1. Introduction
Intracellular pathogens are organisms that are capable of growing and reproducing inside host cells. These pathogens can be divided into facultative intracellular parasites and obligate intracellular parasites [1].
Intracellular microorganisms are very important because they cause many human diseases, resulting in significant morbidity and mortality. Some examples of infectious diseases of global importance that are caused by intracellular microorganisms include tuberculosis, leprosy, typhoid, listeriosis, Legionnaire’s disease, malaria, leishmaniasis, Chagas’ disease, and toxoplasmosis. The course of infection is frequently long lasting and eventually results in chronic disease [2–4].
Facultative intracellular parasites, for example, bacteria such as Francisella tularensis, Listeria monocytogenes, Salmonella typhi, Mycobacterium spp., and Neisseria meningitidis, are capable of living and reproducing either inside or outside host cells. Obligate intracellular parasites cannot reproduce outside their host cell, which means that the parasite’s reproduction is entirely reliant on intracellular resources. Obligate intracellular parasites that infect humans include all viruses; certain bacteria such as Chlamydia and Rickettsia; certain protozoa such as Trypanosoma spp., Plasmodium, and Toxoplasma; and fungi such as Pneumocystis jirovecii [3].
Facultative intracellular bacteria invade host cells when they can gain a selective advantage in the host. Bacteria that can enter and survive within eukaryotic cells are shielded from humoral antibodies and can be eliminated only by a cellular immune response [5]. Moreover, once inside host cells, bacteria must utilize specialized mechanisms to protect themselves from the harsh environment of the lysosomal enzymes encountered within the cells.
Some examples include the bacterium Legionella pneumophila, which prefers the intracellular environment of macrophages for growth so it induces its own uptake and blocks lysosomal fusion by an undefined mechanism [6]; Rickettsia, which destroys the phagosomal membranes (with which the lysosomes fuse); and Salmonella and Mycobacterium spp., which are resistant to intracellular killing by phagocytic and other cells [2].
Other facultative intracellular bacteria include enteroinvasive Escherichia coli, Listeria monocytogenes, Neisseria spp., and Shigella spp. [2, 7]. Obligate intracellular bacteria cannot live outside the host cell. Chlamydial cells are unable to carry out energy metabolism and lack many biosynthetic pathways and therefore are entirely dependent on the host cell to supply them with ATP (adenosine triphosphate) and other intermediate molecules [8].
Obligate intracellular bacteria cannot be grown in artificial media (agar plates/broths) in laboratories but require viable eukaryotic host cells (e.g., cell culture, embryonated eggs, and susceptible animals). Additional obligate intracellular bacteria include Coxiella burnetii, Rickettsia spp., and others [8, 9]. Microbial access to host nutrients is a fundamental aspect of infectious diseases. Pathogens face complex dynamic nutritional host microenvironments that change with increasing inflammation and local hypoxia. Because the host can actively limit microbial access to its nutrient supply, pathogens have evolved various metabolic adaptations to successfully exploit available host nutrients to facilitate their own proliferation [10].
Iron (Fe) is a key global regulator of cellular metabolism, which makes Fe acquisition a focal point of the biology of pathogen systems. In the host environment, the success or failure of Fe uptake processes impacts the outcome of pathogenesis [11]. After phagocytosis by macrophages, intracellular bacteria are located in a membrane-bound vacuole (phagosome), but the ensuing trafficking of this vacuole and subsequent bacterial survival strategies vary considerably. If the ingested bacteria have no intracellular survival mechanisms, the bacteria-containing phagosomes fuse with the lysosomal compartment, and bacteria are digested within 15– 30 min.
For this reason, the majority of intracellular bacteria and other parasites must keep host cells alive as long as possible while they are reproducing and growing [7, 9]. To grow, intracellular pathogens need nutrients such as the iron, that might be scarce in the cell, because this is usually retained or stored by proteins. Pathogens that infect macrophages require Fe for growth, but, during infection, Fe is required by both the host cell and the pathogen that inhabits the host cell [12].
Macrophages require Fe as a cofactor for the execution of important antimicrobial effector mechanisms, including the NADPH- (nicotinamide adenine dinucleotide phosphate-oxidase-) dependent oxidative burst and the production of nitrogen radicals catalyzed by the inducible nitric oxide synthase [13]. On the other hand, intracellular bacteria such as Legionella pneumophila, Coxiella burnetii, Salmonella typhimurium, and Mycobacterium tuberculosis have an obligate requirement for Fe to support their growth and survival inside host cells [14]. In fact, it has been documented that deprivation of Fe in vivo and in vitro severely reduces the pathogenicity of M. tuberculosis, C. burnetii, L. pneumophila, and S. typhimurium [13–15].
2. Iron in the Human Host
Iron (Fe) is essential for the growth of all organisms. The human body contains 3–5 g of Fe distributed throughout the body in the protein hemoglobin, tissues, muscles, bone marrow, blood proteins, enzymes, ferritin, hemosiderin, and transport in plasma. Iron (approximately 75%) is contained in the protein hemoglobin (Hb) and in other iron-bound proteins that are important for cellular processes, and whatever remains in plasma (approximately 25%) is bound to plasma proteins such as transferrin (Tf) [16]. Dietary Fe has two main forms: heme and nonheme. Plants and iron-fortified foods contain nonheme Fe only, whereas meat, seafood, and poultry contain both heme and nonheme iron. Heme iron, which is formed when Fe combines with protoporphyrin IX, contributes about 10% to 15% of total Fe intakes in western populations [17]. Intestinal absorption is the primary mechanism regulating Fe concentrations in the body. Once ingested, Fe absorption occurs predominantly in the duodenum and upper jejunum. The mechanism of iron transport from the gut into the blood stream remains unknown. The first step of the pathway of iron absorption in the human host involves reduction of ferric Fe3+ to Fe2+ in the intestinal lumen by reductases or cytochrome b and transport of Fe2+ across the duodenal epithelium by the apical transporter DMT1 (divalent metal transporter). In nonintestinal cells most Fe uptake occurs via either the classical clathrin-coated pathway utilizing transferrin receptors or the poorly defined transferrin receptor independent pathway. Tf is the principal Fe storage protein that stores and releases Fe inside cells that express the transferrin receptor (TfR). The delivery of Fe from Tf is mediated by an acidic pH 5.5 of the endocytic vesicles carrying holo-Tf and TfR complexes. Fe is then transported across the endosomal membrane and utilized. Excess intracellular Fe is sequestered into the protein Ft [18, 19]. In a healthy individual Fe is largely intracellular, sequestered within Ft or as a cofactor of heme complexed to Hb within erythrocytes. Any extracellular free Fe is rapidly bound by circulating Tf. Hb or heme that is released as a result of natural erythrocyte lysis is captured by haptoglobin and hemopexin, respectively. Taken together, these factors ensure that vertebrate tissue is virtually devoid of free iron [21]. Maintaining cellular Fe content requires precise mechanisms for regulating its uptake, storage, and export. The iron response elements or iron-responsive elements (IRP1 and IRP2) are the principal regulators of cellular Fe homeostasis in vertebrates. IRPs are cytosolic proteins that bind to Feresponsive elements (IREs) in the 5 or 3 untranslated regions of mRNAs encoding proteins involved in Fe uptake (TfR1, DMT1), sequestration (H-ferritin subunit (FTH1) and L-ferritin subunit (FTL)), and export (ferroportin). When cells are Fe deficient, IRPs bind to 5 IREs in ferritin and ferroportin mRNAs with high affinity to repress translation and to 3 IREs in TfR1 mRNA to block its degradation (Tf is involved in the transport or Fe). When Fe is in excess, IRPs do not bind to IREs, increasing synthesis of Ft and ferroportin (proteins involved in the storage of Fe), while promoting the degradation of TfR1 mRNA. The coordinated regulation of Fe uptake, storage, and export by the IRPs ensures that cells acquire adequate Fe for their needs without reaching toxic levels [22]. The ability of pathogens to obtain Fe from Tf, Lf, Ft, Hb, and other iron-containing proteins of their host is central to whether they live or die [14]. This is because these proteins are the main Fe sources for intracellular pathogens in the macrophage. Iron homeostasis in the macrophage is determined by uptake processes through Lf, Tf, DMT1, and phagocytosis of senescent erythrocytes as well as by export through ferroportin (Fpn), as we have discussed before. Inside infected macrophages, a pathogen’s access to Fe may be limited by natural resistance-associated macrophage protein 1 (SLC11A1, formerly Nramp1). SLC11A1 is a divalent metal transporter, recruited to the late endosomal and phagosomal membrane of macrophages and other professional phagocytes. Although SLC11A1 contributes to macrophages’ efficiency in the recycling of erythrocyte-derived Fe, the main function of SLC11A1 seems to be the protection against microbes [20]. Its gene is present in inbred strains of mice in two allelic forms that determine the resistance or susceptibility to several intracellular pathogens such as Mycobacterium spp., Salmonella spp., and Leishmania spp. [23]. Some groups of researchers have suggested that Fe is transported via this protein into the pathogen-containing phagosome, causing the death of the pathogen by catalyzing the formation of reactive oxygen species (ROS), while others argue for Fe efflux from the phagosome, restricting pathogenic growth by Fe deprivation [23, 24]. Another Fe transporter that is expressed in macrophages is Fpn. This transporter is present in the macrophage cytoplasmic membrane and is responsible for Fe export. Overexpression of Fpn has been reported to inhibit the intramacrophagic growth of M. tuberculosis and Salmonella enterica, presumably through Fe deprivation. The details of this mechanism are unclear [25, 26]. A scheme of Fe sources in the human body and iron homeostasis inside the macrophage is shown in Figure 1.
Figure 1: Iron content in the human body and iron-containing proteins in a macrophage. The average male adult contains approximately 3.5 g of iron. Approximately 2 g of iron is in hemoglobin: 1 g in body is stored predominantly in the liver and the rest in myoglobin and other iron-containing proteins. Approximately 1 to 2 mg of iron is lost each day by epithelial shedding in the gastrointestinal tract and the skin and through blood loss in menstruating women. Western diets contain a much greater amount of iron (10 to 20 mg) than what is absorbed daily under normal circumstances (1 to 2 mg). The macrophage is a key agent in iron homeostasis as well as in inflammatory hypoferremia. Macrophages in the spleen and in the liver (Kupffer cells) and perhaps elsewhere recognize damaged or senescent erythrocytes, phagocytize them, and digest them to extract heme and eventually iron. Macrophages can also scavenge heme and hemoglobin, usually complexed with hemopexin and haptoglobin, respectively, and endocytosed by CD163 and CD91, respectively. Whether phagocytosed in erythrocytes or endocytosed by scavenging, hemoglobin undergoes proteolysis to release heme. Heme is degraded by HO-1 to release iron, which is exported to the cytoplasm by DMT1 and probably also by Nramp1. Cytoplasmic chaperones family deliver iron for storage in the protein ferritin. Alternatively, iron from endosomes or phagolysosomes may also be delivered by an unknown carrier to ferroportin (Fpn) for export [20].
3. Mechanisms Used by Intracellular Pathogens for Obtaining Iron: A General Point of View
During infection, pathogens are capable of altering the battlefield to increase the abundance of potential Fe sources. For example, bacterial cytotoxins damage host cells, leading to the release of Ft, while hemolytic toxins from bacteria can lyse erythrocytes, liberating Hb. The resulting inflammatory response includes the release of Lf from secondary granules contained with polymorphonuclear leukocytes (PMNs) [10, 21, 27]. Pathogens are capable of exploiting these diverse Fe sources through the elaboration of a variety of Fe acquisition systems. In the case of extracellular pathogens, they can acquire Fe through receptor-mediated recognition of Tf, Lf, hemopexin, hemoglobin, or hemoglobin-haptoglobin complexes [19, 27]. Alternatively, secreted siderophores can remove Fe from Tf, Lf, or Ft, whereupon siderophore-iron complexes are recognized by cognate receptors at the bacterial surface. Siderophores are small ferric iron chelators that bind with extremely high affinity (iron formation constants 𝐾𝑑 range from 10−20 to 10−50 M), some of which can extract iron from Tf and Lf [21]. Analogously, secreted hemophores can remove heme from Hb or hemopexin and deliver heme to bacterial cells through binding with hemophore receptors. Siderophore mediated Fe acquisition is inhibited by the innate immune protein siderocalin, which binds siderophores and prevents receptor recognition. This host defense is circumvented through the production of stealth siderophores that are modified in such a way as to prevent siderocalin binding [21, 27]. For proper use of Fe, extracellular or intracellular parasites must possess at least the following systems: (a) Fe sensors for monitoring Fe concentration in the intracellular environment, (b) synthesis and release of high-affinity compounds that can compete with host Fe binding proteins for Fe acquisition and storage, or proteases to degrade these host Fe binding proteins, (c) transportation of these Feloaded molecules and their assimilation, and (d) regulation of the expression of proteins involved in iron metabolism, in order to maintain iron homoeostasis [27, 28]. Once ingested by macrophages, many intracellular parasites are taken up by phagosomes through endocytosis. Thus, the success of intracellular parasites seems to be related mainly to their ability to take up Fe from the proteins Tf, Hb, hemoglobinhaptoglobin, free heme, and Ft. Figure 2 shows intracellular parasites and Fe sources inside a macrophage. In order to take the Fe from Tf, these systems can be divided into three main categories: siderophore-based systems, heme acquisition systems, and transferrin/lactoferrin receptors. Upon removing Fe from host proteins, iron-loaded siderophores are bound by cognate receptors expressed at the bacterial surface. The siderophore-iron complex is then internalized into the bacterium and the Fe is released for use as a nutrient source [21]. Heme acquisition systems typically involve surface receptors that recognize either heme or heme bound to hemoproteins such as hemoglobin or hemopexin. Heme is then removed from hemoproteins and transported through the envelope of bacteria into the cytoplasm. Once inside the cytoplasm, the iron is released from heme through the action of heme oxygenases or reverse ferrochelatase activity. Bacterial pathogens can also elaborate secreted heme-scavenging molecules that remove heme from host hemoproteins. These molecules, known as hemophores, are functionally analogous to siderophores but are proteins that target heme, whereas siderophores are small molecules that target iron atoms [29]. In addition to acquiring Fe from Tf and Lf through siderophore-based mechanisms, some pathogens are capable of direct recognition of these host proteins through receptors [21]. These receptors are modeled to recognize Tf or Lf, leading to Fe removal and subsequent transport into the bacterial cytoplasm. Additionally, acidification of the phagosome permits Fe release from Tf and probably Lf and, in this way, some pathogens can gain access to this element directly [19, 21, 30]. The following sections summarize the Fe acquisition systems used by some intracellular pathogens. Table 1 shows Fe sources, mechanism of uptake, transport and regulation, used by intracellular parasites.
4. Mechanism of Intracellular Pathogens for Obtaining Iron from Host Sources
4.1. Francisella tularensis. F. tularensis, the bacterial cause of tularemia, is a virulent intracellular pathogen that can replicate in multiple cell types. Acidification of the phagosome and acquisition of Fe is essential for growth of F. tularensis [31]. An acidic pH promotes the release of Fe from host cell Tf. To acquire the Fe from Tf, F. tularensis involves a receptor for this protein (Transferrin receptor 1, TfR1), induction of ferrireductases, an iron membrane transporter (DMT-1), and iron regulatory proteins (IRP1 and IRP2); this is an active Fe acquisition system associated with a sustained increase of the labile Fe pool inside the macrophage [31]. In addition, F. tularensis uses high-affinity transportation of ferrous Fe across the outer membrane via the proteins FupA and FslE. FsIe appears to be involved in siderophore-mediated ferric Fe uptake, whereas FupA facilitates high-affinity ferrous Fe uptake [32]. It has been hypothesized that F. tularensis uses the Fe from Lf to sustain its growth; however, the mechanism of Fe acquisition from LF remains undetermined [33]. It is most likely that F. tularensis can infect many types of cells because it contains several strategies for Fe acquisition. It has been reported that the expression of certain F. tularensis virulence genes is clearly regulated by Fe availability [34]. The expression of TfR1 is critical for the intracellular proliferation of Francisella. This contrasts with infection of macrophages by Salmonella typhimurium, which does not require expression of TfR1 for successful intracellular survival. Macrophages infected with Salmonella lack significant induction of DMT-1, Steap3, and IRP1 and maintain their labile Fe pool at normal levels [12]. Authors argue that this might be explained by Salmonella’s intracellular localization within an endosomal structure or perhaps by more efficient Fe acquisition strategies compared to Francisella [12].
4.2. Salmonella spp. Salmonella typhimurium is an invasive pathogen that causes diseases ranging from mild gastroenteritis to enteric fever. To establish a systemic infection, Salmonella spp. must invade the epithelial wall of the intestine before the bacteria are ingested by immune effector cells and transported to lymph nodes, the spleen, and other organs. Salmonella spp. reside within modified phagosomes in macrophages, where replication is promoted and killing is evaded. Fe is an essential micronutrient for replication, and Salmonella spp. harbor various Fe acquisition systems, such as the siderophores enterobactin and salmochelin [35]. As iron sources, Salmonella spp. use Fe2+, Fe3+, heme, ovotransferrin, and Tf [35, 36]. S. Typhimurium acquires Fe2+ from hemophagocytic macrophages and also secretes siderophores via IroC and EntS to bind Fe3+, which is subsequently taken up by outer membrane receptors including IronN and FepA. ABC transporters such as FepBCDG are responsible for the transport of siderophores through the cytoplasmic membrane, whereas molecular iron is taken up via Feomediated transmembrane transport [35, 36]. During the infection process in vivo, S. typhimurium induces a number of virulence genes that are required to circumvent host defenses and/or acquire nutrients from the host. A putative Fe transporter in Salmonella called Pathogenicity Island 1, or sitABCD, has been characterized. The sitABCD operon is induced under Fe-deficient conditions in vitro and is repressed by Fur (ferric uptake regulator).This locus is specifically induced in animal models after invasion of the intestinal epithelium, suggesting that SitABCD plays an important role in Fe acquisition in the animal. To regulate its Fe content, Salmonella enterica serovar Typhimurium possesses four ferritins: bacterioferritin (Bfr), ferritin A (FtnA), ferritin B (FtnB), and Dps. The hemecontaining Bfr accounts for the majority of stored Fe, followed by FtnA. Inactivation of Bfr elevates the free intracellular Fe concentration and enhances susceptibility to H2O2 stress. The DNA-binding Dps protein provides protection from oxidative damage without affecting the free intracellular Fe concentration at steady state. FtnB appears to be particularly important for the repair of Fe-sulfur clusters of aconitase that undergo oxidative damage, and, in contrast to Bfr and FtnA, is required for Salmonella virulence in mice. Moreover, FtnB and Dps are repressed by the Fe-responsive regulator Fur and induced under conditions of Fe limitation, whereas Bfr and FtnA are maximally expressed when Fe is abundant. The absence of a conserved ferroxidase domain and the potentiation of oxidative stress by FtnB in some strains that lack Dps suggest that FtnB serves as a facile cellular reservoir of Fe2+ [37].
4.3. Chlamydia spp. Chlamydia is an infection that is caused by the bacteria Chlamydia trachomatis. It is the most common sexually transmitted disease in the U.S., with nearly 3 million cases reported each year (the actual number of cases is likely much higher). The developmental cycle of C. trachomatis includes two forms: an infectious elementary body (EB) and a reticulate body that multiplies within the inclusion by binary fission. A third developmental form is the persistent form, which exists as a mechanism of survival under stressful conditions. Persistence is induced in response to changes in the culture medium, including amino acid or Fe deprivation, and in the presence of antibiotics or cytokines such as gamma interferon (IFN) [38]. It has been shown that Fe is an essential factor in the growth and survival of C. trachomatis and C. pneumoniae (this bacterium causes pneumonia) [39]. Although homologues for bacterial siderophores are missing in the genome of this bacterium, TfR expression does occur. C. trachomatis also appears to be missing a tonB analogue, which would span the periplasm and is crucial in energy transfer to substrate-specific outer membrane transporters that are used to bring Fe-siderophore complexes to the cell. Considering these apparent gaps in the genome, one could speculate that the C. trachomatis genome would need a reductase on the inclusion membrane to transport Fe2+ from the eukaryotic cytosol into the inclusion. C. trachomatis and C. pneumoniae appear to use the host’s Fe transport pathways by attracting TfR and Ft to the phagosome [39]. A report from Vardhan et al. (2009) showed that C. trachomatis alters the Feregulatory protein-1 (IRP-1) binding capacity and modulates cellular iron homeostasis in HeLa-229 cells, suggesting that Fe homeostasis is modulated in CT-infected HeLa cells at the interface of acquisition and commensal use of Fe [40]. ATP-binding cassette (ABC) transport systems play a role in the acquisition of Fe and Fe-complexes, amino acids, sugars, and other compounds. They consist of a soluble periplasmic protein that binds the targeted molecule and changes conformation to close around the substrate. The periplasmic binding protein moves to and binds the transmembrane protein permease in receptor-ligand mechanisms. An ATP-binding lipoprotein binds to the ATP, creating a conformational change in the permease complex that transports the substrate into the cytoplasm. In other pathogenic bacteria, ABC transport systems that transport Fe, zinc, and manganese into the cytoplasm include Tro from Treponema pallidum, Yfe from Yersinia pestis, and Fbp from Neisseria meningitidis [40]. There is evidence that YtgA secretion occurs in C. trachomatis, and YtgA does have high homology with periplasmic binding proteins of the ABC transport systems. ytaA is a gene of 978 bp that resides in an operon with ytgBVD. YtgB and Ytg have predictable membranespanning domains and most likely form the pore of the ABC transporter. YtgA contains similar metal-binding motifs (e.g., histidine, tyrosine) to other metal-binding periplasmic proteins, suggesting a role for YtgA as an Fe-binding periplasmic protein, in addition to its location on the chlamydial membrane [41].
4.4. Neisseria spp. Acquisition of Fe and Fe-complexes has long been recognized as a major determinant in the pathogenesis of Neisseria spp., and some of their high-affinity iron uptake systems are important virulence factors in bacteria. These have been shown to play a major role in promoting the survival of the meningococcus within the host. Most species are Gram-negative bacteria that are primarily commensal inhabitants or reside in the mucus membranes of mammals. There are 12 Neisseria species of human origin, with N. meningitidis and N. gonorrhoeae being important opportunistic pathogens. These intracellular pathogens contain highaffinity iron uptake systems, which allow meningococci to utilize the human host proteins Tf, Lf, Hb, and haptoglobinhemoglobin as sources of essential Fe [29, 42]. Although the meningococci do not produce siderophores, studies indicate that meningococci may be able to use heterologous siderophores secreted by other bacteria. For some time, it has been reported that the gonococci could utilize ferric enterobactin, enterobactin derivatives, aerobactin, and salmochelin S2 in a FetA- and TonB-dependent manner [29]. In N. gonorrhoeae, an outer membrane protein named FetA (formerly FrpB) was recently described. FetA is an outer membrane transporter and is part of an iron-regulated operon that encodes a periplasmic binding protein and the components of a putative ABC transport system. FetA has demonstrated low binding affinity and the transport of ferric enterobactin. The binding contact of FetA for enterobactin was much lower than that for other enterobactin receptors, and it was therefore proposed that this receptor could interact with high affinity to an as-yet unidentified phenolate siderophore. A homologous protein, with 91% similarity to gonococcal FetA, has been identified in N. meningitidis and presumably functions in a similar manner [30, 43]. Only fetA and not the downstream genes require an iron-regulator MpeR for regulation. MpeR regulation is important because it may aid in gonococcal immune evasion. MpeR was suggested to modulate any change in mtrF expression that is needed for full hydrophobic agent resistance. AraC-like regulators of N. meningitidis are homologues of the N. gonorrhoeae type MpeR that is specific to the pathogenic Neisseria species. Both are induced during Fe limitation, and this regulation is also mediated by the Fur regulator. The presence of MpeR in a regulatory cascade downstream of the Fur master Fe regulator suggests that it is being expressed in the Fe limiting environment of the host, where it may in turn regulate a group of genes, including the divergent Fe transport locus, in response to signals that are important for infection [44]. Two proteins, transferrin-binding protein A (TbpA) and transferrin-binding protein B (TbpB), function as the transferrin receptor in N. meningitidis. TbpA and TbpB are induced along with several other proteins in the outer membranes of N. meningitidis under Fe-restricted conditions [30]. Initially, an affinity isolation procedure using biotinylated transferrin was employed to demonstrate the presence of two transferrin-binding proteins in N. meningitidis. The proteins that bound transferrin were TbpA (formerly Tbp1), which is 98 kDa, and TbpB (formerly Tbp2), which is 68 kDa [45]. Among different meningococcal isolates, the molecular masses of TbpA and TbpB vary, with TbpA ranging from 93 to 98 kDa and the more heterogenetic TbpB varying from 68 to 85 kDa. TbpA can be found in all strains. Although it has not been characterized as well as the Tf receptor, the Lf receptor is believed to be an important meningococcal virulence factor [29]. The Lf receptor of N. meningitidis, like the Tf receptor, consists of two protein components, LbpA and LbpB. Initial experiments using affinity isolation by Lf identified a 98-kDa lactoferrin-binding protein named LbpA, formerly known as IroA [46].
4.5. Legionella pneumophila. Legionella pneumophila, the causative agent of Legionnaire’s disease, is a facultative intracellular parasite of human macrophages and freshwater amoebae. This pathogenic bacterium is commonly found in water, thereby presenting a risk that it could be transmitted to humans via inhalation of contaminated aerosols. L. pneumophila resides in the phagosome, although this phagosome does not fuse with endosomes and lysosomes and is at nearly neutral pH during the early stages of the intracellular life cycle. It appears to fuse with low-pH cellular compartments during the later stages of the infection [47]. The ability of L. pneumophila to acquire host cell Fe is pivotal for the parasite to establish a successful intracellular infection. To occupy its intracellular niche, this pathogen has developed multiple Fe acquisition mechanisms: the ira AB locus, which encodes a transporter for Fe-loaded peptides; the cytochrome c maturation ccm genes; the Feregulated frgA, whose product is homologous to aerobactin synthetases; legiobactin siderophores; and two internal ferric reductases. Robey and Cianciotto (2002) identified and characterized L. pneumophila Feo AB, which bears homology to E. coli and Salmonella enterica serovar Typhimurium FeoAB. In those bacteria, FeoB has been shown to be a ferrous Fe transporter and FeoA is possibly involved in Fe2+ uptake [48]. In 2014, Portier and Cols discovered gene ipp 2867, which was highly induced in Fe-restricted conditions. A sequence analysis predicts that Lpp 2867 is a membrane protein involved directly or indirectly in Fe2+ transport and is also a virulence factor [49].
4.6. Shigella spp. Shigella is a Gram-negative bacterium of the Enterobacteriaceae family and is the etiological agent of bacillary dysentery or shigellosis. Shigella encompasses four subgroups (S. flexneri, S. sonnei, S. dysenteriae, and S. boydii), and all species are able to grow in a variety of environments, including intracellularly in host epithelial cells. Shigella has a number of different Fe transport systems that contribute to the bacterium’s ability to grow in these diverse environments [50]. Siderophore Fe uptake systems, heme transporters, and Fe3+ and Fe2+ transport systems are present in these bacteria, and the genes encoding some of these systems appear to have spread among the Shigella species by horizontal transmission [50, 51]. Fe is not only essential for the growth of Shigella but also plays an important role in the regulation of metabolic processes and virulence determinants in Shigella. This regulation is mediated by the repressor protein Fur and the small RNA RyhB [52]. The only Fe transport system that appears to be common to all members of the E. coli/Shigella group is Feo. Shigella spp. have transport systems for both ferric and ferrous iron. The Fe can be taken up as free Fe or complexed with a variety of carriers. All Shigella species have both the Feo and Sit systems for acquisition of Fe2+, and all have at least one siderophoremediated system for transport of Fe3+ [53]. Several of the transport systems, including Sit, Iuc/IutA (aerobactin synthesis and transport), Fec (ferric di-citrate uptake), and Shu (heme transport), are encoded within pathogenicity islands. The presence and the genomic locations of these islands vary considerably among the Shigella species and even between isolates of the same species [53, 54]. The expression of the Fe transport systems is influenced by the concentration of Fe and by environmental conditions, including the level of oxygen. ArcA and FNR regulate Fe transport gene expression as a function of oxygen tension, with the sit and iuc promoters being highly expressed in aerobic conditions, while the feo Fe2+ transporter promoter is most active under anaerobic conditions [52]. The effects of oxygen are also observed in infection of cultured cells by S. flexneri; the Sit and Iuc systems support plaque formation under aerobic conditions, whereas Feo allows plaque formation to occur anaerobically [52, 53].
4.7. Listeria monocytogenes. L. monocytogenes is a Grampositive, intracellular pathogen responsible for the fatal disease listeriosis. L. monocytogenes is recognized as a significant public health problem.The ability of this bacterium to acquire and utilize Fe is not only essential during infection but can also support its growth and survival in many diverse environmental niches. L. monocytogenes possesses at least 4 mechanisms that enable Fe uptake: (1) acquisition of protein-bound Fe that involves the HupDGC protein (for the uptake of hemin, hemoglobin), or Fhu protein (involved in the uptake of ferrichrome siderophores); inside the cell, then Fe can be bound to the Fri protein (ferritin-like) Fur regulated; (2) extracellular and/or surface-bound Fe reductases; (3) a citrate inducible ferric citrate uptake system; and (4) siderophore and siderophore-like systems [55]. The Listeria life cycle involves escape from the phagosome, which is considered to be Fe-limiting and permits proliferation in the host-cell cytosol, where Fe-saturated Ft is stored. It has been hypothesized that L. monocytogenes has access to Fe through increased expression of the PrfAregulated virulence factors listeriolysin (LLO) and ActA, which are used for phagosomal escape. Increased Fe concentrations result in the upregulation of internalin proteins InlA and InlB, which are required for invasion [56]. Fe homeostasis in Listeria is controlled by the regulatory protein Fur. It has been shown that expression of Fur is negatively regulated by PerR, a Fur homologue that is involved in the oxidative stress response. Fourteen Fur-regulated genes have been identified in L. monocytogenes, including genes that encode Fe2+ transporters and ferrichrome ABC transporters and proteins involved in Fe storage [56, 57].