Iron release from transferrin by pyoverdin and elastase from Pseudomonas aeruginosa

Bacterial siderophores: diversity, uptake pathways and applications

Iron is an essential nutrient for the growth, survival and virulence of almost all bacteria. To access iron, many bacteria produce siderophores, molecules with a high affinity for iron. Research has highlighted substantial diversity in the chemical structure of siderophores produced by bacteria, as well as remarkable variety in the molecular mechanisms involved in strategies for acquiring iron through these molecules. The metal-chelating properties of siderophores, characterized by their high affinity for iron and ability to chelate numerous other metals (albeit with lower affinity compared with iron), have also generated interest in diverse fields. Siderophores find applications in the environment, such as in bioremediation and agriculture, in which emerging and innovative strategies are being developed to address pollution and enhance nutrient availability for plants. Moreover, in medicine, siderophores could be used as a tool for novel antimicrobial therapies and medical imaging, as well as in haemochromatosis, thalassemia or cancer treatments. This Review offers insights into the diversity of siderophores, highlighting their potential applications in environmental and medical contexts.

Antimicrobial activity of iron-depriving pyoverdines against human opportunistic pathogens

The global rise of antibiotic resistance calls for new drugs against bacterial pathogens. A common approach is to search for natural compounds deployed by microbes to inhibit competitors. Here, we show that the iron-chelating pyoverdines, siderophores produced by environmental Pseudomonas spp., have strong antibacterial properties by inducing iron starvation and growth arrest in pathogens. A screen of 320 natural Pseudomonas isolates used against 12 human pathogens uncovered several pyoverdines with particularly high antibacterial properties and distinct chemical characteristics. The most potent pyoverdine effectively reduced growth of the pathogens Acinetobacter baumannii, Klebsiella pneumoniae, and Staphylococcus aureus in a concentration- and iron-dependent manner. Pyoverdine increased survival of infected Galleria mellonella host larvae and showed low toxicity for the host, mammalian cell lines, and erythrocytes. Furthermore, experimental evolution of pathogens combined with whole-genome sequencing revealed limited resistance evolution compared to an antibiotic. Thus, pyoverdines from environmental strains have the potential to become a new class of sustainable antibacterials against specific human pathogens.

Revealing quorum-sensing networks in Pseudomonas aeruginosa infections through internal and external signals to prevent new resistance trends

In the context of growing antibiotic resistance in bacteria, the quorum-sensing (QS) system of Pseudomonas aeruginosa (P. aeruginosa) has become a target for new therapeutic strategies. QS is a crucial communication process and an essential pathogenic mechanism. This comprehensive review explores the critical role of QS in the pathogenesis of P. aeruginosa infections, including lung, burn, bloodstream, gastrointestinal, corneal, and urinary tract infections. In addition, this review delves into the complexity of the bacterial QS communication network and highlights the intricate mechanisms underlying these pathological processes. Notably, in addition to the four main QS systems, bacterial QS can interact with various external and internal signaling networks, such as host environments and nutrients in the external microbiome, as well as internal virulence regulation systems within bacteria. These elements can significantly influence the behavior and virulence of microbial communities. Therefore, this review reveals that inhibitors targeting singular QS pathways may inadvertently promote virulence in other pathways, leading to new trends in drug resistance. In response to evolving resistance challenges, this study proposes more cautious treatment strategies, including multitarget interventions and combination therapies, aimed at combating the escalating issue of resistance.

The heme-responsive PrrH sRNA regulates Pseudomonas aeruginosa pyochelin gene expression

Pseudomonas aeruginosa is an opportunistic pathogen that requires iron for growth and virulence, yet this nutrient is sequestered by the innate immune system during infection. When iron is limiting, P. aeruginosa expresses the PrrF1 and PrrF2 small RNAs (sRNAs), which post-transcriptionally repress expression of nonessential iron-containing proteins, thus sparing this nutrient for more critical processes. The genes for the PrrF1 and PrrF2 sRNAs are arranged in tandem on the chromosome, allowing for the transcription of a longer heme-responsive sRNA, termed PrrH. While the functions of PrrF1 and PrrF2 have been extensively studied, the role of PrrH in P. aeruginosa physiology and virulence is not well understood. In this study, we performed transcriptomic and proteomic studies to identify the PrrH regulon. In shaking cultures, the pyochelin synthesis proteins were increased in two distinct prrH mutants compared to the wild type, while the mRNAs for these proteins were not affected by the prrH mutation. We identified complementarity between the PrrH sRNA and the sequence upstream of the pchE mRNA, suggesting the potential for PrrH to directly regulate the expression of genes for pyochelin synthesis. We further showed that pchE mRNA levels were increased in the prrH mutants when grown in static but not shaking conditions. Moreover, we discovered that controlling for the presence of light was critical for examining the impact of PrrH on pchE expression. As such, our study reports on the first likely target of the PrrH sRNA and highlights key environmental variables that will allow for future characterization of PrrH function. IMPORTANCE In the human host, iron is predominantly in the form of heme, which Pseudomonas aeruginosa can acquire as an iron source during infection. We previously showed that the iron-responsive PrrF small RNAs (sRNAs) are critical for mediating iron homeostasis during P. aeruginosa infection; however, the function of the heme-responsive PrrH sRNA remains unclear. In this study, we identified genes for pyochelin siderophore biosynthesis, which mediates uptake of inorganic iron, as a novel target of PrrH regulation. This study therefore highlights a novel relationship between heme availability and siderophore biosynthesis in P. aeruginosa.

Mechanisms of iron homeostasis in Pseudomonas aeruginosa and emerging therapeutics directed to disrupt this vital process

Pseudomonas aeruginosa is an opportunistic pathogen able to infect any human tissue. One of the reasons for its high adaptability and colonization of host tissues is its capacity of maintaining iron homeostasis through a wide array of iron acquisition and removal mechanisms. Due to their ability to cause life-threatening acute and chronic infections, especially among cystic fibrosis and immunocompromised patients, and their propensity to acquire resistance to many antibiotics, the World Health Organization (WHO) has encouraged the scientific community to find new strategies to eradicate this pathogen. Several recent strategies to battle P. aeruginosa focus on targeting iron homeostasis mechanisms, turning its greatest advantage into an exploitable weak point. In this review, we discuss the different mechanisms used by P. aeruginosa to maintain iron homeostasis and the strategies being developed to fight this pathogen by blocking these mechanisms. Among others, the use of iron chelators and mimics, as well as disruption of siderophore production and uptake, have shown promising results in reducing viability and/or virulence of this pathogen. The so-called 'Trojan-horse' strategy taking advantage of the siderophore uptake systems is emerging as an efficient method to improve delivery of antibiotics into the bacterial cells. Moreover, siderophore transporters are considered promising targets for the developing of P. aeruginosa vaccines.

The heme-responsive PrrH sRNA regulates Pseudomonas aeruginosa pyochelin gene expression

Pseudomonas aeruginosa is an opportunistic pathogen that requires iron for growth and virulence, yet this nutrient is sequestered by the innate immune system during infection. When iron is limiting, P. aeruginosa expresses the PrrF1 and PrrF2 small regulatory RNAs (sRNAs), which post-transcriptionally repress expression of non-essential iron-containing proteins thus sparing this nutrient for more critical processes. The genes for the PrrF1 and PrrF2 sRNAs are arranged in tandem on the chromosome, allowing for the transcription of a longer heme-responsive sRNA, termed PrrH. While the functions of PrrF1 and PrrF2 have been studied extensively, the role of PrrH in P. aeruginosa physiology and virulence is not well understood. In this study, we performed transcriptomic and proteomic studies to identify the PrrH regulon. In shaking cultures, the pyochelin synthesis proteins were increased in two distinct prrH mutants compared to wild type, while the mRNAs for these proteins were not affected by prrH mutation. We identified complementarity between the PrrH sRNA and sequence upstream of the pchE mRNA, suggesting potential for PrrH to directly regulate expression of genes for pyochelin synthesis. We further showed that pchE mRNA levels were increased in the prrH mutants when grown in static but not shaking conditions. Moreover, we discovered controlling for the presence of light was critical for examining the impact of PrrH on pchE expression. As such, our study reports on the first likely target of the PrrH sRNA and highlights key environmental variables that will allow for future characterization of PrrH function.

An Organ System-Based Synopsis of Pseudomonas aeruginosa Virulence

Driven in part by its metabolic versatility, high intrinsic antibiotic resistance, and a large repertoire of virulence factors, Pseudomonas aeruginosa is expertly adapted to thrive in a wide variety of environments, and in the process, making it a notorious opportunistic pathogen. Apart from the extensively studied chronic infection in the lungs of people with cystic fibrosis (CF), P. aeruginosa also causes multiple serious infections encompassing essentially all organs of the human body, among others, lung infection in patients with chronic obstructive pulmonary disease, primary ciliary dyskinesia and ventilator-associated pneumonia; bacteremia and sepsis; soft tissue infection in burns, open wounds and postsurgery patients; urinary tract infection; diabetic foot ulcers; chronic suppurative otitis media and otitis externa; and keratitis associated with extended contact lens use. Although well characterized in the context of CF, pathogenic processes mediated by various P. aeruginosa virulence factors in other organ systems remain poorly understood. In this review, we use an organ system-based approach to provide a synopsis of disease mechanisms exerted by P. aeruginosa virulence determinants that contribute to its success as a versatile pathogen.

Iron limitation by transferrin promotes simultaneous cheating of pyoverdine and exoprotease in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a primary bacterial model to study cooperative behaviors because it yields exoproducts such as siderophores and exoproteases that act as public goods and can be exploited by selfish nonproducers behaving as social cheaters. Iron-limited growth medium, mainly casamino acids medium supplemented with transferrin, is typically used to isolate and study nonproducer mutants of the siderophore pyoverdine. However, using a protein as the iron chelator could inadvertently select mutants unable to produce exoproteases, since these enzymes can degrade the transferrin to facilitate iron release. Here we investigated the evolutionary dynamics of pyoverdine and exoprotease production in media in which iron was limited by using either transferrin or a cation chelating resin. We show that concomitant loss of pyoverdine and exoprotease production readily develops in media containing transferrin, whereas only pyoverdine loss emerges in medium treated with the resin. Characterization of exoprotease- and pyoverdine-less mutants revealed loss in motility, different mutations, and large genome deletions (13-33 kb) including Quorum Sensing (lasR, rsal, and lasI) and flagellar genes. Our work shows that using transferrin as an iron chelator imposes simultaneous selective pressure for the loss of pyoverdine and exoprotease production. The unintended effect of transferrin uncovered by our experiments can help to inform the design of similar studies.

Pseudomonas aeruginosa elastase (LasB) as a therapeutic target

Why is P. aeruginosa LasB elastase an attractive target for antivirulence therapy and what is the state-of-the art in LasB inhibitor design and development?

Organometallic ruthenium (η6-p-cymene) complexes interfering with quorum sensing and biofilm formation: an anti-infective approach to combat multidrug-resistance in bacteria

Organometallic ruthenium complexes of flavonoids as antiquorum sensing agents against pathogens likeChromobacterium violaceumATCC 12472,Pseudomonas aeruginosaPAO1 and methicillin-resistantS. aureus(MRSA).

Sodium salicylate interferes with quorum-sensing-regulated virulence in chronic wound isolates of Pseudomonas aeruginosa in simulated wound fluid

Introduction. An important factor for delayed healing of chronic wounds is the presence of bacteria. Quorum sensing (QS), a cell density-dependent signalling system, controls the production of many virulence factors and biofilm formation in Pseudomonas aeruginosa.Aim. Inhibition by sodium salicylate (NaSa) of QS-regulated virulence expression was evaluated in QS-characterized clinical wound isolates of P. aeruginosa, cultured in serum-containing medium.Methodology. Fourteen clinical P. aeruginosa strains from chronic wounds were evaluated for the production of QS signals and virulence factors. Inhibition of QS by NaSa in P. aeruginosa clinical strains, wild-type PAO1 and QS reporter strains was evaluated using in vitro assays for the production of biofilm, pyocyanin, siderophores, alkaline protease, elastase and stapholytic protease.Results. Six clinical strains secreted several QS-associated virulence factors and signal molecules and two were negative for all factors. Sub-inhibitory concentrations of NaSa downregulated the expression of the QS-related genes lasB, rhlA and pqsA and reduced the secretion of several virulence factors in PAO1 and clinical strains cultured in serum. Compared to serum-free media, the presence of serum increased the expression of QS genes and production of siderophores and pyocyanin but decreased biofilm formation.Conclusions. Pseudomonas aeruginosa from chronic wound infections showed different virulence properties. While very few strains showed no QS activity, approximately half were highly virulent and produced QS signals, suggesting that the targeting of QS is a viable and relevant strategy for infection control. NaSa showed activity as a QS-inhibitor by lowering the virulence phenotypes and QS signals at both transcriptional and extracellular levels.

Synthesis and Biological Evaluation of Cajaninstilbene Acid and Amorfrutins A and B as Inhibitors of the Pseudomonas aeruginosa Quorum Sensing System

The quorum sensing (QS) system inhibitors of Pseudomonas aeruginosa are thought to attenuate bacterial pathogenicity and drug resistance by inhibiting biofilm formation and the production of virulence factors. In this study, a synthetic approach to the natural products cajaninstilbene acid (1) and amorfrutins A (2) and B (3) has been developed and was characterized by the Heck reaction, which was used to obtain the stilbene core and a Pinick oxidation to give the O-hydroxybenzoic acid. The biological activities of these compounds against the P. aeruginosa quorum sensing systems were evaluated. Amorfrutin B (3) showed promising antibiofilm activity against P. aeruginosa PAO1 with a biofilm inhibition ratio of 50.3 ± 2.7. Three lacZ reporter strains were constructed to identify the effects of compound 3 on different QS systems. Suppression efficacy of compound 3 on the expression of lasB-lacZ and pqsA-lacZ as well as on the production of their corresponding virulence factors elastase and pyocyanin was observed.

Interdependence between iron acquisition and biofilm formation in Pseudomonas aeruginosa

Bacterial biofilms remain a persistent threat to human healthcare due to their role in the development of antimicrobial resistance. To combat multi-drug resistant pathogens, it is crucial to enhance our understanding of not only the regulation of biofilm formation, but also its contribution to bacterial virulence. Iron acquisition lies at the crux of these two subjects. In this review, we discuss the role of iron acquisition in biofilm formation and how hosts impede this mechanism to defend against pathogens. We also discuss recent findings that suggest that biofilm formation can also have the reciprocal effect, influencing siderophore production and iron sequestration.

Interference of phosphane copper (I) complexes of β-carboline with quorum sensing regulated virulence functions and biofilm in foodborne pathogenic bacteria: A first report

Foodborne pathogens are one of the major cause of food-related diseases and food poisoning. Bacterial biofilms and quorum sensing (QS) mechanism of cell–cell communication have also been found to be associated with several outbreaks of foodborne diseases and are great threat to food safety. Therefore, In the present study, we investigated the activity of three tetrahedrally coordinated copper(I) complexes against quorum sensing and biofilms of foodborne bacteria. All the three complexes demonstrated similar antimicrobial properties against the selected pathogens. Concentration below the MIC i.e. at sub-MICs all the three complexes interfered significantly with the quorum sensing regulated functions in C. violaceum (violacein), P. aeruginosa (elastase, pyocyanin and alginate production) and S. marcescens (prodigiosin). The complexes demonstrated potent broad-spectrum biofilm inhibition in Pseudomonas aeruginosa, E. coli, Chromobacterium violaceum, Serratia marcescens, Klebsiella pneumoniae and Listeria monocytogenes. Biofilm inhibition was visualized using SEM and CLSM images. Action of the copper(I) complexes on two key QS regulated functions contributing to biofilm formation i.e. EPS production and swarming motility was also studied and statistically significant reduction was recorded. These results could form the basis for development of safe anti-QS and anti-biofilm agents that can be utilized in the food industry as well as healthcare sector to prevent food-associated diseases.

Quorum sensing inhibition in Pseudomonas aeruginosa biofilms: new insights through network mining

Quorum sensing plays a pivotal role in Pseudomonas aeruginosa's virulence. This paper reviews experimental results on antimicrobial strategies based on quorum sensing inhibition and discusses current targets in the regulatory network that determines P. aeruginosa biofilm formation and virulence. A bioinformatics framework combining literature mining with information from biomedical ontologies and curated databases was used to create a knowledge network of potential anti-quorum sensing agents for P. aeruginosa. A total of 110 scientific articles, corresponding to 1,004 annotations, were so far included in the network and are analysed in this work. Information on the most studied agents, QS targets and methods is detailed. This knowledge network offers a unique view of existing strategies for quorum sensing inhibition and their main regulatory targets and may be used to readily access otherwise scattered information and to help generate new testable hypotheses. This knowledge network is publicly available at http://pcquorum.org/ .

Catecholamine-Stimulated Growth of Aeromonas hydrophila Requires the TonB2 Energy Transduction System but Is Independent of the Amonabactin Siderophore

The growth-stimulating effects of catecholamine stress hormones have been demonstrated in many pathogens. However, catecholamine-induced growth and its underlying mechanisms remain poorly understood in Aeromonas hydrophila. The present study sought to demonstrate that norepinephrine (NE), epinephrine (Epi), dopamine (Dopa), and L-dopa stimulate the growth of A. hydrophila in iron-restricted media containing serum. NE exhibited the strongest growth stimulation, which could be blocked by adrenergic antagonists. Furthermore, it was demonstrated that NE could sequester iron from transferrin, thereby providing a more accessible iron source for utilization by A. hydrophila. The deletion of the amoA gene associated with amonabactin synthesis revealed that the amonabactin siderophore is not required for NE-stimulated growth. However, the deletion of the TonB2 energy transduction system resulted in the loss of growth promotion by NE, indicating that a specific TonB-dependent outer membrane receptor might be involved in the transport of iron from transferrin. Collectively, our data show that catecholamine sensing promotes the growth of A. hydrophila in a manner that is dependent on the TonB2 energy transduction system.

Iron acquisition in the cystic fibrosis lung and potential for novel therapeutic strategies

Iron acquisition is vital to microbial survival and is implicated in the virulence of many of the pathogens that reside in the cystic fibrosis (CF) lung. The multifaceted nature of iron acquisition by both bacterial and fungal pathogens encompasses a range of conserved and species-specific mechanisms, including secretion of iron-binding siderophores, utilization of siderophores from other species, release of iron from host iron-binding proteins and haemoproteins, and ferrous iron uptake. Pathogens adapt and deploy specific systems depending on iron availability, bioavailability of the iron pool, stage of infection and presence of competing pathogens. Understanding the dynamics of pathogen iron acquisition has the potential to unveil new avenues for therapeutic intervention to treat both acute and chronic CF infections. Here, we examine the range of strategies utilized by the primary CF pathogens to acquire iron and discuss the different approaches to targeting iron acquisition systems as an antimicrobial strategy.

The hierarchy quorum sensing network in Pseudomonas aeruginosa

Pseudomonas aeruginosa causes severe and persistent infections in immune compromised individuals and cystic fibrosis sufferers. The infection is hard to eradicate as P. aeruginosa has developed strong resistance to most conventional antibiotics. The problem is further compounded by the ability of the pathogen to form biofilm matrix, which provides bacterial cells a protected environment withstanding various stresses including antibiotics. Quorum sensing (QS), a cell density-based intercellular communication system, which plays a key role in regulation of the bacterial virulence and biofilm formation, could be a promising target for developing new strategies against P. aeruginosa infection. The QS network of P. aeruginosa is organized in a multi-layered hierarchy consisting of at least four interconnected signaling mechanisms. Evidence is accumulating that the QS regulatory network not only responds to bacterial population changes but also could react to environmental stress cues. This plasticity should be taken into consideration during exploration and development of anti-QS therapeutics.

Deficiency of α-1-antitrypsin influences systemic iron homeostasis

There is evidence that proteases and antiproteases participate in the iron homeostasis of cells and living systems. We tested the postulate that α-1 antitrypsin (A1AT) polymorphism and the consequent deficiency of this antiprotease in humans are associated with a systemic disruption in iron homeostasis. Archived plasma samples from Alpha-1 Foundation (30 MM, 30 MZ, and 30 ZZ individuals) were analyzed for A1AT, ferritin, transferrin, and C-reactive protein (CRP). Plasma samples were also assayed for metals using inductively coupled plasma atomic emission spectroscopy (ICPAES). Plasma levels of A1AT in MZ and ZZ individuals were approximately 60% and 20% of those for MM individuals respectively. Plasma ferritin concentrations in those with the ZZ genotype were greater relative to those individuals with either MM or MZ genotype. Plasma transferrin for MM, MZ, and ZZ genotypes showed no significant differences. Linear regression analysis revealed a significant (negative) relationship between plasma concentrations of A1AT and ferritin while that between A1AT and transferrin levels was not significant. Plasma CRP concentrations were not significantly different between MM, MZ, and ZZ individuals. ICPAES measurement of metals confirmed elevated plasma concentrations of nonheme iron among ZZ individuals. Nonheme iron concentrations correlated (negatively) with levels of A1AT. A1AT deficiency is associated with evidence of a disruption in iron homeostasis with plasma ferritin and nonheme iron concentrations being elevated among those with the ZZ genotype.

Iron mobilization from transferrin by therapeutic iron chelating agents

BACKGROUND

The bacteriostatic activity of the transferrin family has been known since the early 1960's. The possession of high affinity iron(III)-binding sites and the existence of a specific membrane-bound receptor, have led to the present understanding of serum transferrin acting as the major iron transporter between cells in vertebrate systems. Iron chelators can interact with transferrin, either by directly donating iron or by removing iron from the protein; both interactions have relevance for haematology.

SCOPE OF REVIEW

Urea polyacrylamide gels and HPLC methods have been developed for the resolution and quantification of the four major forms of transferrin, diferric-transferrin, C-mono Fe-transferrin, N-mono Fe-transferrin and apo transferrin.

MAJOR CONCLUSIONS

Negatively charged ligands with pFe values >20 remove iron from transferrin, preferably from the N-lobe iron-binding site. Some siderophores are capable of removing iron from transferrin. 3-Hydroxypyridin-4-ones, lacking a negative charge are able to remove iron from transferrin with a strong preference for the C- lobe iron-binding site. The donation of iron to apo transferrin by hydroxypyridinone iron(III) complexes has relevance to the treatment of clinical anaemias, because the hydroxypyridinones can also mobilize iron from the reticuloendothelial system and so facilitate the redistribution of iron from macrophages to reticulocytes.

GENERAL SIGNIFICANCE

Hydroxypyridinones have excellent potential for facilitating the redistribution of iron and this has relevance to the treatment of many disease types, including neurodegeneration and clinical anaemias. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Hydroxamate siderophores of Scedosporium apiospermum

Scedosporium apiospermum is an emerging pathogen colonizing the airways of patients with cystic fibrosis and causing severe infections in immunocompromised hosts. In order to improve our knowledge on the pathogenic mechanisms of this fungus, we investigated the production of siderophores. Cultivation on CAS medium and specific assays for different classes of siderophores suggested the secretion of hydroxamates. A maximal production was obtained by cultivation of the fungus at alkaline pH in an iron-restricted liquid culture medium. Siderophores were then extracted from the culture filtrate by liquid/liquid extraction, and separated by reverse phase high performance liquid chromatography. Two siderophores, dimerumic acid and Nα-methyl coprogen B, were identified by electrospray ionization-mass spectrometry and MS-MS fragmentation. Finally, comparison of various strains suggested a higher production of Na-methyl coprogen B by clinical isolates of respiratory origin. Studies are initiated in order to determine the potential usefulness of these siderophores as diagnostic markers of scedosporiosis.

Novel inhibitors of the Pseudomonas aeruginosa virulence factor LasB: a potential therapeutic approach for the attenuation of virulence mechanisms in pseudomonal infection

Pseudomonas elastase (LasB), a metalloprotease virulence factor, is known to play a pivotal role in pseudomonal infection. LasB is secreted at the site of infection, where it exerts a proteolytic action that spans from broad tissue destruction to subtle action on components of the host immune system. The former enhances invasiveness by liberating nutrients for continued growth, while the latter exerts an immunomodulatory effect, manipulating the normal immune response. In addition to the extracellular effects of secreted LasB, it also acts within the bacterial cell to trigger the intracellular pathway that initiates growth as a bacterial biofilm. The key role of LasB in pseudomonal virulence makes it a potential target for the development of an inhibitor as an antimicrobial agent. The concept of inhibition of virulence is a recently established antimicrobial strategy, and such agents have been termed "second-generation" antibiotics. This approach holds promise in that it seeks to attenuate virulence processes without bactericidal action and, hence, without selection pressure for the emergence of resistant strains. A potent inhibitor of LasB, N-mercaptoacetyl-Phe-Tyr-amide (K(i) = 41 nM) has been developed, and its ability to block these virulence processes has been assessed. It has been demonstrated that thes compound can completely block the action of LasB on protein targets that are instrumental in biofilm formation and immunomodulation. The novel LasB inhibitor has also been employed in bacterial-cell-based assays, to reduce the growth of pseudomonal biofilms, and to eradicate biofilm completely when used in combination with conventional antibiotics.

The yersiniabactin transport system is critical for the pathogenesis of bubonic and pneumonic plague

Iron acquisition from the host is an important step in the pathogenic process. While Yersinia pestis has multiple iron transporters, the yersiniabactin (Ybt) siderophore-dependent system plays a major role in iron acquisition in vitro and in vivo. In this study, we determined that the Ybt system is required for the use of iron bound by transferrin and lactoferrin and examined the importance of the Ybt system for virulence in mouse models of bubonic and pneumonic plague. Y. pestis mutants unable to either transport Ybt or synthesize the siderophore were both essentially avirulent via subcutaneous injection (bubonic plague model). Surprisingly, via intranasal instillation (pneumonic plague model), we saw a difference in the virulence of Ybt biosynthetic and transport mutants. Ybt biosynthetic mutants displayed an approximately 24-fold-higher 50% lethal dose (LD(50)) than transport mutants. In contrast, under iron-restricted conditions in vitro, a Ybt transport mutant had a more severe growth defect than the Ybt biosynthetic mutant. Finally, a Delta pgm mutant had a greater loss of virulence than the Ybt biosynthetic mutant, indicating that the 102-kb pgm locus encodes a virulence factor, in addition to Ybt, that plays a role in the pathogenesis of pneumonic plague.

Analysis of the high-affinity iron uptake system at the Chlamydomonas reinhardtii plasma membrane

Multicopper ferroxidases play a vital role in iron metabolism in bacteria, fungi, algae, and mammals. Saccharomyces cerevisiae utilizes a channeling mechanism to couple the ferroxidase activity of Fet3p to Fe(3+) transport into the cell by Ftr1p. In contrast, the mechanisms by which mammals couple the ferroxidase reaction to iron trafficking is unclear. The human ferroxidases ceruloplasmin and hephaestin are twice the size of Fet3p and interact with proteins that are not expressed in fungi. Chlamydomonas FOX1 is a homolog of the human ferroxidases but likely supports iron uptake in a manner similar to that of yeast, since Chlamydomonas reinhardtii expresses a ferric iron permease homolog, FTR1. The results presented support this hypothesis. We show that FOX1 is trafficked to the plasma membrane and is oriented with its multicopper oxidase/ferroxidase domain in the exocytoplasmic space. Our analysis of FTR1 indicates its topology is similar to that of S. cerevisiae Ftr1p, with a potential exocytoplasmic iron channeling motif and two potential iron permeation motifs in membrane-spanning regions. We demonstrate that high-affinity iron uptake is dependent on FOX1 and the copper status of the cell. Kinetic inhibition of high-affinity iron uptake by a ferric iron chelator does not reflect the strength of the chelator, supporting a ferric iron channeling mechanism for high-affinity iron uptake in Chlamydomonas. Last, recombinant FOX1 (rFOX1) has been isolated in a partially holo form that exhibits the UV-visible absorbance spectrum of a multicopper oxidase and the catalytic activity of a ferroxidase.

Role of lung iron in determining the bacterial and host struggle in cystic fibrosis

Cystic fibrosis (CF) is the most common lethal genetic disorder in Caucasian populations. It is a multiorgan system disease that affects the lungs, gastrointestinal tract, liver, and pancreas. The majority of morbidity and mortality in CF relates to chronic airway infection with a variety of bacterial species, commencing in very early infancy, which results in lung destruction and ultimately organ failure ( 41 , 43 ). This review focuses on iron homeostasis in the CF lung and its role in determining the success and chronicity of Pseudomonas aeruginosa infection. There have been previous excellent reviews regarding iron metabolism in the lower respiratory tract and mechanisms of P. aeruginosa iron acquisition, and we direct readers to these articles for further background reading ( 31 , 53 , 58 , 77 , 96 ). In this review, we have brought the “two sides of the coin” together to provide a holistic overview of the relationship between host and bacterial iron homeostasis and put this information into the context of current understanding on infection in the CF lung.

The induction of nitric oxide response of carp macrophages by transferrin is influenced by the allelic diversity of the molecule

The central role of transferrin (Tf) as an iron transporting protein has been extended by observations that modified versions of Tf also participate in the regulation of innate immunity. We report on the isolation of two carp Tf proteins (alleles D and G) to purity using rivanol precipitation and ion-exchange chromatography, and describe the activation of head kidney-derived carp macrophages by cleaved Tf. We demonstrate the superiority of the D-type over the G-type Tf in inducing nitric oxide (NO) and confirm previous observations that full-length Tf cannot induce NO in fish macrophages. We believe that cleaved Tf fragments should be considered to be "alarmins". We discuss the possibility that parasites such as Trypanoplasma borreli cleave Tf and use Tf fragments to their advantage by modulating the NO induction in carp macrophages.

Iron acquisition by Pseudomonas aeruginosa in the lungs of patients with cystic fibrosis

The bacterium Pseudomonas aeruginosa is commonly isolated from the general environment and also infects the lungs of patients with cystic fibrosis (CF). Iron in mammals is not freely available to infecting pathogens although significant amounts of extracellular iron are available in the sputum that occurs in the lungs of CF patients. P. aeruginosa has a large number of systems to acquire this essential nutrient and many of these systems have been characterised in the laboratory. However, which iron acquisition systems are active in CF is not well understood. Here we review recent research that sheds light on how P. aeruginosa obtains iron in the lungs of CF patients.

Pseudomonas aeruginosa Alkaline Protease Can Facilitate Siderophore-Mediated Iron-Uptake via the Proteolytic Cleavage of Transferrins

In order to determine whether Pseudomonas aeruginosa alkaline protease AprA is involved in facilitating siderophore-mediated iron-acquisition from human transferrins, we measured bacterial growth, the production of siderophore and AprA, iron-acquisition from transferrins, and the proteolytic cleavage of transferrins in an alkaline minimal medium (pH 8.3) containing human transferrins as an iron source and compared these on a time scale. The growth of P. aeruginosa was found to be stimulated in proportion to the iron-saturation levels of transferrins. AprA production and the proteolytic cleavage of transferrins began concomitantly with siderophore production from the early growth phase when P. aeruginosa was actively growing and consuming most iron for growth. However, the AprA-free, but siderophore-containing, culture ultrafiltrates could also remove iron from transferrin. These results indicate that alkaline protease AprA can facilitate the siderophore-mediated iron-uptake of P. aeruginosa via the proteolytic cleavage of transferrins. However, the proteolytic cleavage by AprA is not essentially required for iron-acquisition from transferrins.

Proteases of a Bacillus subtilis Clinical Isolate Facilitate Swarming and Siderophore-Mediated Iron Uptake via Proteolytic Cleavage of Transferrin

We isolated a highly serine protease-producing Bacillus subtilis strain (PRY) from a clinical sample and identified it through biochemical testing and ribosomal DNA sequencing. The PRY strain exhibited a robust swarming behavior and was able to digest human transferrin efficiently, concomitantly with the production of catechol-siderophore in the exponential growth phase. The growth of PRY was in proportion to increased iron availability resulting from transferrin destruction. These results suggest that proteases of the B. subtilis PRY strain may play a significant role in the pathogenesis of human infections by facilitating siderophore-mediated iron uptake from transferrin and swarming motility.

The Aspergillus fumigatus siderophore biosynthetic gene sidA, encoding L-ornithine N5-oxygenase, is required for virulence

Aspergillus fumigatus is the leading cause of invasive mold infection and is a serious problem in immunocompromised populations worldwide. We have previously shown that survival of A. fumigatus in serum may be related to secretion of siderophores. In this study, we identified and characterized the sidA gene of A. fumigatus, which encodes l-ornithine N(5)-oxygenase, the first committed step in hydroxamate siderophore biosynthesis. A. fumigatus sidA codes for a protein of 501 amino acids with significant homology to other fungal l-ornithine N(5)-oxygenases. A stable DeltasidA strain was created by deletion of A. fumigatus sidA. This strain was unable to synthesize the siderophores N',N",N'''-triacetylfusarinine C (TAF) and ferricrocin. Growth of the DeltasidA strain was the same as that of the wild type in rich media; however, the DeltasidA strain was unable to grow in low-iron defined media or media containing 10% human serum unless supplemented with TAF or ferricrocin. No significant differences in ferric reduction activities were observed between the parental strain and the DeltasidA strain, indicating that blocking siderophore secretion did not result in upregulation of this pathway. Unlike the parental strain, the DeltasidA strain was unable to remove iron from human transferrin. A rescued strain (DeltasidA + sidA) was constructed; it produced siderophores and had the same growth as the wild type on iron-limited media. Unlike the wild-type and rescued strains, the DeltasidA strain was avirulent in a mouse model of invasive aspergillosis, indicating that sidA is necessary for A. fumigatus virulence.

Site-specific rate constants for iron acquisition from transferrin by the Aspergillus fumigatus siderophores N′,N′′,N′′′-triacetylfusarinine C and ferricrocin

Aspergillus fumigatus is an opportunistic fungal pathogen that causes life-threatening infections in immunocompromised patients. Despite low levels of free iron, A. fumigatus grows in the presence of human serum in part because it produces high concentrations of siderophores. The most abundant siderophores produced by A. fumigatus are N',N'',N'''-triacetylfusarinine C (TAF) and ferricrocin, both of which have thermodynamic iron binding constants that theoretically allow them to remove transferrin (Tf)-bound iron. Urea-polyacrylamide gel electrophoresis was used to measure the change in concentration of Tf species incubated with TAF or ferricrocin. The rate of removal of iron from diferric Tf by both siderophores was measured, as were the individual microscopic rates of iron removal from each Tf species (diferric Tf, N-terminal monoferric Tf and C-terminal monoferric Tf). TAF removed iron from all Tf species at a faster rate than ferricrocin. Both siderophores showed a preference for removing C-terminal iron, evidenced by the fact that k(1C) and k(2C) were much larger than k(1N) and k(2N). Cooperativity in iron binding was observed with TAF, as the C-terminal iron was removed by TAF much faster from monoferric than from diferric Tf. With both siderophores, C-terminal monoferric Tf concentrations remained below measurable levels during incubations. This indicates that k(2C) and k(1C) are much larger than k(1N). TAF and ferricrocin both removed Tf-bound iron with second-order rate constants that were comparable to those of the siderophores of several bacterial pathogens, indicating they may play a role in iron uptake in vivo and thereby contribute to the virulence of A. fumigatus.

The Crystal Structure of the Pyoverdine Outer Membrane Receptor FpvA from Pseudomonas aeruginosa at 3.6Å Resolution

The pyoverdine outer membrane receptor FpvA from Pseudomonas aeruginosa translocates ferric-pyoverdine across the outer membrane via an energy consuming mechanism that involves the inner membrane energy transducing complex of TonB-ExbB-ExbD and the proton motive force. We solved the crystal structure of FpvA loaded with iron-free pyoverdine at 3.6 angstroms resolution. The pyoverdine receptor is folded in two domains: a transmembrane 22-stranded beta-barrel domain occluded by an N-terminal domain containing a mixed four-stranded beta-sheet (the plug). The beta-strands of the barrel are connected by long extracellular loops and short periplasmic turns. The iron-free pyoverdine is bound at the surface of the receptor in a pocket lined with aromatic residues while the extracellular loops do not completely cover the pyoverdine binding site. The TonB box, which is involved in intermolecular contacts with the TonB protein of the inner membrane, is observed in an extended conformation. Comparison of this first reported structure of an iron-siderophore transporter from a bacterium other than Escherichia coli with the known structures of the E.coli TonB-dependent transporters reveals a high structural homology and suggests that a common sensing mechanism exists for the iron-loading status in all bacterial iron siderophore transporters.

Cleavage of human transferrin by Porphyromonas gingivalis gingipains promotes growth and formation of hydroxyl radicals

Porphyromonas gingivalis, a gram-negative anaerobic bacterium associated with active lesions of chronic periodontitis, produces several proteinases which are presumably involved in host colonization, perturbation of the immune system, and tissue destruction. The aims of this study were to investigate the degradation of human transferrin by gingipain cysteine proteinases of P. gingivalis and to demonstrate the production of toxic hydroxyl radicals (HO*) catalyzed by the iron-containing transferrin fragments generated or by release of iron itself. Analysis by polyacrylamide gel electrophoresis and Western immunoblotting showed that preparations of Arg- and Lys-gingipains of P. gingivalis cleave transferrin (iron-free and iron-saturated forms) into fragments of various sizes. Interestingly, gingival crevicular fluid samples from diseased periodontal sites but not samples from healthy periodontal sites contained fragments of transferrin. By using (55)Fe-transferrin, it was found that degradation by P. gingivalis gingipains resulted in the production of free iron, as well as iron bound to lower-molecular-mass fragments. Subsequent to the degradation of transferrin, bacterial cells assimilated intracellularly the radiolabeled iron. Growth of P. gingivalis ATCC 33277, but not growth of an Arg-gingipain- and Lys-gingipain-deficient mutant, was possible in a chemically defined medium containing 30% iron-saturated transferrin as the only source of iron and peptides, suggesting that gingipains play a critical role in the acquisition of essential growth nutrients. Finally, the transferrin degradation products generated by Arg-gingipains A and B were capable of catalyzing the formation of HO*, as determined by a hypoxanthine/xanthine oxidase system and spin trapping-electron paramagnetic resonance spectrometry. Our study indicates that P. gingivalis gingipains degrade human transferrin, providing sources of iron and peptides. The iron-containing transferrin fragments or the release of iron itself may contribute to tissue destruction by catalyzing the formation of toxic HO*.

Survival of Aspergillus fumigatus in serum involves removal of iron from transferrin: the role of siderophores

Aspergillus fumigatus is a filamentous fungus which can cause invasive disease in immunocompromised individuals. A. fumigatus can grow in medium containing up to 80% human serum, despite very low concentrations of free iron. The purpose of this study was to determine the mechanism by which A. fumigatus obtains iron from the serum iron-binding protein transferrin. In iron-depleted minimal essential medium (MEM), A. fumigatus growth was supported by the addition of holotransferrin (holoTf) or FeCl(3) but not by the addition of apotransferrin (apoTf). Proteolytic degradation of transferrin by A. fumigatus occurred in MEM-serum; however, transferrin degradation did not occur until late logarithmic phase. Moreover, transferrin was not degraded by A. fumigatus incubated in MEM-holoTf. Urea polyacrylamide gel electrophoresis showed that in MEM-holoTf, holoTf was completely converted to apoTf by A. fumigatus. In human serum, all of the monoferric transferrin was converted to apoTf within 8 h. Siderophores were secreted by A. fumigatus after 8 h of growth in MEM-serum and 12 h in MEM-holoTf. The involvement of small molecules in iron acquisition was confirmed by the fact that transferrin was deferrated by A. fumigatus even when physically separated by a 12-kDa-cutoff membrane. Five siderophores were purified from A. fumigatus culture medium, and the two major siderophores were identified as triacetylfusarinine C and ferricrocin. Both triacetylfusarinine C and ferricrocin removed iron from holoTf with an affinity comparable to that of ferrichrome. These data indicate that A. fumigatus survival in human serum in vitro involves siderophore-mediated removal of iron from transferrin. Proteolytic degradation of transferrin may play a secondary role in iron acquisition.

Iron deficiency leads to inhibition of oxygen transfer and enhanced formation of virulence factors in cultures of Pseudomonas aeruginosa PAO1

Pseudomonas aeruginosa PAO1 was recently found to exhibit two remarkable physiological responses to oxidative stress: (1) a strong reduction in the efficiency of oxygen transfer from the gas phase into the liquid phase, thus causing oxygen limitation in the culture and (2) formation of a clear polysaccharide capsule on the cell surface. In this work, it has been shown that the iron concentration in the culture plays a crucial role in evoking these phenomena. The physiological responses of two P. aeruginosa PAO1 isolates (NCCB 2452 and ATCC 15692) were examined in growth media with varied iron concentrations. In a computer-controlled bioreactor cultivation system for controlled dissolved oxygen tension (pO2), a strong correlation between the exhaustion of iron and the onset of oxygen limitation was observed. The oxygen transfer rate of the culture, characterized by the volumetric oxygen transfer coefficient, kLa, significantly decreased under iron-limited conditions. The formation of alginate and capsule was more strongly affected by iron concentration than by oxygen concentration. The reduction of the oxygen transfer rate and the subsequent oxygen limitation triggered by iron deficiency may represent a new and efficient way for P. aeruginosa PAO1 to adapt to growth conditions of iron limitation. Furthermore, the secretion of proteins into the culture medium was strongly enhanced by iron limitation. The formation of the virulence factor elastase and the iron chelators pyoverdine and pyochelin also significantly increased under iron-limited conditions. These results have implications for lung infection of cystic fibrosis patients by P. aeruginosa in view of the prevalence of iron limitation at the site of infection and the respiratory failure leading to death.

Iron-Mediated regulation of alkaline proteinase production in Pseudomonas aeruginosa

We analyzed the regulation by iron of alkaline proteinase (AP) production in Pseudomonas aeruginosa. Extracellular AP production was detected from the mid-logarithmic to the stationary phase by an antibody-based assay system, and was strongly repressed by iron in the medium. This repression was shown by Northern hybridization and primer extension to occur at the level of transcription. The primer extension analysis revealed that the start point of transcription of AP gene was the nucleotide position -84 from the start point of translation. Furthermore, we investigated whether this transcriptional repression involved PvdS protein. Using the mutant strain of pvdS, the alternative sigma factor gene revealed that the PvdS protein is required for the full expression of AP, and a previous study showed that expression of pvdS is also repressed by iron. Therefore, we thought that one mechanism of repression of AP production operated through reduction of the PvdS protein level. Purified AP decomposed the transferrin, and released iron from it. Purified AP added to the medium containing transferrin as the only iron source enhanced the growth of P. aeruginosa. Moreover, mutation in the AP gene decreased the growth rate in the medium containing the transferrin as the only iron source. These results clearly indicated that AP expression should occur in a free-iron-deficient environment and emphasized the importance of AP to iron acquisition in the infection site.

Acquisition of iron from human transferrin by Porphyromonas gingivalis: a role for Arg- and Lys-gingipain activities

Porphyromonas gingivalis, a key causative agent of adult periodontitis, is known to produce a variety of virulence factors including proteases. The aim of this study was to evaluate the participation of Arg- and Lys-gingipain activities of P. gingivalis in the acquisition of iron from human transferrin and its subsequent utilization in growth. Iron-saturated transferrin was found to support the long-term growth of P. gingivalis. Our results indicated that P. gingivalis does not produce siderophore and does not possess ferric reductase and transferrin-binding activities. Incubating transferrin with P. gingivalis resulted in degradation of the protein, a step that may be critical for the acquisition of iron from transferrin. Spontaneous and site-directed mutants of P. gingivalis deficient in one or several proteases were used to demonstrate the key role of specific enzymes in degradation of transferrin and subsequent utilization for growth. The lack of both Arg- and Lys-gingipain activities (mutants M1 and KDP128) was associated with an absence of degradation of transferrin and the incapacity of bacteria to grow in the presence of transferrin as the sole source of iron. It was also found that the Lys-gingipain activity is more critical than the Arg-gingipain activity since the mutant KDP112 (deficient in Arg-gingipain A and B) could grow whereas the mutant KDP129 (deficient in Lys-gingipain) could not. The fact that growth of mutant KDP112 was associated with a lower final optical density and a generation time much longer compared with the parent strain suggests that the Arg-gingipain activity also participates in the acquisition of iron from transferrin. Selected inhibitors of cysteine proteases (TLCK, leupeptin and cathepsin B inhibitor II) were tested for their capacity to reduce or inhibit the growth of P. gingivalis under different iron conditions. All three inhibitors were found to completely inhibit growth of strain ATCC 33277 in a medium supplemented with transferrin as the source of iron. The inhibitors had no effects when the bacteria were grown in a medium containing hemin instead of transferrin. The ability of P. gingivalis to cleave transferrin may be an important mechanism for the acquisition of iron from this protein during periodontitis.

Iron metabolism in pathogenic bacteria

The ability of pathogens to obtain iron from transferrins, ferritin, hemoglobin, and other iron-containing proteins of their host is central to whether they live or die. To combat invading bacteria, animals go into an iron-withholding mode and also use a protein (Nramp1) to generate reactive oxygen species in an attempt to kill the pathogens. Some invading bacteria respond by producing specific iron chelators-siderophores-that remove the iron from the host sources. Other bacteria rely on direct contact with host iron proteins, either abstracting the iron at their surface or, as with heme, taking it up into the cytoplasm. The expression of a large number of genes (>40 in some cases) is directly controlled by the prevailing intracellular concentration of Fe(II) via its complexing to a regulatory protein (the Fur protein or equivalent). In this way, the biochemistry of the bacterial cell can accommodate the challenges from the host. Agents that interfere with bacterial iron metabolism may prove extremely valuable for chemotherapy of diseases.

Iron-free pyoverdin binds to its outer membrane receptor FpvA in Pseudomonas aeruginosa: a new mechanism for membrane iron transport

Under iron limitation, Pseudomonas aeruginosa secretes a fluorescent siderophore called pyoverdin, which, after complexing iron, is transported back into the cell via its outer membrane receptor FpvA. Previous studies demonstrated co-purification of FpvA with iron-free PaA and reported similar binding affinities of iron-free pyoverdin and ferric-pyoverdin to purified FpvA. The fluorescence resonance energy transfer between iron-free PaA and the FpvA receptor here reveals the existence of an FpvA-pyoverdin complex in P. aeruginosa in vivo, suggesting that the pyoverdin-loaded FpvA is the normal state of the receptor in the absence of iron. Using tritiated ferric-pyoverdin, it is shown that iron-free PaA binds to the outer membrane but is not taken up into the cell, and that in vitro and, presumably, in vivo ferric-pyoverdin displaces the bound iron-free pyoverdin on FpvA-PaA to form FpvA-PaA-Fe complexes. In vivo, the kinetics of formation of this FpvA-PaA-Fe complex are more than two orders of magnitude faster than in vitro and depend on the presence of TonB. In P. aeruginosa, two tonB genes have been identified (tonB1 and tonB2). TonB1 is directly involved in ferric-pyoverdin uptake, and TonB2 seems to be able partially to replace TonB1 in its role in iron acquisition. However, no effect of TonB1 or TonB2 on the apparent affinity of free pyoverdin to FpvA was observed, and a 17-fold difference was measured between the affinities of the two forms of pyoverdin (PaA and PaA-Fe) to FpvA in the absence of TonB1 or TonB2. The mechanism of iron uptake in P. aeruginosa via the pyoverdin pathway is discussed in view of these new findings.

Impact of siderophore production on Pseudomonas aeruginosa infections in immunosuppressed mice

Pseudomonas aeruginosa produces siderophores, pyoverdin and pyochelin, for high-affinity iron uptake. To investigate their contribution to P. aeruginosa infections, we constructed allelic exchange mutants from strain PAO1 which were deficient in producing one or both of the siderophores. When inoculated into the calf muscles of immunosuppressed mice, pyochelin-deficient and pyoverdin-deficient mutants grew and killed the animals as efficiently as PAO1. In contrast, the pyochelin- and pyoverdin-deficient (double) mutant did not show lethal virulence, although it did infect the muscles. On the other hand, when inoculated intranasally, all mutants grew in the lungs and killed immunosuppressed mice. Compared with PAO1, however, the pyoverdin-deficient mutant and the double mutant grew poorly in the lungs, and the latter was significantly attenuated for virulence. Irrespective of the inoculation route, the pyoverdin-deficient and doubly deficient mutants detected in the blood were significantly less numerous than PAO1. Additionally, in vitro examination demonstrated that the growth of the double mutant was extremely reduced under a free-iron-restricted condition with apotransferrin but that the growth reduction was completely canceled by supplementation with hemoglobin as a heme source. These results suggest that both pyoverdin and pyochelin are required for efficient bacterial growth and full expression of virulence in P. aeruginosa infection, although pyoverdin may be comparatively more important for bacterial growth and dissemination. However, the siderophores were not always required for infection. It is possible that non-siderophore-mediated iron acquisition, such as via heme uptake, might also play an important role in P. aeruginosa infections.

Genetics and regulation of two distinct haem-uptake systems, phu and has, in Pseudomonas aeruginosa

A gene cluster similar to haem iron uptake loci of bacterial pathogens was identified in Pseudomonas aeruginosa. This phu locus ('Pseudomonas haem uptake') consisted of the phuR receptor gene and the phuSTUVW operon encoding a typical ABC transporter. Expression of phuR and phuSTUVW from mapped transcriptional-start sites occurred under iron-restricted growth conditions and was directly controlled by the Fur protein. Binding of Fur was demonstrated by DNase footprinting of two adjacent 'Fur boxes' that overlapped both the phuR and phuSTUVW promoters. Two tandem repeats of 154 bp were identified downstream of the phuSTUVW operon, each of which contained a strong Fur-dependent promoter driving expression of iron-regulated RNAs antisense to phuSTUVW. Mutant strains with deletions in phuR and phuSTUV showed greatly reduced growth with either haem or haemoglobin as the only iron source: the defects were complemented by plasmids harbouring the phuR or the phuSTUV genes, respectively. Deletions of phuW or of the tandem repeats had only minor effects on haem utilization. The remaining haem and haemoglobin uptake still observed in the deltaphuR or deltaphuSTUV deletion mutants was due to a second haem-acquisition system, has, which was also under the direct control of Fur. This second haem-receptor gene, hasR, was identified upstream of and in an operon with hasA, encoding a haem-binding extracellular protein. A deltahasR mutant also exhibited decreased utilization of haem and haemoglobin, and a deltaphuR deltahasR double mutant was virtually unable to take up either compound. Both the PhuR and HasR proteins were detected in the outer-membrane fraction of P. aeruginosa grown in low-iron media. Taken together, the evidence suggests that the phu and has loci encode two distinct systems required for the acquisition of haem and haemoglobin in P. aeruginosa.

The response of Pseudomonas aeruginosa to iron: genetics, biochemistry and virulence

During the past decade significant progress has been made towards identifying some of the schemes that Pseudomonas aeruginosa uses to obtain iron and towards cataloguing and characterizing many of the genes and gene products that are likely to play a role in these processes. This review will largely recount what we have learned in the past few years about how P. aeruginosa regulates its acquisition, intake and, to some extent, trafficking of iron, and the role of iron acquisition systems in the virulence of this remarkable opportunistic pathogen. More specifically, the genetics, biochemistry and biology of an essential regulator (Ferric uptake regulator - Fur) and a Fur-regulated alternative sigma factor (PvdS), which are central to these processes, will be discussed. These regulatory proteins directly or indirectly regulate a substantial number of other genes encoding proteins with remarkably diverse functions. These genes include: (i) other regulatory genes, (ii) genes involved in basic metabolic processes (e.g. Krebs cycle), (iii) genes required to survive oxidative stress (e.g. superoxide dismutase), (iv) genes necessary for scavenging iron (e.g. siderophores and their cognate receptors) or genes that contribute to the virulence (e.g. exotoxin A) of this opportunistic pathogen. Despite this recent expansion of knowledge about the response of P. aeruginosa to iron, many significant biological issues surrounding iron acquisition still need to be addressed. Virtually nothing is known about which of the distinct iron acquisition mechanisms P. aeruginosa brings to bear on these questions outside the laboratory, whether it be in soil, in a pipeline, on plants or in the lungs of cystic fibrosis patients.

Receptor-mediated recognition and uptake of iron from human transferrin by Staphylococcus aureus and Staphylococcus epidermidis

Staphylococcus aureus and Staphylococcus epidermidis both recognize and bind the human iron-transporting glycoprotein, transferrin, via a 42-kDa cell surface protein receptor. In an iron-deficient medium, staphylococcal growth can be promoted by the addition of human diferric transferrin but not human apotransferrin. To determine whether the staphylococcal transferrin receptor is involved in the removal of iron from transferrin, we employed 6 M urea-polyacrylamide gel electrophoresis, which separates human transferrin into four forms (diferric, monoferric N-lobe, and monoferric C-lobe transferrin and apotransferrin). S. aureus and S. epidermidis but not Staphylococcus saprophyticus (which lacks the transferrin receptor) converted diferric human transferrin into its apotransferrin form within 30 min. During conversion, iron was removed sequentially from the N lobe and then from the C lobe. Metabolic poisons such as sodium azide and nigericin inhibited the release of iron from human transferrin, indicating that it is an energy-requiring process. To demonstrate that this process is receptor rather than siderophore mediated, we incubated (i) washed staphylococcal cells and (ii) the staphylococcal siderophore, staphyloferrin A, with porcine transferrin, a transferrin species which does not bind to the staphylococcal receptor. While staphyloferrin A removed iron from both human and porcine transferrins, neither S. aureus nor S. epidermidis cells could promote the release of iron from porcine transferrin. In competition binding assays, both native and recombinant N-lobe fragments of human transferrin as well as a naturally occurring human transferrin variant with a mutation in the C-lobe blocked binding of 125I-labelled transferrin. Furthermore, the staphylococci removed iron efficiently from the iron-loaded N-lobe fragment of human transferrin. These data demonstrate that the staphylococci efficiently remove iron from transferrin via a receptor-mediated process and provide evidence to suggest that there is a primary receptor recognition site on the N-lobe of human transferrin.

Peptide siderophores

Siderophores are low molecular weight iron chelators, produced by virtually all bacteria, fungi and some plants. They serve to deliver the essential element iron, barely soluble under aerobic conditions, into microbial cells. Siderophores are therefore important secondary metabolites which are very often based on amino acids and their derivatives. Biosynthesis, transport, regulation and chemical synthesis of natural siderophores and their analogues is of considerable interest for the protein and peptide chemist. This review gives an overview of the structural classes of peptidic siderophores, along with data on their biosynthesis. On a number of representative examples, strategies and schemes of their chemical synthesis are described.