A zero-sum game or an interactive frame? Iron competition between bacteria and humans in infection war

Abstract

Iron is an essential trace element for both humans and bacteria. It plays a vital role in life, such as in redox reactions and electron transport. Strict regulatory mechanisms are necessary to maintain iron homeostasis because both excess and insufficient iron are harmful to life. Competition for iron is a war between humans and bacteria. To grow, reproduce, colonize, and successfully cause infection, pathogens have evolved various mechanisms for iron uptake from humans, principally Fe³⁺-siderophore and Fe²⁺-heme transport systems. Humans have many innate immune mechanisms that regulate the distribution of iron and inhibit bacterial iron uptake to help resist bacterial invasion and colonization. Meanwhile, researchers have invented detection test strips and coupled antibiotics with siderophores to create tools that take advantage of this battle for iron, to help eliminate pathogens. In this review, we summarize bacterial and human iron metabolism, competition for iron between humans and bacteria, siderophore sensors, antibiotics coupled with siderophores, and related phenomena. We also discuss how competition for iron can be used for diagnosis and treatment of infection in the future.

Introduction

Iron is an essential nutrient for almost all organisms. It plays an irreplaceable role in basic life activities such as redox reactions and electron transport. Iron is the most abundant trace element in the human body. It functions physiologically in two main forms: Fe³⁺ and Fe²⁺. Iron participates in the composition of oxidoreductase cofactors, energy metabolism, hemopoiesis, and the immune function of humans.^[1] A significant change in the iron concentration of the body increases the probability of bacterial infection.^[2–5] Iron is also an essential element for bacterial survival and proliferation, and plays roles in, for example, the tricarboxylic acid cycle, ribosome assembly, and biofilm formation.^[6] Bacteria colonizing or invading the human body compete with their host for Fe²⁺ (in heme) or Fe³⁺ (from iron-transporters). In response, humans have evolved various mechanisms—such as secreting high concentration iron-transporters, actively decreasing the serum iron concentration, and secreting hepcidin, lipocalin-2 (Lcn-2), and natural resistance-associated macrophage protein 1 (NRAMP1)—to help win this competition for iron.^[7] Meanwhile, some researchers have used the phenomenon of bacterial iron uptake to produce biosensors for pathogens.^[8] Others have found natural antibiotics that inhibit the synthesis of siderophores (small, high-affinity Fe³⁺-chelating compounds that are secreted by microorganisms to help them accumulate iron). Selectively targeted antibiotics have also been developed by coupling siderophores with antibiotic moieties.^[9]

In this review, we summarize iron metabolism, iron competition between humans and bacteria, siderophore sensors, antibiotics coupled with siderophores, and other related phenomena. We describe the profile and mechanisms of iron competition between humans and bacteria, and the prospects of new technologies in future research, diagnosis, and treatment of infection.

Iron metabolism in humans and bacteria

Iron homeostasis is critically important to human health. The major forms of iron in humans include heme (Fe²⁺) in hemoglobin and myoglobin, and Fe³⁺ combined with iron transport proteins (serum transferrin and lactoferrin) or iron storage proteins (serum ferritin and hemosiderin). Iron is an important cofactor component of oxidoreductases in energy metabolism and immune function, and essential for the synthesis of hemoglobin.^[10] On average, an adult human contains 3 to 4 g of iron, which is mainly absorbed from diet in the duodenum and jejunum. Iron is lost by intestinal excretion (epithelial cells falling off the digestive tract and excreted in feces) and parenteral loss (including in urine, sweat, semen, and bleeding). Iron deficiency brings anemia and decreased immunity. However, the human body lacks an active iron efflux mechanism, and iron becomes toxic if it becomes overloaded in the body. Iron overload may lead to ferroptosis, which is an iron-dependent and regulated form of cell death driven by loss of activity of the lipid repair enzyme glutathione peroxidase 4 (GPX4) and subsequent accumulation of lipid-based reactive oxygen species (ROS), particularly lipid hydroperoxides. Mycobacterium tuberculosis (M. tuberculosis)-induced macrophage necrosis is also associated with ferroptosis.^[11]

The dynamic balance of iron is also essential for bacteria. Iron is an essential metal cofactor in enzymes that are involved in the Krebs cycle, ribosome assembly, oxidative stress, DNA synthesis and repair, and biofilm formation. For example, the terminal oxidase of the mitochondrial respiratory chain, which contains a bimetallic Fe–Cu site and two hemes and reduces O₂ to H₂O, is a key step in the electron transport chain and also an important factor affecting the virulence of bacterial biofilms. In Clostridium difficile, a corrinoid iron-sulfur protein plays a crucial role in CO₂ fixation for acetyl-coenzyme A production in the Wood–Ljungdahl metabolic pathway, one of the pivotal carbon metabolic pathways of anaerobic bacteria.^[6,12,13] However, if there is too much free iron in bacteria, the Fenton reaction occurs, by which Fe²⁺ reacts with H₂O₂ to produce Fe³⁺, hydroxide (OH⁻), and hydroxyl radicals (−OH); the latter are highly damaging to biomolecules including DNA, lipids and proteins.^[14] When Escherichia coli (E. coli) infection is treated using quinolone antibiotics, the cells induce the generation of H₂O₂, then Fe²⁺ participates in the Fenton reaction to generate hydroxyl radicals.^[15]

Because of the requirement for iron, the biology of humans and bacteria is closely connected. They compete intensively for iron. And, no matter whether the conditions are iron-rich or iron-poor, iron metabolism in both humans and bacteria is strictly regulated [Figure 1].

Figure 1.

Panorama of iron competing war between bacterial iron uptake system and human “anti-aggression arms”. NRAMP1: Natural resistance-associated macrophage protein 1.

Iron acquisition and regulatory strategies of bacteria

At physiological pH, iron in human serum exists in the form of Fe³⁺, at about 10⁻²⁴ mol/L.^[16] During infection, the human body decreases the absorption of iron from the gut and increases the content of iron storage proteins, transfers iron from plasma to storage proteins, and thus further decreases the concentration of free iron. Each bacterial cell division requires Fe³⁺ at about 10⁻⁵ to 10⁻⁷ mol/L (at least 10⁻¹⁷ mol/L in the cell).^[17,18] Therefore, bacteria have evolved various mechanisms to take up enough iron during colonization or infection.

Ferric iron (Fe³⁺) acquisition and siderophores

Bacteria have two strategies for taking in ferric iron. The most important is by using siderophores. The name “siderophore” comes from the Greek for “iron carrier”. Siderophores are Fe³⁺-specific chelating agents that are secreted by bacteria in iron-poor environments. They strongly bind with Fe³⁺, but have low affinity for Fe²⁺.^[19] The structures of siderophores are diverse. They can be divided into five categories based on the chemical properties of the siderophore chelating group: catecholate, hydroxamate, carboxylate, phenolate, and mixed type. They can all bind six Fe³⁺ ions that form an octahedron.^[20,21] Usually, bacteria produce many types of siderophore and release them into the host environment to enhance their competitiveness in iron-stressed environments. However, some bacteria rely on plundering the siderophores released by other bacteria to supply themselves with iron for survival and reproduction.^[22]

In the human body, siderophores released by bacteria compete with transferrin and lactoferrin to acquire Fe³⁺ and form Fe³⁺-siderophore complexes. In Gram-negative bacteria, these complexes are specifically recognized by siderophore outer-membrane receptors (OMRs, siderophores have their own corresponding siderophore receptor protein). Then, they are transported to the periplasm and form a complex with an Fe³⁺-siderophore-periplasmic binding protein (PBP). This complex is actively transported into the cytoplasm through adenosine triphosphate-binding-cassette transporters (ABC transporters) on the inner membrane. In the cytoplasm, the Fe³⁺ in the complex is reduced to Fe²⁺ by iron reductase. Because of the low affinity between Fe²⁺ and siderophores, Fe²⁺ ions are released, and then the siderophore is degraded or reused [Figure 2]. The whole process is powered by the TonB system (composed of intimal protein ExbB, ExbD, and periplasmic protein TonB).^[19,23–25]

Figure 2.

Mechanism diagram of bacterial iron (Fe³⁺) uptake system via siderophore pathway in Gram-negative bacteria. ABC transporter: Adenosine triphosphate-binding-cassette transporters; OMRs: Outer-membrane receptors; PBPs: Periplasmic binding proteins; TonB system: Composed of intimal protein ExbB, ExbD, and periplasmic protein TonB.

In Klebsiella pneumoniae (K. pneumoniae), there is a close relationship between siderophore production and virulence as well as carbapenem resistance. This finding also shows that a key breakthrough in the diagnosis and treatment of bacterial infection may be achieved by studying the interaction between humans and bacteria in terms of iron metabolism.^[26–28]

Gram-positive bacteria have a relatively simple Fe³⁺-siderophore uptake system because they have no outer membrane. For example, Staphylococcus aureus (S. aureus) has only one membrane, on which siderophore-binding protein (SBP) is expressed. After binding with SBP, the Fe³⁺-siderophore complex passes into the cytoplasm, in which Fe³⁺ is reduced to Fe²⁺ by iron reductase for further use.^[19,29]

In addition, most Gram-negative bacteria can produce transferrin-binding proteins (TBPs) or lactoferrin-binding proteins (LBPs). In a few cases, TBPs and LBPs of bacteria can directly bind host transferrin or lactoferrin to plunder Fe³⁺ into the bacterial cytoplasm under iron stress. The process is also powered by the TonB system.^[30,31]

Systems for acquisition of heme and ferrous iron (Fe²⁺)

More than 80% of the iron in the human body is in heme. Therefore, when bacteria infect the human body, obtaining heme is an important strategy for bacteria to uptake iron, including via direct heme transport systems, indirect heme transport systems, and the iron reductase system. Most Gram-positive bacteria have a transport system to directly obtain heme, while Gram-negative bacteria can obtain Fe²⁺ through either direct transport of heme, or via indirect hemophore-dependent heme transport system.

Gram-positive bacteria such as S. aureus can obtain Fe²⁺ directly from hemoglobin via a typical iron-regulated surface determinant system on the cell membrane. A similar system has been observed in Listeria monocytogenes.^[32]

In Gram-negative bacteria, the heme receptor on the outer membrane can directly bind heme or hemoglobin and transport the heme or hemoglobin to the periplasm; heme or hemoglobin is then transported to the cytoplasm through an ABC transporter. The heme is degraded by heme oxidase to release Fe²⁺. The process is powered by the TonB system.^[33] Gram-negative bacteria such as Pseudomonas aeruginosa (P. aeruginosa), E. coli, and Yersinia pestis have been found to have similar direct heme transport systems.^[34]P. aeruginosa can also secrete the heme acquisition system A (HasA)-type hemophore (heme carrier) to capture heme through the indirect heme transport system to absorb Fe²⁺.^[35] Typical hemophore-mediated indirect heme transport systems operate as follows: The hemophore is released into the host environment by Gram-negative bacteria and binds to hemoglobin to capture heme. The hemophore-bound heme complex binds to a corresponding receptor on the bacterial outer membrane to enter the periplasm. After that, the complex binds to a PBP and passes through an ABC transporter into the cytoplasm for degradation and use of the iron [Figure 3].^[34] Hemophores can be divided into at least three types based on their delivery mechanism: HasA (heme acquisition system A), HxuA (hemopexin uptake system A), and HmuY (heme outer-membrane utilization system Y). HxuA hemophore of Haemophilus influenzae can deliver hemoglobin to a heme receptor and release heme for Fe²⁺ uptake.^[36,37]

Figure 3.

Mechanism diagram of bacterial iron (Fe²⁺) uptake system via direct heme/hemophore pathway in Gram-negative bacteria. ABC transporter: Adenosine triphosphate-binding-cassette transporters; TonB system: Composed of intimal protein ExbB, ExbD, and periplasmic protein TonB.

In addition, most Gram-negative bacteria can reduce Fe³⁺ to Fe²⁺ using an iron reductase, which is iron uptake by the Feo transport system (encoded by the feo operon). The Feo iron reductase system is composed of two soluble cytoplasmic proteins (FeoA and FeoC) and membrane protein FeoB.^[38] Studies have found that the Feo system plays an important role in iron metabolism in Acinetobacter baumannii, Stenotrophomonas maltophilia, and K. pneumoniae.^[39–42]

Regulators of iron metabolism

Bacteria maintain iron homeostasis using a strict dynamic regulatory system for iron uptake. The ferric uptake regulator (Fur) is a vital and widespread protein in bacteria for regulation of iron uptake and use. It is a transcriptional regulator with Fe²⁺ as a cofactor.^[43] The DNA sequence and protein structure of Fur have high homology in the bacterial domain. Fur is a homodimer composed of two subunits, which contains two domains: a DNA binding domain at the N-terminal end, and histidine enrichment at the C-terminal end containing a dimerized domain. The protein contains multiple metal ion-binding sites (including ferrous iron ion-binding sites). In an environment where there is sufficient iron for the bacterial cell, monomeric Fur binds Fe²⁺ to form an Fe²⁺–Fur complex, and then dimerizes. The dimeric Fur protein can bind to an A and T nucleotide-rich palindromic sequence in the promoter region of regulatory genes of Fur; this sequence is highly conserved, and is known as the Fur box. Binding of the Fe²⁺–Fur complex to the Fur box blocks the promoter recognition site of the gene and thus inhibits the expression of iron uptake-related genes. However, if intracellular iron is deficient, the dimer depolymerizes, the inhibition of expression of iron uptake-related genes is relieved, and the ability of the bacteria to take up iron is improved.^[44,45]

Fur can regulate the Feo transport system, ABC transport system, and TonB system in bacterial iron uptake.^[45,46] For example, the expression of siderophore synthesis genes (D-fep/E-fep/C-fep/G-fep) and siderophore receptor OMRs (FecA/FepA/FhuA/FhuE) in E. coli is regulated by Fur.^[47,48] It also regulates small RNAs to indirectly regulate the iron uptake and storage system by affecting the oxidation reaction enzyme system.^[49,50]

In addition, Fur regulates the acid resistance of bacteria. For example, Fur regulates the increased expression of acid oxidoreductase YdeP in gastric acid to increase the acid tolerance.^[51] FurA and FurB in M. tuberculosis participate in the regulation of iron homeostasis, and its acid resistance is related to the joint action of lysyl-transfer RNA (tRNA) synthase, mycobacterial acid-resistant protease, and the ABC transporter DrrA/DrrB.^[52] Although Fur is involved in the regulation of the ABC protein, there is no direct evidence to show the role of Fur in acid resistance of M. tuberculosis, or whether Fur regulates acid resistance in an Fe²⁺ concentration-dependent manner.^[53] Furthermore, the application of p-aminosalicylic acid (PAS) in anti-tuberculosis treatment is also precisely dependent on the inhibition mechanism of siderophore synthesis to exert its efficacy.^[54] This potential phenomenon suggests that regulation of iron metabolism plays a possibly important role in M. tuberculosis, which is worthy of further study for providing a new anti-tuberculosis treatment strategy.

In addition to the regulation of iron homeostasis by Fur, the RstA/RstB two-component regulatory system also regulates iron homeostasis through the Feo transport system. The regulation of RstA/RstB system has a clear regulatory mechanism in Salmonella enterica, while not all bacteria are regulated by RstA/RstB system. Researchers from the Taiwan province of China had also found RstA structural mechanism in K. pneumoniae, but it is not yet clear whether the RstA/RstB two-component regulatory system also regulates iron homeostasis through the Feo transport system in that species.^[55,56]

The arms tactics of humans to defend, identify, and resist

Humans have many innate immune mechanisms to resist bacterial invasion or colonization. They are important components that regulate iron distribution and inhibit bacterial iron uptake. In addition, researchers have invented detection test strips and coupled antibiotics to siderophores to produce tools that take advantage of the war for iron between pathogens and humans.

Human innate defense systems in iron competition war

Regulating iron distribution and inhibiting bacterial iron uptake mean that the human body regulates the subcellular location and the concentration of iron ions.^[57] Strict control is required to prevent hyperferremia and iron deficiency anemia. In the non-infected state, >80% of bodily iron is sequestered in the hemoglobin in red blood cells (RBCs). Some pathogens can release toxins to lyse RBCs to release hemoglobin to obtain divalent iron ions (Fe²⁺) from heme, but they face competition—after RBC lysis, the hemoglobin and heme are respectively bound by haptoglobin and hemopexin in the blood to achieve iron reuse.^[58] During infection, the content of iron storage proteins in the human host is increased, and free iron ions bind to iron-transporters (eg, transferrin) that proactively maintain a lower serum iron level to limit iron uptake by pathogens. The human body also decreases the absorption of iron from the intestine. Together, these mechanisms limit the iron available to pathogens and hence inhibit bacterial reproduction in the body.^[57,59]

Serum transferrin is the most important iron transporter in the human body. Transferrin has high affinity for serum iron, which increases the difficulty of bacterial iron uptake.^[60] However, when the binding capacity of transferrin is saturated, it can also maintain the advantages of the body in the war of iron competition by binding to some macromolecules with relatively low affinity for iron in plasma, such as albumin, amino acids, and citrate.^[61] Lactoferrin, which is present in the body's mucosal secretions and neutrophils, is also considered to be a key antibacterial protein through iron competition.^[60] Lactoferrin maintains its iron-binding ability when the blood pH is low. Studies found that lactoferrin has the ability to prevent antibiotic-induced C. difficile infection, which suggests that lactoferrin plays a major role in limiting bacterial iron uptake in infection-caused acidic environments.^[62]

Functional peptides also play an important role in combating bacterial iron uptake.^[60] Hepcidin, a liver-produced and secreted antibacterial peptide, is regulated by the serum iron level. Hepcidin binds to transferrin, causing conformational changes that inhibit iron outflow; it also binds to membrane ferroportin 1 (an iron export-related protein) to decrease the serum iron concentration. These functions regulate human iron homeostasis and prevent bacterial iron uptake.^[63] Studies have found that, on infection with Chlamydia pneumoniae, liver secretion of hepcidin was upregulated and the serum iron level was decreased.^[64,65]

Lcn2, also known as neutrophil gelatinase-associated lipocalin or siderocalin, is an antibacterial and immunomodulatory peptide produced by neutrophils, macrophages, and other immune cells. Its main function is to bind catechol siderophores and hence limit the use of Fe³⁺ by pathogenic bacteria.^[66,67] Many studies have shown that Lcn2 plays a regulatory role in infection by Streptococcus pneumoniae, K. pneumoniae, E. coli, and M. tuberculosis.^[68–71] NRAMP1 is an H⁺/divalent cation (Fe²⁺, Zn²⁺, and Mn²⁺) antiporter localized to late endosomes/lysosomes in macrophages and neutrophils. Proinflammatory cytokines decrease the expression of transferrin receptor on the surface of phagocytes, promote the expression of NRAMP1, decrease the iron content in cells, and thus limit the iron available for pathogens in this area.^[72,73] Meta-analysis showed that polymorphism of the NRAMP1 regulatory region was closely related to high risk of tuberculosis infection.^[74]

The innate defense system is the key weapon of humans in the war for iron—but this is not enough. Scientists also need to develop rapid detection reagents and antibiotics according to the characteristics of bacterial siderophores to subdue our bacterial enemies by exploiting their strengths.

Scouting for the bacterial enemy—detection reagents related to iron uptake

Siderophores are potential biomarkers for pathogen diagnosis. Researchers have developed various pathogen detection methods by using the characteristics of the bacterial Fe³⁺–siderophore uptake system. A siderophore-derivatized chip was applied in the detection of Yersiniae enterocolitis (Y. enterocolitis), while immobilized deferoxamine can distinguish Y. enterocolitis from S. aureus, P. aeruginosa, and other species.^[75]

Each species of bacteria can be identified by its specific spectroscopic fingerprint. An analytical chemistry study reported Raman spectroscopy combined with the siderophore pyoverdine as a capture probe to detect P. aeruginosa and Pseudomonas fluorescens at as low as 5×10³ colony-forming units (CFU)/mL within 2 h.^[76] In another study, multivalent siderophore–1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid amide conjugates were used as a diagnostic tool for bacterial infection imaging and treatment. In mice infected with P. aeruginosa, a Cy5.5 conjugate proved the applicability of probe imaging in vivo.^[77]

Distinguishing between living and dead bacteria is a major challenge in bacterial detection. Wolfenden et al^[78] designed a simple detection method based on the desferrioxamine B (DFOB), which can selectively and quickly identify only viable bacteria in a complex environment. Desf B was combined with a glass slide to specifically capture live bacteria from a mixture of live and dead E. coli.

Kruss et al^[79] developed a group of near-infrared fluorescent nanoscale sensors for remote or direct identification and fingerprinting of important bacteria in clinical practice. They used nine nanosensor combinations (including lipopolysaccharide, siderophore, DNA enzyme, protease, biofilm, and others) in a hydrogel array, which is remotely monitored by NIR sensors. The sensor array could distinguish not only pathogen species but also different strains of the same species from various clinical sources.

Whether the above concepts for pathogen detection sensors are suitable for pathogen detection in more complex biological and clinical samples remains to be evaluated. Testing in real samples is an important first step. Achieving the required sensitivity and selectivity may require optimization and new design strategies. But this is a good beginning, which provides new ideas for the rapid detection and identification of pathogens.

Trojan horse strategy—siderophore-related antibiotics

In the face of increasingly serious and extensive bacterial drug resistance, iron-uptake systems could be a target for antibacterial therapies.^[80] The characteristics of siderophores have generated new ideas in research, development, and clinical application of antibiotics. For example, PAS, a inhibitor for siderophore biosynthesis, could inhibit synthesis of the Mycostatin siderophore in M. tuberculosis.^[54,81] Meanwhile, siderophore–antibiotic conjugates enter the bacterial cytoplasm through the siderophore transport system, delivering the conjugate to the drug target; this increases the activity of the antibacterial drug and expands the antibacterial spectrum.^[9] At present, siderophores coupled with antibacterial drugs can be divided into two categories. One is natural siderophore–antibiotic conjugates, including sideromycins, such as albomycins, salmycins, ferrimycins, and danomycins, most of which are produced by Streptomyces or Actinomyces species.^[82,83] As described earlier, bacteria take up ferric iron via the siderophore-related iron-uptake system. Albomycins are composed of a trihydroxamic acid-type siderophore and an antibacterial nucleoside-analogous thioribosyl pyrimidine moiety. After they enter bacteria through the iron-uptake system, they are cleaved by a peptidase to release the thioribosyl pyrimidine, which inhibits aminoacyl-tRNA synthase and blocks protein synthesis, playing an antibacterial role in both Gram-positive and Gram-negative bacteria.^[84] Salmycins are composed of a trihydroxamate siderophore and an aminoglycoside with antibacterial function. They release the aminoglycoside through an intramolecular cyclization process triggered by iron reduction, inhibit protein synthesis, and have strong antibacterial activity toward S. aureus and Streptococcus. Ferrimycins are synthetic derivatives of salmycins.^[85,86] Danomycins inhibit protein synthesis in Gram-positive bacteria, particularly Staphylococci and Streptococci.^[87,88]

Synthetic siderophore–antibiotic conjugates have been found to be effective in avoiding common mechanisms of antibiotic resistance. Miller's team from the University of Notre Dame has done much research in this field, such as into N⁵-acetyl-N⁵-hydroxy-l-ornithine–loracarbef conjugates, hydroxamate-type siderophore–loracarbef conjugates, and catechol-type siderophore–loracarbef conjugates. Mixed-type siderophore conjugates with catechol and monohydroxamate showed varying degrees of antibacterial activity against E. coli, and strong activity with a relatively low minimum inhibitory concentration (MIC) toward methicillin-resistant S. aureus.^[89–93] Since then, other researchers have studied siderophore-β-lactams, such as catechol-type siderophore enterobactin–ampicillin conjugates, which exerted selective killing of pathogenic E. coli. The MIC toward P. aeruginosa of catechol analog–ureidopenicillin derivative conjugates was much lower than that of piperacillin.^[94–96] In recent studies, it was found that siderophore conjugates of daptomycin are potent inhibitors of carbapenem-resistant strains of A. baumannii,^[97,98] and aztreonam–siderophore conjugates showed significantly expanded activity against Gram-negative bacteria.^[99]

Cefiderocol, a synthetic siderophore–antibiotic conjugate developed by Shionogi of Japan, has been used on a global scale in the clinic since 2019. Cefiderocol is a catechol-type siderophore cephalosporin that can be administered intravenously. It mainly inhibits the synthesis of the outer membrane of Gram-negative bacteria by combining with penicillin-binding protein to exert antibacterial activity. In vitro, it has extensive and strong inhibitory effects toward a broad-spectrum of aerobic Gram-negative pathogens such as carbapenem-resistant A. baumannii, P. aeruginosa, refractory carbapenem-resistant Enterobacter, and Stenotrophomonas maltophilia. It also has strong antibacterial activity in vitro against bacteria that can produce various drug-resistance-related enzymes. An international, multicenter, randomized, double-blind, parallel-group, phase 3, non-inferiority clinical trial, APEKS-NP, aimed at evaluating the efficacy and safety of cefiderocol in the treatment of nosocomial pneumonia caused by Gram-negative pathogens (including hospital-acquired bacterial pneumonia and ventilator-associated bacterial pneumonia).^[100] Compared with patients receiving the best treatment (usually containing myxin) for carbapenem-resistant Gram-negative bacterial infection (including pneumonia, blood infection, and sepsis), the mortality of critically ill patients receiving cefiderocol increased, but the reason for the increase in mortality was unclear. At present, it is recommended that patients receiving cefiderocol should be closely monitored for their response to treatment. However, on balance, cefiderocol proves the principle of using siderophore–antibiotic conjugates as a new type of anti-infection treatment that avoids the current drug resistance mechanisms of bacteria.

Future Perspectives

As an essential metal element for life, iron has attracted vast attention from clinical and scientific researchers. In recent studies, siderophores and their related genes were found to have a close relationship with virulence and drug resistance of K. pneumoniae.^[101,102] However, research into the relationships between siderophores, virulence, and drug resistance is lacking in other bacteria. Siderophores are the key interaction factor between humans and bacteria in the competition for iron. This is an obvious focus of future research. Moreover, although many studies have focused on iron homeostasis at the sites of interaction between humans and bacteria, the mechanisms of iron homeostasis in the human body and in bacteria, and between the human and bacteria, are still not fully clear. Studies at the basic research level have used siderophores as sensors to detect bacteria—these studies should be developed into clinical trials. Siderophore-conjugated antibiotics have been studied for >20 years, but few have been applied in clinical trials. There are still many questions to be answered: For example, how to synthesize and secrete siderophores? Are siderophores necessary for bacteria to uptake iron? Does dietary supplementation including lactoferrin enhance the body's resistance to bacteria? Why is the effect of siderophore-conjugated antibiotics less than expected, and why is their safety profile unsatisfactory? Nevertheless, future studies of human and bacterial iron metabolism and their interactions are expected to be very promising for the development of new treatments to tackle infection.

Acknowledgements

The authors thank Liwen Bianji (Edanz) (www.liwenbianji.cn) for editing the language of a draft of this manuscript.

Funding

This work was supported by grants from the Beijing Key Clinical Specialty Funding (No. 010071); Clinical Cohort Construction Program of Peking University Third Hospital (No. BYSYDL2019007); Clinical Medicine Plus X - Young Scholars Project, Peking University, the Fundamental Research Funds for the Central Universities (No. PKU2022LCXQ009).

Conflicts of interest

None.