Location, Location, Location: Establishing Design Principles for New Antibacterials from Ferric Siderophore Transport Systems

This review strives to assemble a set of molecular design principles that enables the delivery of antibiotic warheads to Gram-negative bacterial targets (ESKAPE pathogens) using iron-chelating siderophores, known as the Trojan Horse strategy for antibiotic development. Principles are derived along two main lines. First, archetypical siderophores and their conjugates are used as case studies for native iron transport. They enable the consideration of the correspondence of iron transport and antibacterial target location. The second line of study charts the rationale behind the clinical antibiotic cefiderocol. It illustrates the potential versatility for the design of new Trojan Horse-based antibiotics. Themes such as matching the warhead to a location where the siderophore delivers its cargo (i.e., periplasm vs. cytoplasm), whether or not a cleavable linker is required, and the relevance of cheaters to the effectiveness and selectivity of new conjugates will be explored. The effort to articulate rules has identified gaps in the current understanding of iron transport pathways and suggests directions for new investigations.

Bacterial Siderophores: Classification, Biosynthesis, Perspectives of Use in Agriculture

Siderophores are synthesized and secreted by many bacteria, yeasts, fungi, and plants for Fe (III) chelation. A variety of plant-growth-promoting bacteria (PGPB) colonize the rhizosphere and contribute to iron assimilation by plants. These microorganisms possess mechanisms to produce Fe ions under iron-deficient conditions. Under appropriate conditions, they synthesize and release siderophores, thereby increasing and regulating iron bioavailability. This review focuses on various bacterial strains that positively affect plant growth and development through synthesizing siderophores. Here we discuss the diverse chemical nature of siderophores produced by plant root bacteria; the life cycle of siderophores, from their biosynthesis to the Fe-siderophore complex degradation; three mechanisms of siderophore biosynthesis in bacteria; the methods for analyzing siderophores and the siderophore-producing activity of bacteria and the methods for screening the siderophore-producing activity of bacterial colonies. Further analysis of biochemical, molecular-biological, and physiological features of siderophore synthesis by bacteria and their use by plants will allow one to create effective microbiological preparations for improving soil fertility and increasing plant biomass, which is highly relevant for sustainable agriculture.

Electrochemical and Solution Structural Characterization of Fe(III) Azotochelin Complexes: Examining the Coordination Behavior of a Tetradentate Siderophore

We report an electrochemical setup comprising a boron-doped diamond (BDD) working electrode for the electrochemical study of iron(III) catecholate siderophores. We demonstrate its successful application in the voltammetric investigation of iron(III) azotochelin, an iron complex of a bis(catecholate) siderophore. Cyclic voltammetry results, when complemented by UV-vis and native electrospray ionization-mass spectrometry (ESI-MS) characterization, reveal the formation of a coordinatively unsaturated tetracoordinate 1:1 complex of Fe:azotochelin (M₁:L₁) at neutral pH, contrary to iron(III) tetradentate siderophore complexes of other classes which favor the hexacoordinate environment of an M₂:L₃ species. A notable effect of pH and buffer composition on the reduction potential of iron(III) azotochelin is demonstrated. Lower pH values and buffers encompassing primary or secondary amines facilitate a positive potential shift of up to +290 mV and +250 mV vs Ag/AgCl 3 M NaCl, respectively. The study was extended to the investigation of the iron(III) complexes of hexadentate siderophores. For tris(catecholate) siderophores, enterobactin and protochelin, the reduction potentials were found to lie beyond the potential window accessible to the BDD electrode; however, we were successful in observing the electrochemical behavior of a tris(hydroxamate) siderophore, ferricrocin.

Novel Insights on Pyoverdine: From Biosynthesis to Biotechnological Application

Pyoverdines (PVDs) are a class of siderophores produced mostly by members of the genus Pseudomonas. Their primary function is to accumulate, mobilize, and transport iron necessary for cell metabolism. Moreover, PVDs also play a crucial role in microbes’ survival by mediating biofilm formation and virulence. In this review, we reorganize the information produced in recent years regarding PVDs biosynthesis and pathogenic mechanisms, since PVDs are extremely valuable compounds. Additionally, we summarize the therapeutic applications deriving from the PVDs’ use and focus on their role as therapeutic target themselves. We assess the current biotechnological applications of different sectors and evaluate the state-of-the-art technology relating to the use of synthetic biology tools for pathway engineering. Finally, we review the most recent methods and techniques capable of identifying such molecules in complex matrices for drug-discovery purposes.

Hydroxypyridinones in nitrogen-fixing bacterial cultures: A metal buffer for molybdenum and simulation of natural conditions

Organic matter regulates the availability of important trace elements in aquatic and terrestrial ecosystems by acting as a source and container for microbes. To overcome the limitation of trace elements, nitrogen-fixing bacteria, for example, release low-molecular-weight chelators (metallophores), which scavenge the essential cofactors of the nitrogenase, iron and molybdenum, via complexation and subsequent uptake. The formation of metallophores is triggered by limiting conditions, which must be replicated in the laboratory in order to study metallophores as a mediator in metal cycling. While EDTA-based buffer systems for metal cations are well established, there is limited knowledge regarding the buffering of oxoanions such as molybdate in a bacterial growth medium. To mimic the availability of molybdenum in nature under laboratory conditions, this study created a Mo-buffer system for bacterial growth media of the model organisms Azotobacter vinelandii and Frankia sp. CH37. We investigated selected hydroxypyridinones (HPs) as potential molybdenum-chelating agents, determining the amount required for efficient molybdenum complexation by calculating speciation plots of the various candidate complexes in artificial growth media at various pH values. The Mo-maltol system was identified as an ideal, non-toxic molybdenum-buffer system. In the presence of the Mo-maltol system, the growth of Frankia sp. was limited under diazotrophic conditions, whereas A. vinelandii could acquire molybdenum through the release of protochelin and subsequent molybdenum uptake. The study paves the way for unravelling molybdenum recruitment and homeostasis under limiting conditions in bacteria.

Roles of SmeYZ, SbiAB, and SmeDEF Efflux Systems in Iron Homeostasis of Stenotrophomonas maltophilia

Stenotrophomonas maltophilia, a nonfermenting Gram-negative rod, is frequently isolated from the environment and is emerging as a multidrug-resistant global opportunistic pathogen. S. maltophilia harbors eight RND-type efflux pumps that contribute to multidrug resistance and physiological functions. Among the eight efflux pumps, SmeYZ pump is constitutively highly expressed. In our previous study, we demonstrated that loss-of-function of the SmeYZ pump results in pleiotropic phenotypes, including abolished swimming motility, decreased secreted protease activity, and compromised tolerance to oxidative stress and antibiotics. In this study, we attempted to elucidate the underlying mechanisms responsible for ΔsmeYZ-mediated pleiotropic phenotypes. RNA-seq transcriptome analysis and subsequent confirmation with qRT-PCR revealed that smeYZ mutant experienced an iron starvation response because the genes involved in the synthesis and uptake of stenobactin, the sole siderophore of S. maltophilia, were significantly upregulated. We further verified that smeYZ mutant had low intracellular iron levels via inductively coupled plasma mass spectrometry (ICP-MS). Also, KJΔYZ was more sensitive to 2,2'-dipyridyl (DIP), a ferrous iron chelator, in comparison with the wild type. The contribution of SmeYZ, SmeDEF, and SbiAB pumps to stenobactin secretion was suggested by qRT-PCR and further verified by Chrome Azurol S (CAS) activity, iron source utilization, and cell viability assays. We also demonstrated that loss-of-function of SmeYZ led to the compensatory upregulation of SbiAB and SmeDEF pumps for stenobactin secretion. The overexpression of the SbiAB pump resulted in a reduction in intracellular iron levels, which may be the key factor responsible for the ΔsmeYZ-mediated pleiotropic phenotypes, except for antibiotic extrusion. IMPORTANCE Efflux pumps display high efficiency of drug extrusion, which underlies their roles in multidrug resistance. In addition, efflux pumps have physiological functions, and their expression is tightly regulated by various environmental and physiological signals. Functional redundancy of efflux pumps is commonly observed, and mutual regulation occurs among these functionally redundant pumps in a bacterium. Stenotrophomonas maltophilia is an opportunistic pathogen that shows intrinsic multi-drug resistance. In this study, we demonstrated that SmeYZ, SbiAB, and SmeDEF efflux pumps of S. maltophilia display functional redundancy in siderophore secretion. Inactivation of smeYZ led to the upregulation of smeDEF and sbiAB. Unexpectedly, sbiAB overexpression resulted in the reduction of intracellular iron levels, which led to pleiotropic defects in smeYZ mutant. This study demonstrates a previously unidentified connection between efflux pumps, siderophore secretion, and intracellular iron levels in S. maltophilia.

Siderophores: an alternative bioremediation strategy?

Siderophores are small molecular weight iron scavengers that are mainly produced by bacteria, fungi, and plants. Recently, they have attracted increasing attention because of their potential role in environmental bioremediation. Although siderophores are generally considered to exhibit high specificity for iron, they have also been reported to bind to various metal and metalloid ions. This unique ability allows siderophores to solubilise and mobilise heavy metals and metalloids from soil, thereby facilitating their bioremediation. In addition, because of their redox nature, they can mediate the production of reactive oxygen species (ROS), and thus promote the biodegradation of organic contaminants. The aim of this review is to summarise the existing knowledge on the developed strategies of siderophore-assisted bioremediation of metals, metalloids, and organic contaminants. Additionally, this review also includes the biosynthesis and classification of microbial and plant siderophores.

Extraction and Detection of Structurally Diverse Siderophores in Soil

Although the biochemistry of bacterial and fungal siderophores has been intensively studied in laboratory cultures, their distribution and impacts on nutrient cycling and microbial communities in soils remain poorly understood. The detection of siderophores in soil is an analytical challenge because of the complexity of the soil matrix and their structural diversity. Liquid chromatography-mass spectrometry (LC-MS) is a suitable method for the sensitive analysis of siderophores in complex samples; however, siderophore extraction into liquid phases for analysis by LC-MS is problematic because of their adsorption to soil particles and organic matter. To determine extraction efficiencies of structurally diverse siderophores, spike-recovery experiments were set up with standards representing the three main siderophore classes: the hydroxamate desferrioxamine B (DFOB), the α-hydroxycarboxylate rhizoferrin, and the catecholate protochelin. Previously used solvent extractions with water or methanol recovered only a small fraction (< 35%) of siderophores, including < 5% for rhizoferrin and protochelin. We designed combinatorial chemical extractions (22 total solutions) to target siderophores associated with different soil components. A combination of calcium chloride and ascorbate achieved high and, for some soils, quantitative extraction of DFOB and rhizoferrin. Protochelin analysis was complicated by potential fast oxidation and interactions with colloidal soil components. Using the optimized extraction method, we detected α-hydroxycarboxylate type siderophores (viz. rhizoferrin, vibrioferrin, and aerobactin) in soil for the first time. Concentrations reached 461 pmol g-1, exceeding previously reported concentrations of siderophores in soil and suggesting a yet unrecognized importance of α-hydroxycarboxylate siderophores for biological interactions and biogeochemical processes in soil.

Effect of salt and metal accumulation on performance of membrane distillation system and microbial community succession in membrane biofilms

Membrane distillation (MD) works as a potential technology for the "zero liquid discharge" water treatment owing to its high concentration brine tolerance. The continuous accumulation of salts and metals in the MD system during the "zero liquid discharge" water treatment inevitably posed remarkable impacts on the biofilm formation as well as the MD performance. Hence, the biofouling mechanism of MD was deeply researched in this study with an emphasis on the roles of salt-stress (NaCl) and metal-stress (Zn and Fe) in biofilm development. The membrane flux decline of MD was effectively mitigated by the appearance of NaCl and ZnO, while that was significantly aggravated under the metal-stress of Fe. Considering the serious membrane scaling caused by NaCl crystals, a sharp flux decline was seen for the NaCl group during the later stage of MD operation. Basing on the 16S rDNA and 16S rRNA analysis, heat-stress, salt-stress, and metal-stress all posed certain impacts on the biofouling development in the MD system, and a more remarkable influence was observed for metal-stress. Under the salt-stress from NaCl, a thin biofilm containing high biovolume of dead cells finally formed, in which the bacterial community mainly consisted of halotolerant and thermophile species. Owing to the Zn²⁺-stress and oxidation-stress mechanisms of ZnO, the bacteria in the MD system were largely dead and live bacterial community in biofilms was dominated by some gram-negative species. Under the metal-stress from Fe, a rather thick biofilm containing higher biovolume of live cells clearly developed, in which the prevailing species could secret large amounts of EPS and accumulate metabolites around cells as biological surfactants, inducing aggravated membrane biofouling and high risk of membrane wetting.

Metal binding ability of microbial natural metal chelators and potential applications

Metallophores can chelate many different metal and metalloid ions next to iron, make them valuable for many applications.

Evaluation of the Efficacy of Two New Biotechnological-Based Freeze-Dried Fertilizers for Sustainable Fe Deficiency Correction of Soybean Plants Grown in Calcareous Soils

Currently, fertilization with synthetic chelates is the most effective agricultural practice to prevent iron (Fe) deficiencies in crops, especially in calcareous soils. Because these compounds are not biodegradable, they are persistent in the environment, and so, there is the risk of metal leaching from the soils. Thus, new, more environment-friendly efficient solutions are needed to solve iron-deficiency-induced chlorosis (IDIC) in crops grown in calcareous soils. Therefore, the central aim of this work was to prepare new freeze-dried Fe products, using a biotechnological-based process, from two siderophores bacterial (Azotobacter vinelandii and Bacillus subtilis) cultures (which previously evidenced high Fe complexation ability at pH 9) and test their capacity for amending IDIC of soybean grown in calcareous soils. Results have shown that A. vinelandii iron fertilizer was more stable and interacted less with calcareous soils and its components than B. subtilis one. This behavior was noticeable in pot experiments where chlorotic soybean plants were treated with both fertilizer products. Plants treated with A. vinelandii fertilizer responded more significantly than those treated with B. subtilis one, when evaluated by their growth (20% more dry mass than negative control) and chlorophyll development (30% higher chlorophyll index than negative control) and in most parameters similar to the positive control, ethylenediamine-di(o-hydroxyphenylacetic acid). On average, Fe content was also higher in A. vinelandii-treated plants than on B. subtilis-treated ones. Results suggest that this new siderophore-based formulation product, prepared from A. vinelandii culture, can be regarded as a possible viable alternative for replacing the current nongreen Fe-chelating fertilizers and may envisage a sustainable and environment-friendly mending IDIC of soybean plants grown in calcareous soils.

The complex of ferric-enterobactin with its transporter from Pseudomonas aeruginosa suggests a two-site model

Bacteria use small molecules called siderophores to scavenge iron. Siderophore-Fe3+ complexes are recognised by outer-membrane transporters and imported into the periplasm in a process dependent on the inner-membrane protein TonB. The siderophore enterobactin is secreted by members of the family Enterobacteriaceae, but many other bacteria including Pseudomonas species can use it. Here, we show that the Pseudomonas transporter PfeA recognises enterobactin using extracellular loops distant from the pore. The relevance of this site is supported by in vivo and in vitro analyses. We suggest there is a second binding site deeper inside the structure and propose that correlated changes in hydrogen bonds link binding-induced structural re-arrangements to the structural adjustment of the periplasmic TonB-binding motif.

Calcareous soil interactions of the iron(III) chelates of DPH and Azotochelin and its application on amending iron chlorosis in soybean (Glycine max)

In order to find new greener solutions for iron (Fe) induced chlorosis, two new chelating agents, N,N-dihydroxy-N,N'-diisopropylhexanediamide (DPH) and Azotochelin (AZO), were assessed for its effectiveness in mending induced chlorosis in soybean (Glycine max). DPH-Fe and AZO-Fe complexes were firstly tested for their soil interactions and capability to maintain Fe in a bioavailable form. Secondly, 57Fe-chelates of DPH and AZO were applied to the soil in a pot experiment with chlorotic soybean plants. Their growth, SPAD chlorophyll index, and the Fe concentration in plant tissues and the remaining soil were evaluated. An isotope deconvolution analysis by using the concentration of the Fe isotopes was used to distinguish the Fe coming from soil and from the 57Fe labelled fertilizer treatments. AZO and DPH have shown different interactions with soil and its components, with AZO showing less interaction than DPH. The application of AZO and DPH resulted in SPAD increase and Fe content. However, it was found that the Fe in plants had not come from the fertilizer application, but instead from natural sources. This is likely due to dissolution phenomena aided by the chelates added. Overall, AZO and DPH have shown good results in amending Fe induced chlorosis in calcareous soils and for this reason should be regarded as good green-candidates for Fe plant nutrition in calcareous soils.

Multiple siderophores: bug or feature?

It is common for bacteria to produce chemically diverse sets of small Fe-binding molecules called siderophores. Studies of siderophore bioinorganic chemistry have firmly established the role of these molecules in Fe uptake and provided great insight into Fe complexation. However, we still do not fully understand why microbes make so many siderophores. In many cases, the release of small structural variants or siderophore fragments has been ignored, or considered as an inefficiency of siderophore biosynthesis. Yet, in natural settings, microbes live in complex consortia and it has become increasingly clear that the secondary metabolite repertoires of microbes reflect this dynamic environment. Multiple siderophore production may, therefore, provide a window into microbial life in the wild. This minireview focuses on three biochemical routes by which multiple siderophores can be released by the same organism-multiple biosynthetic gene clusters, fragment release, and precursor-directed biosynthesis-and highlights emergent themes related to each. We also emphasize the plurality of reasons for multiple siderophore production, which include enhanced iron uptake via synergistic siderophore use, microbial warfare and cooperation, and non-classical functions such as the use of siderophores to take up metals other than Fe.

Azotobacter vinelandii: the source of 100 years of discoveries and many more to come

Azotobacter vinelandii has been studied for over 100 years since its discovery as an aerobic nitrogen-fixing organism. This species has proved useful for the study of many different biological systems, including enzyme kinetics and the genetic code. It has been especially useful in working out the structures and mechanisms of different nitrogenase enzymes, how they can function in oxic environments and the interactions of nitrogen fixation with other aspects of metabolism. Interest in studying A. vinelandii has waned in recent decades, but this bacterium still possesses great potential for new discoveries in many fields and commercial applications. The species is of interest for research because of its genetic pliability and natural competence. Its features of particular interest to industry are its ability to produce multiple valuable polymers - bioplastic and alginate in particular; its nitrogen-fixing prowess, which could reduce the need for synthetic fertilizer in agriculture and industrial fermentations, via coculture; its production of potentially useful enzymes and metabolic pathways; and even its biofuel production abilities. This review summarizes the history and potential for future research using this versatile microbe.

Metals in fungal virulence

Metals are essential for life, and they play a central role in the struggle between infecting microbes and their hosts. In fact, an important aspect of microbial pathogenesis is the 'nutritional immunity', in which metals are actively restricted (or, in an extended definition of the term, locally enriched) by the host to hinder microbial growth and virulence. Consequently, fungi have evolved often complex regulatory networks, uptake and detoxification systems for essential metals such as iron, zinc, copper, nickel and manganese. These systems often differ fundamentally from their bacterial counterparts, but even within the fungal pathogens we can find common and unique solutions to maintain metal homeostasis. Thus, we here compare the common and species-specific mechanisms used for different metals among different fungal species-focusing on important human pathogens such as Candida albicans, Aspergillus fumigatus or Cryptococcus neoformans, but also looking at model fungi such as Saccharomyces cerevisiae or A. nidulans as well-studied examples for the underlying principles. These direct comparisons of our current knowledge reveal that we have a good understanding how model fungal pathogens take up iron or zinc, but that much is still to learn about other metals and specific adaptations of individual species-not the least to exploit this knowledge for new antifungal strategies.

Enterobactin as Part of the Oxidative Stress Response Repertoire

Microorganisms produce siderophores to facilitate iron uptake and even though this trait has been extensively studied, there is growing evidence suggesting that siderophores may have other physiological roles aside from iron acquisition. In support of this notion, we previously linked the archetypal siderophore enterobactin with oxidative stress alleviation. To further characterize this association, we studied the sensitivity of Escherichia coli strains lacking different components of the enterobactin system to the classical oxidative stressors hydrogen peroxide and paraquat. We observed that strains impaired in enterobactin production, uptake and hydrolysis were more susceptible to the oxidative damage caused by both compounds than the wild-type strain. In addition, meanwhile iron supplementation had little impact on the sensitivity, the reducing agent ascorbic acid alleviated the oxidative stress and therefore significantly decreased the sensitivity to the stressors. This indicated that the enterobactin-mediated protection is independent of its ability to scavenge iron. Furthermore, enterobactin supplementation conferred resistance to the entE mutant but did not have any protective effect on the fepG and fes mutants. Thus, we inferred that only after enterobactin is hydrolysed by Fes in the cell cytoplasm and iron is released, the free hydroxyl groups are available for radical stabilization. This hypothesis was validated testing the ability of enterobactin to scavenge radicals in vitro. Given the strong connection between enterobactin and oxidative stress, we studied the transcription of the entE gene and the concomitant production of the siderophore in response to such kind of stress. Interestingly, we observed that meanwhile iron represses the expression and production of the siderophore, hydrogen peroxide and paraquat favour these events even if iron is present. Our results support the involvement of enterobactin as part of the oxidative stress response and highlight the existence of a novel regulation mechanism for enterobactin biosynthesis.

The Siderophore Metabolome of Azotobacter vinelandii

In this study, we performed a detailed characterization of the siderophore metabolome, or "chelome," of the agriculturally important and widely studied model organism Azotobacter vinelandii. Using a new high-resolution liquid chromatography-mass spectrometry (LC-MS) approach, we found over 35 metal-binding secondary metabolites, indicative of a vast chelome in A. vinelandii. These include vibrioferrin, a siderophore previously observed only in marine bacteria. Quantitative analyses of siderophore production during diazotrophic growth with different sources and availabilities of Fe showed that, under all tested conditions, vibrioferrin was present at the highest concentration of all siderophores and suggested new roles for vibrioferrin in the soil environment. Bioinformatic searches confirmed the capacity for vibrioferrin production in Azotobacter spp. and other bacteria spanning multiple phyla, habitats, and lifestyles. Moreover, our studies revealed a large number of previously unreported derivatives of all known A. vinelandii siderophores and rationalized their origins based on genomic analyses, with implications for siderophore diversity and evolution. Together, these insights provide clues as to why A. vinelandii harbors multiple siderophore biosynthesis gene clusters. Coupled with the growing evidence for alternative functions of siderophores, the vast chelome in A. vinelandii may be explained by multiple, disparate evolutionary pressures that act on siderophore production.

Azotobacter Genomes: The Genome of Azotobacter chroococcum NCIMB 8003 (ATCC 4412)

The genome of the soil-dwelling heterotrophic N2-fixing Gram-negative bacterium Azotobacter chroococcum NCIMB 8003 (ATCC 4412) (Ac-8003) has been determined. It consists of 7 circular replicons totalling 5,192,291 bp comprising a circular chromosome of 4,591,803 bp and six plasmids pAcX50a, b, c, d, e, f of 10,435 bp, 13,852, 62,783, 69,713, 132,724, and 311,724 bp respectively. The chromosome has a G+C content of 66.27% and the six plasmids have G+C contents of 58.1, 55.3, 56.7, 59.2, 61.9, and 62.6% respectively. The methylome has also been determined and 5 methylation motifs have been identified. The genome also contains a very high number of transposase/inactivated transposase genes from at least 12 of the 17 recognised insertion sequence families. The Ac-8003 genome has been compared with that of Azotobacter vinelandii ATCC BAA-1303 (Av-DJ), a derivative of strain O, the only other member of the Azotobacteraceae determined so far which has a single chromosome of 5,365,318 bp and no plasmids. The chromosomes show significant stretches of synteny throughout but also reveal a history of many deletion/insertion events. The Ac-8003 genome encodes 4628 predicted protein-encoding genes of which 568 (12.2%) are plasmid borne. 3048 (65%) of these show > 85% identity to the 5050 protein-encoding genes identified in Av-DJ, and of these 99 are plasmid-borne. The core biosynthetic and metabolic pathways and macromolecular architectures and machineries of these organisms appear largely conserved including genes for CO-dehydrogenase, formate dehydrogenase and a soluble NiFe-hydrogenase. The genetic bases for many of the detailed phenotypic differences reported for these organisms have also been identified. Also many other potential phenotypic differences have been uncovered. Properties endowed by the plasmids are described including the presence of an entire aerobic corrin synthesis pathway in pAcX50f and the presence of genes for retro-conjugation in pAcX50c. All these findings are related to the potentially different environmental niches from which these organisms were isolated and to emerging theories about how microbes contribute to their communities.

Catechin as a new improving agent for a photo-Fenton-like system at near-neutral pH for the removal of inderal

During rapid ligand-to-metal charge transfer (LMCT) of the Fe(iii)–catechin complex at near-neutral pH, the species Fe²⁺ and H₂O₂ is generated in the presence of oxygen, and formed photo-Fenton reaction to produce ˙OH.

An antioxidant role for catecholate siderophores in Salmonella

Iron acquisition is an important aspect of the host–pathogen interaction. In the case of Salmonella it is established that catecholate siderophores are important for full virulence. In view of their very high affinity for ferric iron, functional studies of siderophores have been almost exclusively focused on their role in acquisition of iron from the host. In the present study, we investigated whether the siderophores (enterobactin and salmochelin) produced by Salmonella enterica sv. Typhimurium could act as antioxidants and protect from the oxidative stress encountered after macrophage invasion. Our results show that the ability to produce siderophores enhanced the survival of Salmonella in the macrophage mainly at the early stages of infection, coincident with the oxidative burst. Using siderophore biosynthetic and siderophore receptor mutants we demonstrated that salmochelin and enterobactin protect S. Typhimurium against ROS (reactive oxygen species) in vitro and that siderophores must be intracellular to confer full protection. We also investigated whether other chemically distinct siderophores (yersiniabactin and aerobactin) or the monomeric catechol 2,3-dihydroxy-benzoate could provide protection against oxidative stress and found that only catecholate siderophores have this property. Collectively, the results of the present study identify additional functions for siderophores during host–pathogen interactions.

Vanadium requirements and uptake kinetics in the dinitrogen-fixing bacterium Azotobacter vinelandii

Vanadium is a cofactor in the alternative V-nitrogenase that is expressed by some N(2)-fixing bacteria when Mo is not available. We investigated the V requirements, the kinetics of V uptake, and the production of catechol compounds across a range of concentrations of vanadium in diazotrophic cultures of the soil bacterium Azotobacter vinelandii. In strain CA11.70, a mutant that expresses only the V-nitrogenase, V concentrations in the medium between 10(-8) and 10(-6) M sustain maximum growth rates; they are limiting below this range and toxic above. A. vinelandii excretes in its growth medium micromolar concentrations of the catechol siderophores azotochelin and protochelin, which bind the vanadate oxoanion. The production of catechols increases when V concentrations become toxic. Short-term uptake experiments with the radioactive isotope (49)V show that bacteria take up the V-catechol complexes through a regulated transport system(s), which shuts down at high V concentrations. The modulation of the excretion of catechols and of the uptake of the V-catechol complexes allows A. vinelandii to precisely manage its V homeostasis over a range of V concentrations, from limiting to toxic.

Complexation of oxoanions and cationic metals by the biscatecholate siderophore azotochelin

Azotochelin is a biscatecholate siderophore produced by the nitrogen-fixing soil bacterium Azotobacter vinelandii. The complexation properties of azotochelin with a series of oxoanions [Mo(VI), W(VI) and V(V)] and divalent cations [Cu(II), Zn(II), Co(II) and Mn(II)] were investigated by potentiometry, UV-vis and X-ray spectroscopy. Azotochelin forms a strong 1:1 complex with molybdate (log K=7.6+/-0.4) and with tungstate and vanadate; the stability of the complexes increases in the order Mo<V<W (log KappMo=7.3+/-0.4; log KappV=8.8+/-0.4 and log KappW=9.0+/-0.4 at pH 6.6). The Mo atom in the 1:1 Mo-azotochelin complex is bound to two oxo groups in a cis position and to the two catecholate groups of azotochelin, resulting in a slightly distorted octahedral configuration. Below pH 5, azotochelin appears to form polynuclear complexes with Mo in addition to the 1:1 complex. Azotochelin also forms strong complexes with divalent metals. Of the metals studied, Cu(II) binds most strongly to azotochelin (log betaCuLH2-=-12.9+/-0.1), followed by Zn(II) log betaZnL3-=-24.1+/-0.14, log betaZnLH2-=-17.83+/-0.09), Mn(II) (log betaMnL3-=-29, log betaMnLH2-=-18.6+/-0.8, log betaMnLH2-=-11.5+/-0.7) and Co(II) (log betaCoLH2-=-23.0+/-0.3, log betaCoLH2-=-13.5+/-0.2). Since very few organic ligands are known to bind strongly to oxoanions (and particularly molybdate) at circumneutral pH, the unusual properties of azotochelin may be used for the separation and concentration of oxoanions in the laboratory and in the field. In addition, azotochelin may prove useful for the investigation of the biogeochemistry of Mo, W and V in aquatic and terrestrial systems.

Mechanism of augmentation of organotin decomposition by ferripyochelin: formation of hydroxyl radical and organotin-pyochelin-iron ternary complex

Pyochelin (PCH), a kind of siderophore secreted by Pseudomonas aeruginosa, was recently found to have triphenyltin (TPT)-decomposing capacity. In this work, significant augmentation of TPT decomposition by ferripyochelin (FePCH), the chelating compound of PCH with iron, was demonstrated in Tris-HCl buffer (pH 8.0). The generation of hydroxyl radical (HO.) in the presence of FePCH was observed. Inhibition of HO. generation by adding catalase and HO. scavengers (methanol and dimethyl sulfoxide) decreased TPT decomposition, while an increase in HO. formation in the presence of H(2)O(2) enhanced its decomposition. Our findings indicated that HO. generated in the reaction system was responsible for the enhanced TPT decomposition by FePCH versus PCH. The existence of the TPT-pyochelin-iron ternary complex was demonstrated by electron spray ionization-mass spectrometry, tandem mass spectrometry, and (1)H nuclear magnetic resonance. On the basis of the above results, HO. produced in the presence of FePCH was deduced to be in close proximity to TPT and has more opportunity to attack the Sn-C bond, which resulted in the enhanced organotin decomposition. The information obtained may have considerable environmental significance.

Siderophore cross-utilization amongst rhizospheric bacteria and the role of their differential affinities for Fe3+ on growth stimulation under iron-limited conditions

The majority of bacteria isolated from rhizospheres of Arachis hypogea (Groundnut) and Vigna radiata (Mung bean) predominantly produced catechol-type siderophores except for a few fluorescent pseudomonads that produced hydroxamates in addition to catecholates. The rhizospheric isolates differed in their ability to cross-utilize siderophores produced by other rhizospheric isolates (heterologous); some were highly proficient at utilizing heterologous siderophores, while others were poor cross-utilizers. Isolate G9, which utilized hydroxamate as well as catecholate siderophores, was found to be an efficient siderophore cross-utilizer, while isolates G2 and G6 were poor-utilizers of catecholate and non-utilizers of hydroxamate siderophores. Growth stimulation of two isolates G9 and G6 was seen when grown in the presence of externally supplied heterologous siderophores, which they cross-utilized. The iron-regulated outer membrane protein (IROMP) profiles differed for the most cross-utilizer and the least cross-utilizer strains, but in both the cases no new outer membrane proteins (OMP) were induced in response to the exogenous siderophores supplied. The growth of the organisms in the presence of heterologous siderophores that they failed to cross-utilize led to growth inhibition in the case of isolate G9. This appears to be due to a lower affinity of the siderophore of G9 as compared to the exogenously supplied G6 siderophore. A simple method was devised to measure relative affinities of respective siderophores for iron based on CAS solution decolorization by the siderophore preparations. The effect on the growth of the differential affinities of the siderophores for iron and the interactions of the organisms through cross-utilization is also discussed.

Characterization of the iron superoxide dismutase gene of Azotobacter vinelandii: sodB may be essential for viability

Azotobacter vinelandii contains two superoxide dismutases (SODs), a cytoplasmic iron-containing enzyme (FeSOD), and a periplasmic copper/zinc-containing enzyme (CuZnSOD). In this study, the FeSOD was found to be constitutive, while the activity of CuZnSOD increased as the culture entered the stationary phase. Total SOD (units/mg protein) in stationary phase cells grown under nitrogen-fixing conditions was not significantly different from those grown under non-nitrogen-fixing conditions. The gene encoding FeSOD (sodB) was isolated from an A. vinelandii cosmid library. A 1-kb fragment containing the coding region and 400 base pairs of upstream sequence was cloned and sequenced. The nucleotide sequence and the deduced amino acid sequence had a high degree of homology with other bacterial FeSODs, particularly with P. aeruginosa. Attempts to construct a sodB mutant by recombination of a sodB::kan insertion mutation into the multicopy chromosome of A. vinelandii were unsuccessful even in the presence of SOD mimics or nutritional supplements. These results suggest that FeSOD may be essential for the growth and survival of A. vinelandii, and that the periplasmic CuZnSOD cannot replace the function of FeSOD.

ThecsbXgene ofAzotobacter vinelandiiencodes an MFS efflux pump required for catecholate siderophore export

The csbX gene of Azotobacter vinelandii was regulated in an iron-repressible manner from a divergent promoter upstream of the catecholate siderophore biosynthesis (csb) operon and was predicted to encode an efflux pump of the major facilitator superfamily. Other proteins that were most similar to CsbX were encoded by genes found in the catecholate siderophore biosynthesis operons of Aeromonas hydrophila and Stigmatella aurantiaca. Inactivation of csbX resulted in 57-100% decrease in the amount of catecholates released when compared to the wild-type in iron-limited medium. CsbX was most important for the export of the high affinity chelator protochelin with the majority of the catecholates released by csbX mutants being the protochelin intermediates azotochelin and aminochelin.

Vanadium interferes with siderophore-mediated iron uptake in Pseudomonas aeruginosa

Vanadium is a metal that under physiological conditions can exist in two oxidation states, V(IV) (vanadyl ion) and V(V) (vanadate ion). Here, it was demonstrated that both ions can form complexes with siderophores. Pseudomonas aeruginosa produces two siderophores under iron-limiting conditions, pyoverdine (PVD) and pyochelin (PCH). Vanadyl sulfate, at a concentration of 1-2 mM, strongly inhibited growth of P. aeruginosa PAO1, especially under conditions of severe iron limitation imposed by the presence of non-utilizable Fe(III) chelators. PVD-deficient mutants were more sensitive to vanadium than the wild-type, but addition of PVD did not stimulate their growth. Conversely, PCH-negative mutants were more resistant to vanadium than the wild-type strain. Both siderophores could bind and form complexes with vanadium after incubation with vanadyl sulfate (1:1, in the case of PVD; 2:1, in the case of PCH). Although only one complex with PVD, V(IV)-PVD, was found, both V(IV)- and V(V)-PCH were detected. V-PCH, but not V-PVD, caused strong growth reduction, resulting in a prolonged lag phase. Exposure of PAO1 cells to vanadium induced resistance to the superoxide-generating compound paraquat, and conversely, exposure to paraquat increased resistance to V(IV). Superoxide dismutase (SOD) activity of cells grown in the presence of V(IV) was augmented by a factor of two. Mutants deficient in the production of Fe-SOD (SodB) were particularly sensitive to vanadium, whilst sodA mutants deficient for Mn-SOD were only marginally affected. In conclusion, it is suggested that V-PCH catalyses a Fenton-type reaction whereby the toxic superoxide anion O(2)- is generated, and that vanadium compromises PVD utilization.

Dual regulation of catecholate siderophore biosynthesis in Azotobacter vinelandii by iron and oxidative stress

Azotobacter vinelandii forms both catecholate and azotobactin siderophores during iron-limited growth. Azotobactin is repressed by about 3 microM iron, but catecholate siderophore synthesis continues up to a maximum of 10 microM iron. This suggests that catecholate siderophore synthesis is regulated by other factors in addition to the ferric uptake repressor (Fur). In this study the first gene required for catecholate siderophore biosynthesis, which encodes an isochorismate synthase (csbC), was isolated. The region upstream of csbC contained a typical sigma(70) promoter, with an iron-box overlapping the -35 sequence and a Sox-box (Box 1) overlapping the -10 sequence. Another Sox-box was found further upstream of the -35 sequence (Box 2). Also upstream, an unidentified gene (orfA) was detected which would be transcribed from a divergent promoter, also controlled by an iron-box. The activity of csbC and a csbC::luxAB fusion was negatively regulated by iron availability and upregulated by increased aeration and by superoxide stress. The iron-box in the csbC promoter was 74% identical to the Fur-binding consensus sequence and bound the Fur protein of Escherichia coli with relatively high affinity. Both Box 1 and Box 2 were in good agreement with the consensus sequence for binding the SoxS protein of E. coli and Box 1 was in very good agreement with the Sox-box found in the fpr promoter of A. vinelandii, which is also regulated by superoxide stress. Both Sox-boxes bound a protein found in A. vinelandii cell extracts, with Box 1 exhibiting the higher binding affinity. The Sox protein identified in this assay appeared to be constitutive, rather than inducible by superoxide stress. This indicates that the Sox response in A. vinelandii is different from that in E. coli. These data support the hypothesis that catecholate siderophore biosynthesis is under dual control, repressed by a Fur-iron complex and activated by another DNA-binding protein in response to superoxide stress. The interaction between these regulators is likely to account for the delay in ferric repression of catecholate siderophore production, since these siderophores have an additional role to play in the protection of iron-limited cells against oxidative damage.

Role of molybdate and other transition metals in the accumulation of protochelin by Azotobacter vinelandii

Both molybdate and iron are metals that are required by the obligately aerobic organism Azotobacter vinelandii to survive in the nutrient-limited conditions of its natural soil environment. Previous studies have shown that a high concentration of molybdate (1 mM) affects the formation of A. vinelandii siderophores such that the tricatecholate protochelin is formed to the exclusion of the other catecholate siderophores, azotochelin and aminochelin. It has been shown previously that molybdate combines readily with catecholates and interferes with siderophore function. In this study, we found that the manner in which each catecholate siderophore interacted with molybdate was consistent with the structure and binding potential of the siderophore. The affinity that each siderophore had for molybdate was high enough that stable molybdo-siderophore complexes were formed but low enough that the complexes were readily destabilized by Fe(3+). Thus, competition between Fe(3+) and molybdate did not appear to be the primary cause of protochelin accumulation; in addition, we determined that protochelin accumulated in the presence of vanadate, tungstate, Zn(2+), and Mn(2+). We found that all five of these metal ions partially inhibited uptake of (55)Fe-protochelin and (55)Fe-azotochelin complexes. Also, each of these metal ions partially inhibited the activity of ferric reductase, an enzyme important in the deferration of ferric siderophores. Our results suggest that protochelin accumulates in the presence of molybdate because protochelin uptake and conversion into its component parts, azotochelin and aminochelin, are inhibited by interference with ferric reductase.

Mutation of cytochrome bd quinol oxidase results in reduced stationary phase survival, iron deprivation, metal toxicity and oxidative stress in Azotobacter vinelandii

Azotobacter vinelandii cydAB mutants lacking cytochrome bd lost viability in stationary phase, irrespective of temperature, but microaerobiosis or iron addition to stationary phase cultures prevented viability loss. Growth on solid medium was inhibited by a diffusible factor from neighbouring cells, and by iron chelators, In(III) or Ga(III); microaerobic growth overcame inhibition by the extracellular factor. Siderophore production and total Fe(III)-chelating activity were not markedly affected in Cyd(-) mutants, and remained responsive to iron repression. Cyd(-) mutants were hypersensitive to Cu(II), Zn(II), and compounds exerting oxidative stress. Failure to synthesise haemoproteins does not explain the complex phenotype since mutants retained significant catalase activity. We hypothesise that Cyd(-) mutants are defective in maintaining the near-anoxic cytoplasm required for reductive iron metabolism and nitrogenase activity.