Homeostasis in Pseudomonas aeruginosa

Iron homeostasis offers a significant bacterial vulnerability because pathogens obtain essential iron from their mammalian hosts, but host-defenses maintain vanishingly low levels of free iron. Although pathogens have evolved mechanisms to procure host-iron, these depend on well-regulated iron homeostasis. To disrupt iron homeostasis, our work has targeted iron mobilization from the iron storage protein bacterioferritin (BfrB) by blocking a required interaction with its cognate ferredoxin partner (Bfd). The blockade of the BfrB–Bfd complex by deletion of the bfd gene (Δbfd) causes iron to irreversibly accumulate in BfrB. In this study we used mass spectrometry and NMR spectroscopy to compare the proteomic response and the levels of key intracellular metabolites between wild type (wt) and isogenic ΔbfdP. aeruginosa strains. We find that the irreversible accumulation of unusable iron in BfrB leads to acute intracellular iron limitation, even if the culture media is iron-sufficient. Importantly, the iron limitation and concomitant iron metabolism dysregulation trigger a cascade of events that lead to broader metabolic homeostasis disruption, which includes sulfur limitation, phenazine-mediated oxidative stress, suboptimal amino acid synthesis and altered carbon metabolism.

Agrobacterium tumefaciens ferritins play an important role in full virulence through regulating iron homeostasis and oxidative stress survival

Ferritins are a large family of iron storage proteins, which are used by bacteria and other organisms to avoid iron toxicity and as a safe iron source in the cytosol. Agrobacterium tumefaciens, a phytopathogen, has two ferritin-encoding genes: atu2771 and atu2477. Atu2771 is annotated as a Bfr-encoding gene (Bacterioferritin, Bfr) and atu2477 as a Dps-encoding gene (DNA binding protein from starved cells, Dps). Three deletion mutants (Δbfr, Δdps, and bfr-dps double-deletion mutant ΔbdF) of these two ferritin-encoding genes were constructed to investigate the effects of ferritin deficiency on the iron homeostasis, oxidative stress resistance, and pathogenicity of A. tumefaciens. Deficiency of two ferritins affects the growth of A. tumefaciens under iron starvation and excess. When supplied with moderate iron, the growth of A. tumefaciens is not affected by the deficiency of ferritin. Deficiency of ferritin significantly reduces iron accumulation in the cells of A. tumefaciens, but the effect of Bfr deficiency on iron accumulation is severer than Dps deficiency and the double mutant ΔbdF has the least intracellular iron content. All three ferritin-deficient mutants showed a decreased tolerance to 3 mM H2 O2 in comparison with the wild type. The tumour induced by each of three ferritin-deficient mutants is less than that of the wild type. Complementation reversed the effects of ferritin deficiency on the growth, iron homeostasis, oxidative stress resistance, and tumorigenicity of A. tumefaciens. Therefore, ferritin plays an important role in the pathogenesis of A. tumefaciens through regulating iron homeostasis and oxidative stress survival.