Microbial Metallomics and Parkinson’s Disease: A Unified Metal-Driven Framework Linking Ferroptosis, Dysbiosis, and α-Synuclein Pathology

Overview

Parkinson’s disease (PD) is widely interpreted through isolated mechanisms such as mitochondrial dysfunction, α-synuclein aggregation, or gut dysbiosis, yet these findings align more coherently as consequences of a single upstream disturbance: heavy metal dyshomeostasis. Toxic metal and pesticide exposure activates NCOA4 mediated ferritinophagy, releasing stored iron into dopaminergic neurons and priming them for ferroptosis, while a profound loss of transferrin in the substantia nigra disables iron export and accelerates oxidative injury. These same metals mismetallate critical enzymes, collapse antioxidant defenses, and drive misfolding of α-synuclein and SOD1, linking environmental exposure directly to the canonical neuropathology of PD. Systemically, metal overload selects for Gram-negative, metal-resistant gut pathobionts such as Desulfovibrionaceae and Enterobacteriaceae that exploit nickel and zinc-dependent virulence systems, degrade the mucus barrier, liberate iron from host proteins, and generate persistent inflammation. This inflammatory pressure propagates through circulation and the vagus nerve, activates microglia, and seeds misfolded α-synuclein originating in the enteric nervous system before it ascends to the brain, a process further amplified by magnetite-producing Desulfovibrio. Together, these mechanisms reveal heavy metal dyshomeostasis as the initiating event that unites ferroptosis, microbial virulence, neuroinflammation, and α-synuclein pathology into a single explanatory framework for PD.

Introduction

Chronic exposure to heavy metals and certain pesticides sets off a cascade of iron dysregulation in Parkinson’s disease (PD). Environmental neurotoxins like the herbicide paraquat trigger ferroptosis in dopamine neurons by inducing ferritinophagy – the autophagic degradation of ferritin that unleashes its stored iron.¹  In experimental models, paraquat causes iron to accumulate in the cytosol and mitochondria via NCOA4-mediated ferritin breakdown, sensitizing cells to iron-driven lipid peroxidation and cell death.¹  At the same time, PD patients show a loss of iron-binding capacity in the brain: the iron-transport protein transferrin is markedly decreased (≈35% lower) in the substantia nigra of PD brains.² 

Normally, transferrin and its partner, the ferroportin-ceruloplasmin complex, export excess iron out of cells. ³ A deficiency of transferrin in PD means that nigral iron cannot be adequately sequestered and exported, promoting iron overload in affected neurons.^{2 3} 

Together, toxin-induced ferritinophagy and impaired iron export result in elevated extracellular and cytosolic iron levels in PD-vulnerable regions. This labile iron fuels Fenton chemistry and reactive oxygen species (ROS) generation, setting the stage for oxidative damage. Notably, transferrin supplementation in PD models can reverse nigral iron buildup, underscoring that iron dyshomeostasis is a key early event. ² In essence, environmental toxicants initiate PD pathogenesis by tipping the balance toward unbound iron – a trigger for oxidative stress and neuronal injury.

Mismetallation, Oxidative Stress, and Protein Misfolding

Excess heavy metals in tissues also disrupt the delicate balance of essential trace elements, leading to the mismetallation of proteins. In PD, chronic exposure to metals such as iron, manganese, copper, lead, and mercury can displace the normal metal cofactors in metalloenzymes, impairing their function and promoting protein misfolding. ^{4 5} For example, manganese (a metal linked to welding-induced Parkinsonism)⁶ binds directly to the α-synuclein protein and accelerates its misfolding and aggregation.^{7 8}  

Researchers found that Mn exposure causes α-synuclein to misfold into pathogenic conformers that get packaged into exosomes, which can spread between cells and provoke neuroinflammation.⁷  Heavy metals similarly interfere with antioxidant enzymes. Recent evidence shows that “mismetallation” of wild-type superoxide dismutase (SOD1) – i.e. loading SOD1 with the wrong metal – can induce its misfolding and fibrillar deposition in a Parkinson-like neurodegenerative processs.⁵  Such metal-induced misfolded proteins are prone to aggregate and form toxic oligomers. The excess free iron released by ferritinophagy further exacerbates this by catalyzing ROS generation that oxidatively damages proteins and lipids.⁹  This oxidative stress, combined with aberrant metal binding, promotes pathological α- synuclein aggregation: the defining proteinopathy of PD.⁶ 

In short, heavy metal dyshomeostasis creates a perfect storm for protein misfolding. Metalloenzymes lose function, antioxidant defenses falter, and aggregation-prone proteins like α-synuclein and SOD1 adopt toxic conformations. This links environmental metal burden to the classic mitochondrial, oxidative, and protein misfolding theories of PD. ^{5 9}

Heavy Metals Drive Gut Dysbiosis

Metal overload in PD does not remain a local brain issue. Instead, it reverberates systemically, particularly impacting the gut ecosystem. Elevated levels of iron, manganese, copper, lead, and other metals in the intestines exert a strong selective pressure on the gut microbiome.^{11 12} Metal-resistant microbes with efficient efflux pumps and metal-cofactor utilization systems thrive under these conditions, while more sensitive commensal bacteria are depleted.^{11 12}   Human studies and animal models confirm that exposure to heavy metals perturbs the gut microbiota composition, often enriching Gram-negative pathobionts. For instance, chronic arsenic exposure increases the relative abundance of Proteobacteria (Gram-negative opp-ortunists) and specifically blooms sulfate-reducing Desulfovibrionaceae and metal- tolerant Escherichia coli in the gut.¹²  Mercury exposure similarly elevates Desulfovibrio and methanogenic archaea, while lead has been linked to an expansion of Burkholderiales( a Gram-negative order) in adults.^{13 14}

These shifts reflect a general pattern: toxic metals in the diet or water can damage the intestinal epithelium and disrupt immune-microbial homeostasis, leading to dysbiosis.¹⁵ Heavy metals directly injure the gut lining, increasing permeability (“leaky gut”) and oxidative stress in the mucosa.^{16 17}  The microbiome responds by favoring hardy bacteria that can withstand metal toxicity – often organisms that are also pro-inflammatory. Notably, Desulfovibrio, which is highly tolerant to metal stress, has emerged as a pathobiont associated with PD.¹⁸ Elevated Desulfovibrio levels have been reported in the fecal microbiota of PD patients, and this genus is known to reduce sulfate and heavy-metal ions in anaerobic conditions.^{18 19 20}

Thus, heavy metal dyshomeostasis creates a dysbiotic gut milieu characterized by loss of beneficial microbes and overgrowth of Gram-negative, metal-resistant bacteria. This dysbiosis is not benign. It actively contributes to pathology via inflammation and metabolic disturbances, essentially connecting environmental metal exposure to the gut-brain axis of Parkinson’s Disease. ^{21 11}

Metals and Microbial Virulence

The metal-selected microbes in the dysbiotic PD gut often possess virulence factors that exploit these very metals. In particular, nickel (Ni) and zinc (Zn) become double-edged swords – they are toxic to many bacteria, but the survivors repurpose Ni and Zn as cofactors to enhance their survival and pathogenicity. Nickel is an essential cofactor for several microbial enzymes that contribute to virulence.^{22 23}  The most notable are Ni-dependent ureases and [NiFe]-hydrogenases, which have been shown to aid colonization and infection in pathogens.²³  For example, Helicobacter pylori relies on a Ni-urease to survive stomach acidity, and Salmonella uses multiple Ni-hydrogenases to thrive in the gut. ²³ Many Enterobacteriaceae (e.g. Proteus mirabilis, Klebsiella) produce ureases that require nickel and contribute to their virulence in urinaryand GI infections. ²³

PD studies have noted an overrepresentation of Enterobacteriaceae in the gut microbiome of patients, which could be linked to metal-driven selection of such urease-positive bacteria. Nickel also activates Ni-superoxide dismutase and Ni-glyoxalase I in certain bacteria, enzymes that detoxify ROS and reactive carbonyls, respectively.^{24 25}  These systems can allow pathogens to better withstand the oxidative burst of phagocytes. In fact, glyoxalase (which detoxifies methylglyoxal produced during inflammation) is critical for bacterial survival against neutrophils – Group B Streptococcus mutants lacking glyoxalase are cleared more easily by neutrophils.^{26 27}

Many bacteria encode two classes of glyoxalase I: one requiring Zn2+ and another using Ni2+ or Co2+.²⁸  If nickel is abundant (or if the bacterium has high Ni affinity uptake), the Ni-dependent glyoxalase can greatly enhance resilience to immune attack. The human host attempts to counter this by sequestering Ni: neutrophils release calprotectin, a metal-binding protein that avidly chelates zinc and prefers Ni(II), effectively starving microbes of nickel.²⁹  However, metal-tolerant pathogens often encode high-affinity Ni transporters and storage proteins. In heavy-metal-rich conditions, such pathogens can maintain their Ni-cofactored enzymes despite calprotectin, thereby evading neutrophil killing.²⁹  Zinc, likewise, is a critical micronutrient that pathogenic bacteria weaponize in the dysbiotic gut. Many secreted bacterial metalloproteases are Zn-dependent enzymes that degrade host tissues and immune factors.³⁰ Extracellular zinc metalloproteases are widely distributed among human pathogens and exhibit diverse pathological actions, from cleaving mucus and extracellular matrix to inactivating host defense proteins.³⁰  In the PD gut, an overgrowth of proteolytic, Gram-negative bacteria (e.g. some Bacteroides or Clostridiales species) can lead to excessive protease activity. For example, toxic Bacteroides fragilis strains secrete a Zn-metalloprotease (fragilysin) that cleaves tight-junction proteins, causing barrier permeability. More generally, bacterial metalloproteases target host iron-binding proteins to scavenge nutrients – they can cleave transferrin and lactoferrin into fragments, liberating the iron for bacterial use.³⁰ 

This means a heavy-metal-adapted microbe can simultaneously resist metal toxicity and exploit metal- binding host proteins to obtain iron. The net effect is a further increase in local free iron and tissue damage. In PD, where transferrin is already deficient, such microbial proteases exacerbate the problem by destroying any remaining iron-sequestration capacity. Additionally, Zn metalloproteases and other virulence factors (e.g. hemolysins) can directly injure the gut epithelium, helping pathogens penetrate or inflame the mucosa.³⁰ 

In effect, the dysbiotic bacteria enriched by metal exposure are not passive bystanders – they actively use Ni and Zn-dependent virulence strategies to persist and to extract nutrients from the host. Nickel-based enzymes (urease, hydrogenase, Ni-SOD, Ni-glyoxalase) allow them to withstand host immunity, while zinc metalloproteases and other secreted factors enable them to breach host barriers and feed on host tissues, all of which amplify inflammation. ³¹

Gut Epithelium Integrity and Chronic Neuroinflammation

As metal-resistant, protease-secreting Gram-negative bacteria flourish, the integrity of the gut lining is progressively compromised. These microbes produce high levels of endotoxin (lipopolysaccharide, LPS) and hydrogen sulfide (H2S), among other metabolites, which trigger inflammation and damage the epithelium. Desulfovibrio, for instance, is a dominant sulfate-reducer in the colon that generates H2S as an end-product.³²  H2S is a double-edged sword: at low levels it serves as a neuromodulator, but at the concentrations produced in dysbiosis it becomes severely toxic.³²   Excess H2S can directly reduce disulfide bonds in the mucin layer, eroding the protective mucus barrier.³²    By stripping the mucus and damaging tight junction proteins, H2S increases gut permeability.³²  Meanwhile, the overgrowth of mucin- degrading bacteria (e.g. an increase in Akkermansia noted in PD microbiomes) also thins the mucus layer, further exposing the epithelium to insult.^,32 

As the intestinal barrier loses its integrity, bacterial products translocate into the gut wall and bloodstream. Lipopolysaccharide (LPS) from Gram-negatives potently activates innate immune receptors (TLR4) on resident immune cells. Indeed, LPS alone can reproduce many features of PD when chronically administered: it induces intestinal and brain inflammation, activates microglia, and even causes selective loss of dopaminergic neurons in animal models.³²  In PD patients, gene pathways for LPS synthesis are elevated in the gut microbiome, and circulating LPS levels are found to be higher, indicating a continuous endotoxin load.³² 

The endotoxin and H2S influx ignites chronic mucosal inflammation. The gut lamina propria floods with neutrophils and macrophages trying to contain the microbial breach. These immune cells release pro-inflammatory cytokines (TNF-α, IL-1β, IL-6, etc.) which not only damage local tissue but can enter circulation to exert systemic effects.³²

The inflamed gut also shows increased production of reactive nitrogen and oxygen species, compounding the epithelial injury. Studies have found that PD patients have elevated fecal calprotectin (a neutrophil inflammation marker) and increased serum levels of gut permeability markers like zonulin and α1-antitrypsin, reflecting this barrier dysfunction and inflammation.³² Crucially, chronic gut inflammation does not stay confined to the GI tract. 

It propagates along the gut– brain axis, an inter-connected highway of neural and humoral com-munication. Pro-inflammatory cytokines and microbial toxins from the gut can reach the brain via circulation (especially if the blood-brain barrier is also impaired by systemic inflammation).³²  Additionally, the vagus nerve provides a direct neural pathway: inflammatory signals and even whole pathogens in the gut can activate vagal afferents, or in the case of misfolded proteins, physically travel retrograde into the brainstem. The constant stimulation of the vagus and the infiltration of inflammatory mediators lead to microglial activation in the brain, essentially mirroring the peripheral immune activation.^{32 33}

This neuroinflammatory state in the CNS creates a toxic milieu for neurons, particularly the delicate dopamine neurons of the substantia nigra which are highly susceptible to oxidative and inflammatory damage. In sum, heavy metal-triggered dysbiosis instigates a feed-forward loop of gut inflammation and a barrier that, over time, spills into systemic and brain inflammation. The gut becomes a chronic reservoir of endotoxin and other noxious stimuli, continually “fueling the fire” of neuroinflammation that drives neurogenerative disease and PD progression. ^{32  33}

 α-Synuclein Aggregation and the Final Common Pathway to PD

One of the most compelling links between peripheral dysbiosis and PD is the misfolding and aggregation of α-synuclein, a neuronal protein that forms Lewy body deposits in PD. Both heavy metals and microbial inflammation converge on this final pathway.³² Excess iron and other metals in neurons can directly promote α-synuclein aggregation via metal binding and oxidation 40 . Lab studies show that iron(III) can bind to α-syn and foster its assembly into fibrils, while magnetite (Fe3O4) nanoparticles, which Desulfovibrio bacteria produce in the gut, can seed α-synuclein oligomerization .²⁰  

Desulfovibrio isolated from PD patients was recently shown to induce Lewy-body-like aggregates and motor impairment when fed to animals, likely due to its H2S and magnetite output. H2S not only damages the gut, but when absorbed it can enter enteric neurons and elevate intracellular iron by releasing iron from ferritin stores.³² The combination of high cytosolic iron and inflammatory free radicals in enteric neurons promotes α-synuclein misfolding in the gut’s nervous system (ENS).³² Biopsies from PD patients show misfolded α-syn accumulations in enteric ganglia years before brain symptoms arise. Chronic gut inflammation (from LPS, H2S, etc.) upregulates α-syn expression as well. α-Syn is thought to be part of an intrinsic defense against microbes, so its synthesis in enteric neurons may increase in response to the dysbiosis, inadvertently increasing the pool of protein that can misfold.³²  

Once α-synuclein adopts a pathologically aggregated form in the gut or peripheral nerves, it can propagate in a prion-like fashion along the vagus nerve to the brainstem.³² The inflamed gut thus becomes the launching pad for α-synuclein pathology that ascends this neural axis. Simultaneously, circulating misfolded α-syn, released in exosomes from stressed neurons, can breach a leaky blood-brain barrier and seed aggregation in the brain.³⁸  Heavy metals again further facilitate this spread. Manganese exposure caused misfolded α-syn to be packaged into vesicles that traveled from neuron to neuron, inducing neurodegeneration in a manner very similar to how PD progresses.⁷  In welders with high manganese, elevated misfolded α-syn levels are detectable in blood serum, highlighting this systemic transport of the toxic protein.⁸

Ultimately, the culmination of these events is the death of dopaminergic neurons and the clinical onset of PD motor symptoms. The feed-forward cycle can be summarized as follows:^{11 34}

Heavy metal exposure leads to metal mismanagement (excess iron, manganese, etc.) and mismetallation, causing oxidative stress and misfolded proteins. This metal dyshomeostasis simultaneously alters the gut microbiome, enriching heavy-metal–resistant, Gram-negative taxa that produce inflammatory and neurotoxic factors (LPS, H2S, proteases). Those factors cause gut inflammation and barrier dysfunction, which in turn seeds systemic and CNS inflammation, and triggers α-synuclein pathology both in the gut and brain. The misfolded α-synuclein then spreads and accumulates, exacerbating neuronal iron accumulation and oxidative injury, as aggregated α-syn can bind metals and impede their normal handling.³²  This cycle of metal toxicity, microbial virulence, and protein aggregation keeps reinforcing itself, driving the progression of neuro-degeneration.^{35 11} The heavy metal–dysbiosis hypothesis thus unifies multiple threads of PD etiology into a single mechanistic narrative: environmental metal dys-homeostasis sets the stage, and the resulting dysbiotic microbiome executes the final lethal script – through nutrient piracy, tissue destruction, chronic inflammation, and ultimately the prion-like spread of misfolded proteins that kill neurons and give rise to Parkinson’s disease.^{36 37} 

References

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This section broadens the paraquat-heavy-metal narrative by showing that iron overload, ferritinophagy, and ferroptosis are central, druggable mechanisms in PD. Clinical chelators, ferroptosis inhibitors, CISD1-targeting drugs, and HO-1 modulation all counteract iron-driven mitochondrial damage and neuroinflammation, reinforcing iron dysregulation as a core therapeutic target.

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This section provides the mechanistic and therapeutic proof-of-concept that reduced transferrin likely drives substantia nigra iron overload. This section shows that experimentally: Tf is reduced in PD nigra, Tf supplementation corrects neuronal and MPTP iron overload and motor deficits, but also causes systemic iron depletion, highlighting both its relevance and its translational limitation.

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This section mechanistically explains why transferrin loss in PD promotes iron overload. It details how the ceruloplasmin–ferroportin–Tf system normally coordinates both iron uptake and efflux, with Tf acting as the central shuttle. Thus, when Tf is reduced in PD, cells can neither properly sequester nor export iron, amplifying iron accumulation and vulnerability to ferritinophagy-driven ferroptosis.

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Imlay’s review describes how reactive oxygen species oxidize ferrous iron in mononuclear iron enzymes, displacing iron and allowing zinc to bind instead, which results in inactive enzymes and metabolic bottlenecks. Cells respond by importing manganese to replace the catalytic iron. These observations illustrate how oxidative stress and exposure to non‑cognate metals lead to mismetallation of metalloenzymes.

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The Frontiers review notes that manganese binds to the C‑terminal domain of α‑synuclein at Asp‑121, Asn‑122 and Glu‑123. This Mn binding alters α‑syn structure and can induce oxidative stress and oligomerization. Binolfi et al.’s NMR work demonstrated that divalent metal ions, including Mn²⁺, interact with α‑synuclein and influence its folding and aggregation kinetics.

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The previous Frontiers review [6] cites Uversky et al.’s findings that α‑synuclein has affinity for various metal ions and that metals such as aluminium, copper, cadmium, iron, manganese and zinc can trigger α‑synuclein aggregation and cross‑linking. Uversky et al. showed that several di‑ and trivalent metals (Al³⁺ > Cu²⁺ > Fe³⁺ ≈ Mn²⁺) markedly accelerate α‑synuclein fibrillation, establishing a mechanistic link between metal exposure and protein misfolding.

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Sánchez-Corona et al. [10] posit that heavy metals are widespread pollutants that exert selection pressure on microbial populations due to their toxicity and persistence, leading to the evolution of heavy metal resistance genes (HMRGs). These genes are part of the resistome, and their spread often occurs via mobile genetic elements that allow co-selection with antibiotic and biocide resistance genes. The selection pressure exerted by HM promotes the spread of multidrug-resistant strains and thus increases ecological and health risks.

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In this human study, higher urinary Pb levels were associated with shifts in the microbiome. After adjustment, only taxa within the phylum Proteobacteria and the order Burkholderiales remained significantly associated with Pb exposure. The authors report that Pb level was linked to increased colonization by Proteobacteria, specifically Burkholderiales. 

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Singh et al. describe Desulfovibrio species as opportunistic pathobionts that may overgrow in disease states. They note that Desulfovibrio can reduce heavy‑metal sulfates to precipitate heavy metals, reflecting their tolerance to metal stress

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In this human case–control study, all PD patients harbored Desulfovibrio in their fecal microbiota and levels were higher than in controls; abundance correlated with PD severity. The authors explain that
Desulfovibrio bacteria produce hydrogen sulfide and lipopolysaccharide and can reduce ferric iron to ferrous iron via a periplasmic [FeFe]-hydrogenase, producing magnetite which may promote α‑synuclein aggregation. The review notes that Desulfovibrio reduce soluble heavy‑metal sulfates so the metals precipitate out of solution. The npj meta‑analysis further reports that Desulfovibrio is a dominant sulfate‑reducing bacterium whose relative abundance and contribution to sulfate‑reduction genes are significantly increased in PD and that these bacteria can reduce ferric iron to ferrous iron and produce Fe₃O₄.

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The review summarizes how metal dyshomeostasis and gut dysbiosis contribute to PD. It states that heavy metals promote oxidative stress, alter gut‑barrier permeability and cause inflammation, leading to increased metal absorption and traffic into the brain. The gut microbiota modulates metal absorption and can bioremediate metals, so interventions that reduce dietary toxic metals may lessen the inflammatory burden on beneficial microbes and potentially delay PD onset. 

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This review notes that nickel is an essential cofactor for several microbial virulence factors, highlighting that nickel-dependent enzymes such as urease and [NiFe]-hydrogenase are major virulence determinants

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The review further identifies several nickel‑requiring enzymes beyond urease and hydrogenase, including Ni-acireductone dioxygenase, Ni-superoxide dismutase (Ni‑SOD) and Ni‑glyoxalase I, which participate in metabolic and detoxifying processes.

[26] Akbari MS, Joyce LR, Spencer BL, Brady A, McIver KS, Doran KS. Identification of glyoxalase A in group B Streptococcus and its contribution to methylglyoxal tolerance and virulence. Infect Immun. 2025 Apr 8;93(4):e0054024. doi: 10.1128/iai.00540-24  

In neutrophil-killing assays, Group B Streptococcus lacking glyoxalase A (ΔgloA) showed significantly decreased survival compared with the wild-type, demonstrating that glyoxalase detoxification helps bacteria resist neutrophil killing.

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In vivo, neutrophil depletion rescued the attenuated virulence of the ΔgloA mutant, highlighting that glyoxalase is essential for bacterial survival against neutrophil-mediated immunity

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The study shows that glyoxalase I exists as two metal-specific forms: a Ni/Co-dependent version found mainly in prokaryotes and a Zn-dependent version in eukaryotes while some organisms encode both forms. This study suggests that horizontal gene-transfer and gene-fusion led to co-existence of different metal-ion specific glyoxalase I.

[29] Maier RJ, Benoit SL. Role of Nickel in Microbial Pathogenesis. Inorganics. 2019; 7(7):80. https://doi.org/10.3390/inorganics7070080  

The review explains that neutrophils secrete calprotectin, a metal-binding protein that coordinates Zn(II) and has an even higher affinity for Ni(II); calprotectin thereby sequesters Ni and inhibits urease activity in pathogens such as Staphylococcus aureus and Klebsiella pneu-moniae.

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The study revealed that enterococci use several ways to acquire iron from TR and LF, such as iron chelating siderophores, iron reduction, facilitated iron release, protein degradation – promoted iron release, and receptor mediated capture of the iron-host protein complexes. The broad spectrum of iron acquisition mechanisms used by enterococci play a significant role in the colonization of the human body and the resulting pathogenicity.

[32] Nie, S., Wang, J., Deng, Y. et al. Inflammatory microbes and genes as potential biomarkers of Parkinson’s disease. npj Biofilms Microbiomes 8, 101 (2022). https://doi.org/10.1038/s41522-022-00367-z

[33] Tizabi Y, Bennani S, El Kouhen N, Getachew B, Aschner M. Heavy Metal Interactions with Neuroglia and Gut Microbiota: Implications for Huntington’s Disease. Cells. 2024; 13(13):1144. https://doi.org/10.3390/cells13131144

[34] Forero-Rodríguez LJ, Josephs-Spaulding J, Flor S, Pinzón A, Kaleta C. Parkinson’s Disease and the Metal-Microbiome-Gut-Brain Axis: A Systems Toxicology Approach. Antioxidants (Basel). 2021 Dec 28;11(1):71. doi: 10.3390/antiox11010071 

This review proposes a feed‑forward cycle linking metals, dysbiosis and α‑synuclein pathology. It explains that microbial dysbiosis and free heavy metal ions leaking from the gut promote oxidative stress in the enteric nervous system, leading to α‑synuclein misfolding and aggregation; the misfolded protein can propagate via the vagus nerve, and chronic oxidative stress ultimately causes dopaminergic neuronal death and PD motor symptoms.

[35] Forero-Rodríguez LJ, Josephs-Spaulding J, Flor S, Pinzón A, Kaleta C. Parkinson’s Disease and the Metal-Microbiome-Gut-Brain Axis: A Systems Toxicology Approach. Antioxidants (Basel). 2021 Dec 28;11(1):71. doi: 10.3390/antiox11010071 

In the same paper, the authors describe how heavy metals promote oxidative stress, alter gut barrier permeability, and provoke inflammation, leading to increased metal absorption into the brain.They note that dysbiosis amplifies these effects and that interventions to mitigate dietary heavy metals could reduce the inflammatory burden and potentially delay PD onset.

[36] Johnson ME, Stecher B, Labrie V, Brundin L, Brundin P. Triggers, Facilitators, and Aggravators: Redefining Parkinson’s Disease Pathogenesis. Trends Neurosci. 2019 Jan;42(1):4-13. doi: 10.1016/j.tins.2018.09.007 

This framework article proposes that gut dysbiosis and increased intestinal permeability trigger systemic inflammation that can spread to the CNS. The authors speculate that inflammation from altered microbiota leads to α‑synuclein aggregation in enteric nerves, with propagation along the vagus nerve to the brain; dysbiosis‑driven inflammatory mediators and metabolites can activate microglia and worsen motor deficits.

[37 Cao L, Jha SK, Gupta N, Chen X, Soni R, Yuan L, Srivastava R, Chen ZS. The Aging Gut-Brain Axis: Effects of Dietary Polyphenols and Metal Exposure. Chronic Dis Transl Med. 2025 Oct 15;11(4):251-268. doi: 10.1002/cdt3.70026  

This review explicitly frames PD as a gut-flora–associated neurodegenerative disease, noting that it affects ~1% of older adults and up to 5% of middle-aged individuals, with classic motor symptoms (bradykinesia, resting tremor, rigidity, postural instability) and prominent non-motor GI complaints such as nausea, hypersalivation and dysphagia. 

The authors summarize evidence that gut dysbiosis drives pro-inflammatory signaling and α-synuclein-linked pathology, connecting pathological actions of gut flora directly to PD development. Crucially, they state that intake of toxic heavy metals is a risk factor for dysbiosis and hence for PD, citing Forero-Rodríguez et al. as support, and further highlight work showing that heavy metal toxicity alters gut flora composition and impacts multiple organs. Together, this review explicitly ties heavy-metal exposure, microbiome disruption, and PD pathogenesis into a single gut–brain axis framework, strongly reinforcing the heavy metal–dysbiosis hypothesis we aim to advance.

[38] Pinnell JR, Cui M, Tieu K. Exosomes in Parkinson disease. J Neurochem. 2021 May;157(3):413-428. doi: 10.1111/jnc.15288

The Neurochemistry review notes that exosomes can permeate biological membranes, including the blood–brain barrier, enabling intercellular delivery of proteins and nucleic acidspmc.ncbi.nlm.nih.gov. Together, these findings support the statement that misfolded α‑syn-containing exosomes can breach a leaky blood–brain barrier and seed aggregation in the brain.