Caister Academic Press

Iron Metabolism in Microorganisms

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Iron Uptake and Metabolism in Microorganisms

from Iron Uptake and Homeostasis in Microorganisms by Cornelis, P and Andrews, SC (2010)

Iron is essential for almost all living organisms as it is involved in a wide variety of important metabolic processes. However, iron is not readily available and microorganisms therefore employ various iron uptake systems to secure sufficient supplies from their surroundings. There is considerable variation in the range of iron transporters and iron sources utilised by different microbial species. Pathogens, in particular, require efficient iron acquisition mechanisms to enable them to compete successfully for iron in the highly iron-restricted environment of the host's tissues and body fluids.

Siderophores from Bacteria and from Fungi

In the early days of life on earth due to the emergence of photosynthesis, the atmosphere became oxidative and so iron prevailed in its trivalent form. The consequence was that most microorganisms live today in a surrounding where the concentration of soluble iron compounds is much too low to satisfy their physiological needs. One of the possibilities to circumvent this problem is the production of so-called siderophores, compounds which can form water soluble complexes with Fe3+. They are released in situations of iron deficiency. Fe3+ possesses six coordination sites arranged in the shape of an octahedron which can accommodate three bidentate ligands. The major ligand types are catecholates, hydroxamates and alpha-hydroxycarboxylates, in the ideal case connected by adequate spacers which keep them in the correct position. Many siderophores have elaborate structures which can bind to the cell surface of the producing species. In this way pirating by competing microorganisms is rendered more difficult.

Heme Uptake and Iron Extraction by Bacteria

Heme is ubiquitous, abundant and necessary for energy metabolism. Most bacteria have a heme biosynthesis pathway, but nevertheless, since heme is a major source of iron (an essential metal), microbes take up exogenous heme to retrieve iron. To grab heme, microbes extract it from host hemoproteins. This is achieved by two non-exclusive distinct pathways. One pathway involves proteins secreted by bacteria (hemophores) that scavenge heme from host hemoproteins. The second pathway involves microbial cell surface receptors that catch hemoproteins circulating in the vicinity of the cell surface. Both pathways lead to heme docking to cell surface receptors. In Gram-negative bacteria, docked heme is transported through the outer membrane by an energy-dependent process. In Gram-positive bacteria, docked heme is transferred to membrane-anchored heme binding lipoproteins. In all thus far described systems, heme is actively transported through the plasma membrane by an ATP hydrolysis-powered ABC transporter. Heme is either degraded into biliverdin, CO and iron by heme oxygenases, or iron is retrieved from heme, keeping the tetrapyrrol ring intact by recently identified enzymes. As excess heme is toxic, heme uptake, efflux and degradation are usually highly regulated. In most cases, intracytoplasmic heme or iron released during heme degradation are cofactors along with transcriptional regulators. In several cases, heme uptake and efflux are regulated by extracellular heme.

Iron in the Rhizobia

The rhizobia live as free-living soil bacteria or in symbiosis with leguminous plants. The success of these organisms in each milieu involves the ability to sense the environment to assess the availability of nutrients, and to optimize cellular systems for their acquisition. Iron in the rhizosphere is mostly inaccessible due to low solubility, and microorganisms must compete for this limited nutrient. Rhizobia belong to the alpha-Proteobacteria, a diverse taxonomic group that includes numerous species that form close or intracellular associations with eukaryotic hosts in a symbiotic or pathogenic context. Thus, in addition to their agricultural and economic importance, rhizobia are model organisms that have given new insights into related, but less tractable animal pathogens. In particular, genetic control of iron homeostasis in the rhizobia and other alpha-Proteobacteria has moved away from the Fur paradigm to an iron sensing mechanism responding to the metal indirectly. Moreover, utilization of heme as an iron source is not unique to animal pathogens, but is an acquisition strategy employed by the rhizobia with some interesting novel features.

Iron in Bordetella

Upon colonization of the mammalian respiratory epithelium by mucosal pathogens of the genus Bordetella, the host-pathogen interaction causes inflammatory changes, immune activation, and host cell injury. In this dynamic environment, Bordetella cells scavenge the nutritional iron necessary for growth. The three classical Bordetella species produce the siderophore alcaligin. In addition, they can utilize xenosiderophores that could be produced by commensals or other microbes that transiently inhabit the nasopharynx. As infection progresses, extravasation of immune cells, erythrocytes and serum to the mucosal surface can occur, exacerbated by the damaging action of Bordetella toxins, thus providing iron sources such as transferrin and heme compounds to the microbe. The three characterized Bordetella iron systems for utilization of alcaligin, enterobactin and heme are each inducible by the cognate iron source. The ability to sense and respond to the presence of available iron sources allows these pathogens to adapt to temporal changes in iron source availability, and this ability is important for successful in vivo growth.

Iron Shigella and E. coli

Shigella spp. and pathogenic E. coli are characterized by a variety and abundance of iron transport systems. Although members of this group of bacteria are closely related genetically, they differ widely in the iron transport systems they use. This may reflect the different niches occupied by different strains and the nature of the source of iron available in a specific environment. Only the ferrous iron transporter Feo is common to all the commensals and pathogens. All members of this group produce one or more siderophore, but no single siderophore is produced by all. Other iron transport systems include heme transporters and the ferrous iron transporters Sit and Efe. With the exception of the genes for enterobactin and the Feo system, the iron transport genes in the enterics are found within pathogenicity islands or on plasmids and their presence often increases pathogenicity or colonization of niches within the host.

Iron inErwinia

The critical role of iron in host-pathogen relationships has been elucidated in infectious diseases of mammals, where the importance of siderophores in microbial pathogenesis has been demonstrated. Our group has established the role of iron and its ligands in the virulence of the plant pathogenic bacteria Dickeya dadantii (Erwinia chrysanthemi) and Erwinia amylovora. The genomes of the two pectinolytic enterobacterial species Pectobacterium atrosepticum SCRI1043 and D. dadantii 3937 have been sequenced and annotated. This review focuses on the functions involved in iron acquisition in both species. Besides the production and utilization of siderophores, P. atrosepticum and D. datantii have the capacity to use other iron sources. Indeed, both species are able to use haem iron, whereas only P. atrosepticum can transport the ferric citrate complex and only D. dadantii can acquire ferrous iron. These different modes of iron capture indicate that these species have to cope with various environmental and ecological conditions during their pathogenic life cycle.

Iron in Vibrio and Aeromonas

Vibrio and Aeromonas species are ubiquitous bacteria in aquatic environments worldwide. Many of the species are important pathogens for humans and/or aquatic animals. Several iron acquisition strategies have been developed by vibrios and aeromonads in order to get this essential element for surviving in their host and in aquatic habitats. All species studied so far have the ability to synthesize siderophores to sequester iron from the cell environment and transport it through their respective cognate outer membrane receptors. It has been demonstrated that this capacity is a relevant virulence factor for human and animal pathogens. Furthermore, all species studied can utilize exogenous siderophores, made by other bacteria. Another iron acquisition system described in both genera involves the use of heme as a source of iron, by a mechanism very well conserved among all species, which involves a heme transporter that includes a specific TonB-dependent outer membrane receptor(s) and an ABC-type inner membrane transporter. Alternative systems based on ferrous or ferric iron transporters have been reported in V. cholerae. How the different iron acquisition systems work together to supply iron to the cell and how they are used in the different environments where vibrios and aeromonads can be found is still an open question.

Iron in Francisella

Francisella tularensis is unusual among Gram-negative bacteria in that its genome does not encode orthologs for TonB, ExbB and ExbD that typically energize the uptake of iron across the outer membrane. This organism secretes however a siderophore similar in structure to rhizoferrin. The fsl operon of six genes encodes functions for biosynthesis and uptake of the siderophore. Two of these genes encode a siderophore synthetase belonging to the nonribosomal peptide synthetase (NRPS)-independent synthetase (NIS)-family and a protein belonging to the pyridoxyl phosphate-dependent decarboxylase family, and both are required for siderophore production. Siderophore utilization involves the product of the fslE gene, a protein unique to Francisella species that could function as a siderophore receptor. Additionally, genes related in sequence to fslE also play a role in siderophore acquisition. The mechanism for TonB-independent iron uptake in this microorganism remains to be elucidated.

Iron in Bacteroides

Bacteroides spp. have an essential requirement for heme and non-heme iron. They cannot synthesize the tetrapyrrole macrocycle ring due to a lack of genes for the heme biosynthetic pathway. It is remarkable that heme-dependent organisms outnumber heme-independent organisms in the lower intestinal tract suggesting that heme biosynthesis is not essential for colonization of the colonic environment. However, this colonization advantage may be due to the fact that under anaerobic conditions in the presence of heme, B. fragilis can generate nearly the double amount of ATP than Escherichia coli per mol of glucose. This high energy yield is linked to a rudimentary heme-induced fumarate reductase and cytochrome b-dependent electron transport energy metabolism pathway which uses fumarate as the terminal electron acceptor. Moreover, Bacteroides spp. can incorporate iron-deuteroporphyrin and iron-mesoporphyrin into a functional type-b cytochrome. Heme can be demetalated without cleaving the tetrapyrrole ring releasing free iron and free protoporphirin IX. The ability of the opportunistic human pathogen B. fragilis to cause infections seems to be due in part to its ability to scavenge heme and iron from host proteins. The in-frame translated intergenic region of the fused FeoAB proteins are exclusively present in gastro-intestinal colonizers belonging to the Bacteroidetes, Firmicutes and Actinobacteria phyla. Several members of the Bacteroides group have three orthologs of the mammalian-type bacterial ferritin gene, ftnA. FtnA may play an important role in protection against iron-induced oxidative stress in this group of highly aerotolerant anaerobes.

Iron in Campylobacter

Iron is known to catalyze a wide range of biochemical reactions essential for most living organisms, including Campylobacter jejuni. Paradoxically, this iron reactivity is also responsible for the generation of hydroxyl radicals (·OH), which are particularly biotoxic. In order to avoid iron toxicity, microorganisms must achieve an effective iron homeostasis by tightly regulating the expression of genes encoding the proteins involved in iron acquisition, metabolism and oxidative stress defences in response to iron availability. Interestingly, in addition to the classical ferric uptake regulator Fur, C. jejuni carries another member of the Fur family of metalloregulators, PerR. PerR is a peroxide-sensing regulator and typically regulates peroxide stress response in Gram-positive bacteria. Recent work indicates that the regulatory functions of Fur and PerR extend beyond their classically ascribed roles. These diverse functions include energy metabolism, protein glycosylation and flagella biogenesis. Moreover, the Fur and PerR regulons appear to overlap and co-regulate key genes at specific junctions.

Iron in Cyanobacteria

Cyanobacteria are dependent on but can also be compromised by metals such as iron. On the one hand the demand for iron for photosystem functionality represents a challenge for the iron uptake machinery in iron limiting environments. On the other hand intoxication by iron causes a severe problem for growth and reproduction. To overcome this dilemma cyanobacteria have developed a regulatory network controlling iron uptake. They produce siderophores, which are distinct from that of other bacteria. Furthermore, the iron metabolism is linked to the nitrogen metabolism as documented for example in Anabaena sp. PCC 7120.

Iron in Bacillus

Bacillus subtilis is a metabolically versatile soil microbe and Gram-positive model organism that displays a sophisticated adaptive response to conditions of iron limitation. The endogenous siderophore of B. subtilis is bacillibactin, a trimeric catecholate siderophore similar in structure to enterobactin. In addition to bacillibactin, B. subtilis can obtain iron from several xenosiderophores, ferric citrate, heme, and through a newly discovered elemental iron permease. The regulation of iron homeostasis in B. subtilis is complex and involves a ferric uptake regulator (Fur) protein as master regulator and at least two subsidiary regulatory systems. The most significant of these is an iron-sparing/prioritization response controlled by the small RNA FsrA and three auxiliary proteins (FbpABC). In addition, the bacillibactin uptake system is transcriptionally activated by an AraC family activator, Btr that directly senses bacillibactin. Iron uptake and homeostasis systems in B. anthracis and related organisms are largely similar to those in B. subtilis with some additional components. These include a second siderophore synthesis operon for petrobactin, which is important for virulence, and a more elaborate (or at least better understood) heme uptake system.

Iron in Staphylococci

Staphylococcus aureus causes a significant amount of human morbidity and mortality. The ability of S. aureus to cause disease is dependent upon its acquisition of iron from the host. S. aureus can obtain iron from various sources during infection, including heme and transferrin. The most abundant iron source in humans is heme-iron bound by hemoglobin contained within erythrocytes. S. aureus is known to lyse erythrocytes through secretion of pore-forming toxins, providing access to host hemoglobin. Proteins of the iron-regulated surface determinant (Isd) system bind host hemoproteins, remove the heme cofactor, and shuttle heme into the cytoplasm for use as a nutrient iron source. Deletion of Isd system components decreases staphylococcal virulence, underscoring the importance of heme-iron acquisition during infection. In addition to heme, S. aureus can utilize transferrin-iron through the secretion of siderophores. Several staphylococcal siderophores have been described, some of which have defined roles during the pathogenesis of staphylococcal infections. A greater understanding of staphylococcal iron acquisition may lead to the development of novel therapeutic strategies that target nutrient uptake and decrease the threat of this increasingly drug-resistant bacterial pathogen.

Iron in Yeasts

Yeasts take up iron by three main mechanisms. In the reductive uptake mechanism, specialized flavo-hemoproteins (Fre) dissociate extracellular ferric complexes by reduction involving trans-plasma membrane electron transfer. The resulting free iron is then imported by a high-affinity permease system (Ftr), coupled to a copper-dependent oxidase (Fet), which channels iron through the plasma membrane. As a consequence, iron uptake by this mechanism is dependent on the availability of copper. In the siderophore-mediated mechanism, siderophores excreted by the cells or produced by other bacterial or fungal species are taken up without prior dissociation, via specific, copper-independent high-affinity receptors. The iron is then dissociated from the siderophores intracellularly, probably by reduction. In the heme uptake mechanism, free heme or heme bound to hemoglobin is taken up as such, probably by endocytosis. Iron is released intracellularly after hydrolysis of the porphyrin ring catalyzed by heme oxygenase. Within the cell, iron is stored in vacuoles or in siderophores. Iron can be mobilized from vacuoles by a reductive mechanism homologous to that found at the plasma membrane. Regulation of iron uptake and iron use are mediated by transcriptional regulators acting either as activators in iron-deficient conditions or as repressors in iron-rich conditions, according to the yeast species; these regulators thus adjust the iron uptake flux to the cell's requirements. In the baker's yeast, Saccharomyces cerevisiae, a post-transcriptional mechanism is active under low iron conditions, involving the degradation of RNAs encoding inessential iron-utilizing proteins. Other fungi have mechanisms serving a similar purpose at the transcriptional level. Studies in S. cerevisiae show that mitochondria are central to regulating cellular iron homeostasis, through the synthesis of iron-sulfur clusters.

from Iron Uptake and Homeostasis in Microorganisms by Cornelis, P and Andrews, SC (2010)


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