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SIDEROPHORES: KEY PLAYERS IN BACTERIAL PATHOGENESIS WITH
BROAD MEDICAL AND INDUSTRIAL USES
Bazarova Gulnora Rustamovna
Associate Professor at the “Alfraganus “ university’s medical faculty
Email:gulnorabazarova599@gmail.com
Orcid Id:0009-0006-7621-2642
https://doi.org/10.5281/zenodo.14905933
Abstract
Iron is a vital element required by all microorganisms. In response to iron scarcity, both
Gram-positive and Gram-negative bacteria synthesize siderophores—low molecular weight
compounds with high iron-binding affinity—that serve as crucial virulence factors. Research
has shown that impairments in the production or function of these molecules, as well as in
overall iron acquisition systems, are associated with reduced bacterial pathogenicity. Given
their involvement in diverse biological processes, siderophores have attracted considerable
attention as important secondary metabolites. They also function as biosensors, monitoring
iron levels in various environments. In the medical field, siderophores are exploited to deliver
antibiotics into resistant bacteria via a Trojan horse strategy and are employed in the treatment
of diseases such as cancer and malaria. This review examines iron acquisition mechanisms in
both Gram-positive and Gram-negative bacteria, the significance of siderophore production in
bacterial pathogenesis, the classification of siderophores, and their primary applications in
medicine and industry.
Keywords:
Siderophore, Trojan horse, Iron, Pathogenesis, Cancer, Medicine, Industry
Introduction
Iron is an indispensable element that underpins a wide range of cellular processes. It
plays a critical role in key metabolic pathways such as the tricarboxylic acid cycle, electron
transport, oxidative phosphorylation, nitrogen fixation, and the synthesis of aromatic
compounds. Iron also contributes to the formation of important metabolites including
porphyrins, toxins, antibiotics, cytochromes, pigments, and siderophores, meaning that
fluctuations in iron levels can significantly influence these biosynthetic processes. Additionally,
many essential enzymes—like peroxidase, catalase, and certain forms of superoxide
dismutase—rely on iron to neutralize harmful radicals and maintain cellular integrity. As a
cofactor for numerous other enzymes, iron directly affects overall cell composition. When iron
is deficient, DNA and RNA synthesis diminish, bacterial growth slows, and sporulation is
impaired, often leading to notable morphological changes. For example, in Escherichia coli, iron
is necessary for the function of a key ribonucleotide reductase involved in deoxyribonucleotide
synthesis, and its absence hampers DNA replication. Similar effects have been observed in other
bacteria, where reduced iron availability results in lower nucleic acid production and even
structural changes in RNA molecules.
Iron also influences the virulence of pathogenic bacteria by regulating the expression of
several toxins and other virulence factors. Toxins such as Shiga toxin, diphtheria toxin, and
exotoxin A are modulated by iron levels or by iron-responsive regulatory systems. Moreover,
bacteria exhibit distinct phenotypic and genetic changes when forming biofilms compared to
their planktonic state. Biofilm-associated cells typically require lower concentrations of iron
than free-floating cells, both in laboratory experiments and in natural environments.
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Studies have further demonstrated that iron is essential for biofilm formation, as it
controls cell surface properties and stabilizes the polysaccharide matrix. Under conditions of
iron scarcity, bacterial surface hydrophobicity decreases and the composition of surface
proteins shifts, leading to impaired biofilm development. For instance, research on
Staphylococcus epidermidis has shown that in low-iron conditions, bacteria produce
siderophores and grow more slowly in a planktonic state, while higher iron concentrations can
trigger the early expression of genes linked to biofilm formation. Additional studies have
confirmed that the expression of genes in operons related to iron acquisition is sensitive to both
iron and manganese levels. Furthermore, experiments with compounds that chelate iron, such
as PGG, have demonstrated that restoring iron availability can enhance the expression of iron-
regulated genes and promote biofilm formation in Staphylococcus aureus, underscoring the
critical role of iron in biofilm development.
Iron Acquisition Systems in Pathogenic Bacteria
Both Gram-positive and Gram-negative bacteria produce siderophores when faced with
iron deficiency. In Gram-negative organisms, specialized outer membrane receptors recognize
specific Fe³⁺-siderophore complexes on the cell surface. These ferric-siderophore complexes
are then actively transported across the cell membrane via an energy-dependent system that
includes outer membrane receptors, periplasmic binding proteins, and inner membrane
transport proteins.
A common mechanism in Gram-negative bacteria involves β-barrel receptors on the outer
membrane that recognize iron-loaded siderophores. Once the ligand binds, the receptors
undergo conformational changes, allowing the iron-siderophore complex to be translocated
into the periplasmic space. This process is powered by the TonB complex, composed of TonB,
ExbB, and ExbD proteins, which harness energy from the proton motive force. Subsequently,
ATP-binding cassette (ABC) transporters in the inner membrane facilitate the transfer of the
iron-siderophore complex into the cytosol, where iron reduction occurs.
The TonB-ExbB-ExbD system, well-characterized in Escherichia coli, requires direct
interaction between TonB and outer membrane receptors for energy transfer. In E. coli, TonB
consists of three domains: an N-terminal domain (residues 1–33) that anchors the protein to
the inner membrane and interacts with ExbB-ExbD; a central domain (residues 34–154)
enriched with Pro-Glu and Pro-Lys repeats and located in the periplasm; and a C-terminal
domain (residues 155–239) in the periplasm that interacts with the N-terminal region of ferric
siderophore receptors. Under iron-limited conditions, TonB expression is upregulated. ExbB, a
26-kDa membrane protein with three transmembrane domains, and ExbD, a 17-kDa protein
with both a transmembrane and a periplasmic domain, are essential for this energy
transduction process. While E. coli employs a single TonB-ExbB-ExbD system, other bacteria,
such as Vibrio cholerae, possess multiple TonB proteins (e.g., TonB1 and TonB2) that appear to
be specific for different outer membrane receptors. These receptors enhance the uptake of
ferric-siderophore complexes, enabling bacteria to efficiently scavenge iron from their
environment.
The Importance of Siderophore Production in Bacterial Pathogenesis
Siderophores are among a variety of virulence factors—including bacterial toxins,
adhesion molecules, and protective capsules—that facilitate pathogen colonization and
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exacerbate disease severity. Microbes have evolved sophisticated mechanisms to acquire iron
from their hosts, notably by producing siderophores that sequester iron from host-bound
proteins. Disrupting or deleting genes involved in siderophore synthesis or other iron-
harvesting systems often leads to reduced virulence in pathogens such as Salmonella and
Staphylococcus, while strains capable of producing high levels of siderophores tend to exhibit
increased pathogenicity. Conversely, bacteria that cannot synthesize or secrete siderophores
are generally less effective at colonizing and causing disease.
Both Gram-positive and Gram-negative bacteria ramp up siderophore production under
conditions of iron deficiency. These molecules not only help secure iron but can also disrupt
host cellular iron homeostasis, triggering changes in transcription and cellular behavior. For
instance, iron chelation by siderophores can steer cells toward mitophagy—a regulated process
of mitochondrial autophagy. Additionally, siderophores may induce a hypoxic response in host
cells even when oxygen levels are normal by stabilizing hypoxia-inducible factor 1-alpha (HIF-
1α). This stabilization leads to the upregulation of various genes, including those encoding
antimicrobial peptides and inflammatory cytokines, thereby influencing the host immune
response.
Beyond their role as iron scavengers, siderophores can act as toxins and immune
modulators. However, it remains unclear whether the activation of host pathways, such as
those governed by HIF-1α, ultimately benefits the host or the invading pathogen. For example,
studies investigating siderophores produced by Klebsiella pneumoniae—such as enterobactin,
yersiniabactin, salmochelin, and aerobactin—have shown that these compounds can enhance
the expression of autophagy-related markers, induce mitochondrial membrane proteins, and
elevate levels of reactive oxygen species (ROS) and apoptosis indicators in platelets. In
particular, enterobactin has been noted for its significant impact on activating mitophagy
pathways.
Siderophore production also influences the anatomical site and pattern of infection. In
animal models of pneumonia, infection with yersiniabactin-producing strains of Klebsiella
pneumoniae has been linked to bronchopneumonia with moderate bacterial loads in both the
lungs and spleen. In organisms like Acinetobacter baumannii, siderophores are essential for
surface adhesion and the synthesis of extracellular polysaccharides, which facilitate biofilm
formation and the establishment of microbial communities that rely on shared iron resources.
Similarly, Pseudomonas aeruginosa produces pyoverdine and pyochelin; while pyochelin is
associated with biofilm formation and chronic lung infections, pyoverdine has been shown to
cause mitochondrial toxicity in model organisms, leading to mitochondrial fragmentation and
cell death.
In addition to their deleterious effects, siderophores can also protect bacteria by
mitigating the impact of reactive oxygen species generated by ultraviolet light or antibiotic
stress. For instance, pyoverdine can absorb UV radiation, thereby reducing ROS formation. In
uropathogenic Escherichia coli (UPEC), the ability to produce various siderophores—including
enterobactin, salmochelin, aerobactin, and yersiniabactin—plays a critical role in establishing
urinary tract infections. Research in animal models indicates that non-catecholate siderophore
receptors are important for the formation of bacterial colonies during progressive urinary tract
infections. Specific receptors, such as IroN, have been implicated in urinary tract colonization
and even in the invasion of urethral cells, while IroN has also been identified as a key factor in
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the pathogenicity of E. coli strains causing neonatal meningitis. The production of salmochelin
is frequently associated with urinary isolates and may contribute to recurring or resistant
infections, whereas aerobactin serves as an epidemiological marker, correlating with enhanced
bacterial growth under iron-limited conditions in serum and urine. Studies have further
demonstrated that UPEC strains harboring genes for yersiniabactin and aerobactin exhibit
more efficient iron uptake and increased growth rates in minimal media.
Moreover, the catecholate siderophore enterobactin produced by E. coli has been found
to inhibit the generation of ROS and the formation of neutrophil extracellular traps (NETs),
while also reducing degranulation and phagocytic activity in neutrophils without affecting their
migration. In Salmonella, enterobactin aids bacterial survival by chelating intracellular iron
pools, thereby disrupting critical iron-dependent antimicrobial mechanisms within
macrophages.
Finally, in Yersinia species such as Yersinia pestis, the synthesis of yersiniabactin is crucial
for full virulence. The gene cluster responsible for yersiniabactin production is tightly
regulated, and deletion of key biosynthetic genes has been shown to abolish virulence in animal
models, underscoring the pivotal role of siderophores in bacterial pathogenesis.
Classification of Siderophores Based on Chemical Structure
Since the early discovery of siderophores such as mycobactin, ferrichrome, and coprogen
in the early 1950s, over 500 distinct siderophores have been identified. Bacterial siderophores
are generally classified into three primary families according to the chemical groups involved
in binding iron: catecholates, hydroxamates, and carboxylates. In addition, there are
siderophores with mixed ligands, which can be further divided into four categories:
catecholate-hydroxamate,
phenolate-hydroxamate,
citrate-catecholate,
and
citrate-
hydroxamate. Although all these families utilize negatively charged oxygen atoms to chelate
ferric iron, each group possesses unique characteristics that influence its iron-binding affinity.
Conclusion
Siderophores hold great promise as biomarkers for a wide range of therapeutic and
industrial applications. By leveraging iron chelators and the mechanisms bacteria use to
acquire iron, it is possible to develop innovative vaccines and antibiotics—such as conjugates
that act as "Trojan horses" against antibiotic-resistant strains, a pressing health challenge
today. Essentially, bacterial products can be repurposed to combat the pathogens that produce
them, offering a novel strategy to counteract treatment-resistant infections. Moreover, these
versatile molecules could pave the way for new cancer therapies, potentially overcoming many
of the limitations associated with conventional treatments. However, further studies in animal
and human models are necessary to deepen our understanding of iron uptake pathways and
the therapeutic actions of siderophores, ensuring that the full potential of iron chelators is
harnessed for specific medical applications and beyond.
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