Emergence of Ferrichelatase Activity in a Siderophore-Binding Protein Supports an Iron Shuttle in Bacteria

A Siderophore Mimicking Gelation Component for Capturing and Self-Separation of Fe(III) from an Aqueous Solution of Mixture of Metal Ions

The carbohydrazide-based gelation component N2,N4,N6-(1,3,5-triazine-2,4,6-triyl)tris(benzene-1,3,5-tricarbohydrazide) (CBTC) was synthesized and characterized using various spectroscopic tools. CBTC and trimesic acid (TMA) get self-assembled to form metallogel with Fe3+, specifically through various noncovalent interactions in a DMSO and H2O mixture. The self-assembly shows remarkable specificity toward Fe(III) among different transition metal salts. It is pertinent to point out that the binding specificity for Fe3+ can also be found in nature in the form of siderophores, as they are mainly involved in scavenging iron selectively from the surroundings. DFT studies have been used to investigate the possible interaction between the different components of the iron metallogel. To determine the selectivity of CBTC for iron, CBTC, along with trimesic acid, is used to interact with other metal ions, including Fe(III) ions, in a single system. The gelation components CBTC and TMA selectively bind with iron(III), which leads to the formation of metallogel and gets separated as a discrete layer, leaving the other metal ions in the solution. Therefore, CBTC and TMA together show iron-scavenging properties. This selective scavenging property is explored through FE-SEM, XPS, PXRD, IR, and ICP-AES analysis. The FE-SEM analysis shows a flower-petal-like morphology for the Fe(III) metallogel. The resemblance in the CBTC-TMA-Fe metallogel and metallogel obtained from the mixture of different metal salts is established through FE-SEM images and XPS analysis. The release of iron from the metallogel is achieved with the help of ascorbic acid, which converts Fe3+ to Fe2+. In biological systems, iron also gets released similarly from siderophores. This is the first report where the synthesized gelation component CBTC molecule is capable of scavenging out iron in the form of metallogel and self-separating from the aqueous mixture in the presence of various other metal ions.

Reduction-cleavable desferrioxamine B pulldown system enriches Ni(ii)-superoxide dismutase from a Streptomyces proteome

Two resins with the hydroxamic acid siderophore desferrioxamine B (DFOB) immobilised as a free ligand or its Fe(iii) complex were prepared to screen the Streptomyces pilosus proteome for proteins involved in siderophore-mediated Fe(iii) uptake. The resin design included a disulfide bond to enable the release of bound proteins under mild reducing conditions. Proteomics analysis of the bound fractions did not identify proteins associated with siderophore-mediated Fe(iii) uptake, but identified nickel superoxide dismutase (NiSOD), which was enriched on the apo-DFOB-resin but not the Fe(iii)-DFOB-resin or the control resin. While DFOB is unable to sequester Fe(iii) from sites deeply buried in metalloproteins, the coordinatively unsaturated Ni(ii) ion in NiSOD is present in a surface-exposed loop region at the N-terminus, which might enable partial chelation. The results were consistent with the notion that the apo-DFOB-resin formed a ternary complex with NiSOD, which was not possible for either the coordinatively saturated Fe(iii)-DFOB-resin or the non-coordinating control resin systems. In support, ESI-TOF-MS measurements from a solution of a model Ni(ii)-SOD peptide and DFOB showed signals that correlated with a ternary Ni(ii)-SOD peptide-DFOB complex. Although any biological implications of a DFOB-NiSOD complex are unclear, the work shows that the metal coordination properties of siderophores might influence an array of metal-dependent biological processes beyond those established in iron uptake.

Microbial interactions within beneficial consortia promote soil health

By ecologically interacting with various biotic and abiotic agents acting in soil ecosystems, highly diverse soil microorganisms establish complex and stable assemblages and survive in a community context in natural settings. Besides facilitating soil microbiome to maintain great levels of population homeostasis, such microbial interactions drive soil microbes to function as the major engine of terrestrial biogeochemical cycling. It is verified that the regulative effect of microbe-microbe interplay plays an instrumental role in microbial-mediated promotion of soil health, including bioremediation of soil pollutants and biocontrol of soil-borne phytopathogens, which is considered an environmentally friendly strategy for ensuring the healthy condition of soils. Specifically, in microbial consortia, it has been proven that microorganism-microorganism interactions are involved in enhancing the soil health-promoting effectiveness (i.e., efficacies of pollution reduction and disease inhibition) of the beneficial microbes, here defined as soil health-promoting agents. These microbial interactions can positively regulate the soil health-enhancing effect by supporting those soil health-promoting agents utilized in combination, as multi-strain soil health-promoting agents, to overcome three main obstacles: inadequate soil colonization, insufficient soil contaminant eradication and inefficient soil-borne pathogen suppression, all of which can restrict their probiotic functionality. Yet the mechanisms underlying such beneficial interaction-related adjustments and how to efficiently assemble soil health-enhancing consortia with the guidance of microbe-microbe communications remain incompletely understood. In this review, we focus on bacterial and fungal soil health-promoting agents to summarize current research progress on the utilization of multi-strain soil health-promoting agents in the control of soil pollution and soil-borne plant diseases. We discuss potential microbial interaction-relevant mechanisms deployed by the probiotic microorganisms to upgrade their functions in managing soil health. We emphasize the interplay-related factors that should be taken into account when building soil health-promoting consortia, and propose a workflow for assembling them by employing a reductionist synthetic community approach.

Iron acquisition by a commensal bacterium modifies host nutritional immunity during Salmonella infection

During intestinal inflammation, host nutritional immunity starves microbes of essential micronutrients, such as iron. Pathogens scavenge iron using siderophores, including enterobactin; however, this strategy is counteracted by host protein lipocalin-2, which sequesters iron-laden enterobactin. Although this iron competition occurs in the presence of gut bacteria, the roles of commensals in nutritional immunity involving iron remain unexplored. Here, we report that the gut commensal Bacteroides thetaiotaomicron acquires iron and sustains its resilience in the inflamed gut by utilizing siderophores produced by other bacteria, including Salmonella, via a secreted siderophore-binding lipoprotein XusB. Notably, XusB-bound enterobactin is less accessible to host sequestration by lipocalin-2 but can be "re-acquired" by Salmonella, allowing the pathogen to evade nutritional immunity. Because the host and pathogen have been the focus of studies of nutritional immunity, this work adds commensal iron metabolism as a previously unrecognized mechanism modulating the host-pathogen interactions and nutritional immunity.

Biofortification revisited: Addressing the role of beneficial soil microbes for enhancing trace elements concentration in staple crops

Trace element deficiency is a pervasive issue contributing to malnutrition on a global scale. The primary cause of this hidden hunger is related to low dietary intake of essential trace elements, which is highly prevalent in numerous regions across the world. To address deficiency diseases in humans, fortification of staple crops with vital trace elements has emerged as a viable solution. Current methods for fortifying crops encompass chemical amendments, genetic breeding, and transgenic approaches, yet these approaches possess certain limitations, constraining their agricultural application. In contrast, fortifying staple crops through the utilization of soil-beneficial microbes has emerged as a promising and economically feasible approach to enhance trace element content in crops. A specific subset of these beneficial soil microbes, referred to as plant growth-promoting microbes, have demonstrated their ability to influence the interactions between plants, soil, and minerals. These microbes facilitate the transport of essential soil minerals, such as zinc, iron, and selenium, into plants, offering the potential for the development of tailored bioinoculants that can enhance the nutritional quality of cereals, pulses, and vegetable crops. Nevertheless, further research efforts are necessary to comprehensively understand the molecular mechanisms underlying the uptake, transport, and augmentation of trace element concentrations in staple crops. By delving deeper into these mechanisms, customized bioinoculants of soil-beneficial microbes can be developed to serve as highly effective strategies in combating trace element deficiency and promoting global nutritional well-being.

Microbial siderophores as molecular shuttles for metal cations: sources, sinks and application perspectives

Iron is one of the highly abundant elements on the earth's crust, an essential micronutrient for a majority of life forms, and exists in two frequent oxidation states such as ferrous (Fe2+) and ferric (Fe3+). These two oxidation states are interconvertible by redox reactions and form complexes with a wide range of siderophores. At neutral pH in soil, Fe2+ is highly soluble upto 100 mM but have less biological value, whereas Fe3+ is less soluble upto 10-9 M. This reduced bioavailability of Fe3+ induces competition among microorganisms. As many microorganisms need at least 10-6 M of Fe3+ form of iron for their growth, siderophores from these microbes readily withdraw Fe3+ iron from a variety of habitats for their survival. In this review, we bring into light the several recent investigations related to diverse chemistry of microbial siderophores, mechanisms of siderophore uptake, biosynthetic gene clusters in microbial genomes, various sources of heavy metal cations in soil, siderophore-binding protein receptors and commercialisation perspectives of siderophores. Besides, this review unearths the recent advancements in the characterisation of novel siderophores and its heavy metal complexes alongside the interaction kinetics with receptors.

Iron acquisition by a commensal bacterium modifies host nutritional immunity during Salmonella infection

During intestinal inflammation, host nutritional immunity starves microbes of essential micronutrients such as iron. Pathogens scavenge iron using siderophores, which is counteracted by the host using lipocalin-2, a protein that sequesters iron-laden siderophores, including enterobactin. Although the host and pathogens compete for iron in the presence of gut commensal bacteria, the roles of commensals in nutritional immunity involving iron remain unexplored. Here, we report that the gut commensal Bacteroides thetaiotaomicron acquires iron in the inflamed gut by utilizing siderophores produced by other bacteria including Salmonella, via a secreted siderophore-binding lipoprotein termed XusB. Notably, XusB-bound siderophores are less accessible to host sequestration by lipocalin-2 but can be "re-acquired" by Salmonella , allowing the pathogen to evade nutritional immunity. As the host and pathogen have been the focus of studies of nutritional immunity, this work adds commensal iron metabolism as a previously unrecognized mechanism modulating the interactions between pathogen and host nutritional immunity.

Draft genome of Raoultella planticola, a high lead resistance bacterium from industrial wastewater

Isolation of heavy metals-resistant bacteria from their original habitat is a crucial step in bioremediation. Six lead (Pb) resistant bacterial strains were isolated and identified utilizing 16S rRNA to be Enterobacter ludwigii FACU 4, Shigella flexneri FACU, Microbacterium paraoxydans FACU, Klebsiella pneumoniae subsp. pneumonia FACU, Raoultella planticola FACU 3 and Staphylococcus xylosus FACU. It was determined that all these strains had their Minimum inhibitory concentration (MIC) to be 2500 ppm except R. planticola FACU 3 has a higher maximum tolerance concentration (MTC) up to 2700 ppm. We evaluated the survival of all six strains on lead stress, the efficiency of biosorption and lead uptake. It was found that R. planticola FACU 3 is the highest MTC and S. xylosus FACU was the lowest MTC in this evaluation. Therefore, transmission electron microscopy (TEM) confirmed the difference between the morphological responses of these two strains to lead stress. These findings led to explore more about the genome of R. planticola FACU 3 using illumine Miseq technology. Draft genome sequence analysis revealed the genome size of 5,648,460 bp and G + C content 55.8% and identified 5526 CDS, 75 tRNA and 4 rRNA. Sequencing technology facilitated the identification of about 47 genes related to resistance to many heavy metals including lead, arsenic, zinc, mercury, nickel, silver and chromium of R. planticola FACU 3 strain. Moreover, genome sequencing identified plant growth-promoting genes (PGPGs) including indole acetic acid (IAA) production, phosphate solubilization, phenazine production, trehalose metabolism and 4-hydroxybenzoate production genes and a lot of antibiotic-resistant genes.

The role of bacterial ATP-binding cassette (ABC) transporters in pathogenesis and virulence: Therapeutic and vaccine potential

The ATP-binding cassette (ABC) transporter superfamily is found in all domains of life, facilitating critical biological processes through the translocation of a wide variety of substrates from, ions to proteins, across cellular membranes in an ATP-coupled process. The role of ABC transporters in eukaryotes has been well established: the facilitation of genetic diseases and multi-drug resistance (MDR) in cancer patients. In contrast, the role of ABC transporters in prokaryotes has been ambiguous due to their diverse functions and the sheer number of organisms in which they reside. This review examines the role of bacterial ABC transporters in pathogenesis and virulence, and their potential for therapeutic and vaccine application. We demonstrate how ABC transporters play a vital role in the virulence and pathogenesis of several pathogenic bacteria through the import of essential molecules, such as metal ions, amino acids, peptides, vitamins and osmoprotectants, as well as, the export of virulent determinants involved in glycoconjugate biosynthesis and Type I secretion. Furthermore, ABC exporters facilitate the persistence of pathogenic bacteria through the export of toxic xenobiotic substances, thus, contributing to the development of antimicrobial resistance. We also show that ABC transporters display considerable potential for therapeutic application through immunisation and resistance reversal. In conclusion, bacterial ABC transporters play an immense role in virulence and pathogenesis and display desirable traits for clinical use, therefore, potentially aiding in the battle against MDR.

ThIV-Desferrioxamine: characterization of a fluorescent bacterial probe

Diversifying our ability to guard against emerging pathogenic threats is essential for keeping pace with global health challenges, including those presented by drug-resistant bacteria. Some modern diagnostic and therapeutic innovations to address this challenge focus on targeting methods that exploit bacterial nutrient sequestration pathways, such as the desferrioxamine (DFO) siderophore used by Staphylococcus aureus (S. aureus) to sequester Fe^III. Building on recent studies that have shown DFO to be a versatile vehicle for chemical delivery, we show proof-of-principle that the Fe^III sequestration pathway can be used to deliver a potential radiotherapeutic. Our approach replaces the Fe^III nutrient sequestered by H₄DFO⁺ with Th^IV and made use of a common fluorophore, FITC, which we covalently bonded to DFO to provide a combinatorial probe for simultaneous chelation paired with imaging and spectroscopy, H₃DFO_FITC. Combining insight provided from FITC-based imaging with characterization by NMR spectroscopy, we demonstrated that the fluorescent DFO_FITC conjugate retained the Th^IV chelation properties of native H₄DFO⁺. Fluorescence microscopy with both [Th(DFO_FITC)] and [Fe(DFO_FITC)] complexes showed similar uptake by S. aureus and increased intercellular accumulation as compared to the FITC and unchelated H₃DFO_FITC controls. Collectively, these results demonstrate the potential for the newly developed H₃DFO_FITC conjugate to be used as a targeting vector and bacterial imaging probe for S. aureus. The results presented within provide a framework to expand H₄DFO⁺ and H₃DFO_FITC to relevant radiotherapeutics (like ²²⁷Th).

Synthetic Mimics of Native Siderophores Disrupt Iron Trafficking in Acinetobacter baumannii

Many pathogenic bacteria biosynthesize and excrete small molecule metallophores, known as siderophores, that are used to extract ferric iron from host sources to satisfy nutritional need. Native siderophores are often structurally complex multidentate chelators that selectively form high-affinity octahedral ferric iron complexes with defined chirality recognizable by cognate protein receptors displayed on the bacterial cell surface. Simplified achiral analogues can serve as synthetically tractable siderophore mimics with potential utility as chemical probes and therapeutic agents to better understand and treat bacterial infections, respectively. Here, we demonstrate that synthetic spermidine-derived mixed ligand bis-catecholate monohydroxamate siderophores (compounds 1-3) are versatile structural and biomimetic analogues of two native siderophores, acinetobactin and fimsbactin, produced by Acinetobacter baumannii, a multidrug-resistant Gram-negative human pathogen. The metal-free and ferric iron complexes of the synthetic siderophores are growth-promoting agents of A. baumannii, while the Ga(III)-complexes are potent growth inhibitors of A. baumannii with MIC values <1 μM. The synthetic siderophores compete with native siderophores for uptake in A. baumannii and maintain comparable apparent binding affinities for ferric iron (K_Fe) and the siderophore-binding protein BauB (K_d). Our findings provide new insight to guide the structural fine-tuning of these compounds as siderophore-based therapeutics targeting pathogenic strains of A. baumannii.