Overview
Pathogens deploy dedicated cellular machinery to import, store, regulate, and when necessary export transition metals. These systems complement the extracellular siderophores metallophores that scavenge metals from the host environment by providing the membrane transport, intracellular buffering, and transcriptional control needed to maintain metal homeostasis in the face of host nutritional immunity. The sophistication and redundancy of these systems -- often multiple transporters for a single metal, dedicated storage proteins, and exquisitely sensitive metal-sensing regulators -- reflects the intense evolutionary pressure imposed by host metal restriction.
Import Systems
ABC Transporters (ATP-Binding Cassette)
The highest-affinity metal import systems in bacteria. Consist of a periplasmic/surface-associated binding protein, membrane permease, and cytoplasmic ATPase.
Nickel-specific:
- NikABCDE (E. coli): The prototypical bacterial nickel importer. NikA is the periplasmic binding protein; NikB/C form the transmembrane channel; NikD/E provide the ATPase. High affinity, capable of scavenging nickel at very low concentrations [maier 2019 nickel microbial pathogenesis].
- NiuBDE (H. pylori): ABC-type nickel transporter that operates at acidic pH -- essential for the gastric niche where low pH is constant [maier 2019 nickel microbial pathogenesis].
- UreMQO (S. salivarius): The only characterized nickel transporter in Streptococci. Part of the Ni-dependent urease operon [akbari 2022 metal homeostasis streptococci].
Iron-specific:
- PitABCD (Streptococci): Iron import system.
- FeoABC: Ferrous iron (Fe2+) transport; widely distributed across Gram-negative and some Gram-positive pathogens [akbari 2022 metal homeostasis streptococci].
- PiaA/PiuA (S. pneumoniae): Iron acquisition proteins contributing to virulence in pneumococcal infection.
- Siderophore ABC transporters: Import siderophore-Fe complexes (e.g., staphyloferrin A/B uptake in S. aureus) [cassat 2012 metal acquisition staphylococcus aureus].
Zinc-specific:
- AdcABC/AdcAII (Streptococci): Primary zinc import system. AdcA and AdcAII are two distinct zinc-binding lipoproteins with complementary roles. Mutants show attenuated colonization across multiple infection models (nasopharynx, tooth, meningitis, skin) [akbari 2022 metal homeostasis streptococci].
- Lmb (S. agalactiae): Laminin-binding protein that also functions as zinc-binding lipoprotein for import.
Manganese-specific:
- MntABC/SloABC (Streptococci, Staphylococci): High-affinity Mn import critical for superoxide dismutase activity and oxidative stress defense [cassat 2012 metal acquisition staphylococcus aureus], [akbari 2022 metal homeostasis streptococci].
Metallophore-metal ABC transporters:
- CntABCDF (S. aureus): Imports staphylopine-metal complexes (Ni, Zn, Cu, Co) after the metallophore captures metals extracellularly [maier 2019 nickel microbial pathogenesis].
NiCoT Secondary Transporters
Single-component, secondary (proton motive force-driven) transporters specific for nickel and/or cobalt.
- NixA (H. pylori): The best-characterized NiCoT. A high-affinity Ni-only transporter. NixA works alongside the NiuBDE ABC system, providing redundant nickel import -- evidence of how critical nickel acquisition is for H. pylori [maier 2019 nickel microbial pathogenesis].
- NixA homologs: Found in other Ni-dependent pathogens. Also characterized in engineered probiotics as a target for metal-sequestering therapy [chen 2022 living microorganisms detoxification heavy metals].
MntH (NRAMP Family)
- Secondary Mn2+ transporters homologous to host NRAMP1/SLC11A1.
- Found in Streptococci (S. pyogenes, S. pneumoniae) and other pathogens [akbari 2022 metal homeostasis streptococci].
- The evolutionary irony: the same NRAMP protein family is used by hosts (NRAMP1 to export metals from phagolysosomes, starving engulfed pathogens) and by pathogens (MntH to import metals for survival).
ECF (Energy-Coupling Factor) Transporters
- Modular ABC-type systems that can switch substrate specificity by exchanging the substrate-binding component.
- Nickel ECF transporters identified in some pathogens but less well characterized than NikABCDE or NixA.
TonB-Dependent Outer Membrane Receptors
- In Gram-negative bacteria, large beta-barrel proteins in the outer membrane that use TonB/ExbBD energy transduction to import siderophore-metal complexes and heme.
- FpvA: Pyoverdine-Fe receptor in P. aeruginosa [braud 2010 siderophores pseudomonas metal tolerance].
- FptA: Pyochelin-Fe receptor in P. aeruginosa [braud 2010 siderophores pseudomonas metal tolerance].
- FrpB4: TonB-dependent receptor proposed for nickel transport in some species.
Heme Uptake Systems
Dedicated machinery to capture host hemoglobin/heme as an iron source.
- Isd system (S. aureus): IsdB captures hemoglobin on the cell surface, passes heme through the cell wall (IsdC) and membrane (IsdDEF) into the cytoplasm, where IsdG/IsdI degrade heme to release iron. Heme is the preferred iron source during infection. S. aureus hemolysins actively lyse red blood cells to liberate hemoglobin [cassat 2012 metal acquisition staphylococcus aureus].
- Shp/Shr system (Streptococci): Heme relay system; Shr is the surface receptor, Shp the chaperone [akbari 2022 metal homeostasis streptococci].
- 22 kDa and 37 kDa proteins (S. pneumoniae): The first identified hemoglobin/heme-binding membrane proteins in pneumococcus. Both share the KVAFDH motif essential for heme binding. S. pneumoniae can use Hb and heme but NOT transferrin or lactoferrin as iron sources [romero espejel 2013 streptococcus pneumoniae iron].
Storage Systems
Nickel Storage
Best characterized in helicobacter pylori:
- Hpn: Extraordinary small His-rich protein -- 47% of residues are histidine. Forms 20-mers, each monomer binding 5 Ni(II) ions. Present in all gastric Helicobacter species. Functions as the primary nickel reservoir, buffering against fluctuations in nickel availability [maier 2019 nickel microbial pathogenesis].
- HpnI (Hpn-like): 25% histidine, binds 2 Ni(II) per monomer. Restricted to H. pylori and H. acinonychis. Competes with Hpn for nickel under low-nickel conditions.
- Recent work reveals Hpn/HpnI interact with a much wider array of proteins than expected, including urease/hydrogenase maturation enzymes (delivering nickel to these virulence factors), AmiE (aliphatic amidase), and PepA (aminopeptidase). They function as central nickel distribution hubs in the cell [maier 2019 nickel microbial pathogenesis].
- HspA: A GroES (chaperonin) homolog with a unique His-rich C-terminus for nickel binding. Dual function: protein folding chaperone and nickel storage. Candidate for anti-H. pylori vaccine [maier 2019 nickel microbial pathogenesis].
Iron Storage
- Ferritins: Ubiquitous iron storage cages (24 subunits, up to 4,500 Fe atoms per cage). Found across bacterial phyla.
- Bacterioferritins: Bacterial-specific ferritin homologs with a heme cofactor. Protect against iron-mediated Fenton chemistry by sequestering free iron.
- Dps (DNA-binding protein from starved cells): Miniferritin (12 subunits); protects DNA from oxidative damage by sequestering Fe2+ and preventing Fenton reaction.
Zinc/Other Metal Storage
- Pht proteins (Streptococci): Polyhistidine triad proteins that bind zinc and serve as extracellular zinc reservoirs/trafficking proteins [akbari 2022 metal homeostasis streptococci].
- Metallothionein-like proteins: Small cysteine-rich proteins that bind multiple metals; found in some pathogenic bacteria.
Regulation: Metal-Sensing Transcription Factors
Pathogens use exquisitely sensitive metal-responsive regulators to match transporter expression to metal availability.
NikR (Nickel-Responsive)
- Ni2+-sensing transcriptional regulator. In H. pylori, NikR both activates and represses genes depending on nickel levels.
- At high Ni: activates urease (ureA) expression, nickel storage (hpn), and nickel efflux (cznABC).
- At low Ni: depresses import systems.
- NikR is essential for balancing nickel acquisition with toxicity avoidance [maier 2019 nickel microbial pathogenesis].
Fur (Ferric Uptake Regulator)
- The master Fe2+-responsive regulator in most bacteria. When iron is sufficient, Fe-Fur represses siderophore biosynthesis and iron import genes. When iron is scarce, derepression allows maximal iron acquisition.
- Also regulates virulence factors, acid resistance, and oxidative stress defense.
- Fur homologs control manganese (Mur) and zinc (Zur) in some species.
Zur (Zinc Uptake Regulator)
- Zn2+-responsive repressor. Represses zinc import (adcABC) when zinc is sufficient.
- In Streptococci, Zur also influences Pht protein expression and zinc trafficking [akbari 2022 metal homeostasis streptococci].
MntR (Manganese Transport Regulator)
- Mn2+-responsive activator/repressor. Coordinates manganese import with export to maintain homeostasis.
- In S. pneumoniae, MntR and the manganese-sensing SczA regulate the balance between Mn import (MntABC) and Mn efflux (MntE) [akbari 2022 metal homeostasis streptococci].
CadR/MerR-Type Regulators
- CadR (A. baumannii): Highly attuned cadmium sensor; activates czcE expression ~480-fold upon Cd exposure [alquethamy 2021 acinetobacter cadmium resistance].
- CopY (Streptococci): Cu-responsive repressor controlling copper efflux via CopA [akbari 2022 metal homeostasis streptococci].
Export and Detoxification Systems
When metals are too abundant (from host copper killing, environmental contamination, or mis-metallation), pathogens must export them.
CDF (Cation Diffusion Facilitator) Family
- CzcD (Streptococci): Zinc exporter [akbari 2022 metal homeostasis streptococci].
- CzcE (A. baumannii): Primary cadmium exporter; translocates Cd from cytoplasm to periplasm. Mutants are 30-fold more sensitive to cadmium [alquethamy 2021 acinetobacter cadmium resistance].
- MntE (Streptococci): Manganese exporter preventing Mn toxicity [akbari 2022 metal homeostasis streptococci].
HME (Heavy Metal Efflux) RND Systems
- CzcCBA (A. baumannii, H. pylori): Three-component system spanning inner membrane, periplasm, and outer membrane. Exports Cd, Zn, and Ni from periplasm to extracellular space, completing a two-step translocation (CDF: cytoplasm-->periplasm, HME: periplasm-->exterior) [alquethamy 2021 acinetobacter cadmium resistance].
- CznABC (H. pylori): Cadmium, zinc, and nickel efflux. Critical for surviving in the metal-variable gastric environment [maier 2019 nickel microbial pathogenesis].
P-Type ATPases
- CopA (Streptococci): Cu-exporting P-type ATPase; essential for surviving host copper toxicity in phagosomes [akbari 2022 metal homeostasis streptococci].
- PmtA (S. pyogenes, S. suis): Iron-exporting ATPase [akbari 2022 metal homeostasis streptococci].
- ZccE (S. mutans): Unique zinc-exporting P-type ATPase [akbari 2022 metal homeostasis streptococci].
- P-type ATPases are also upregulated in Enterococcus under cadmium stress for Cd export [cheng 2021 cadmium enterococcus metabolic].
Metal Efflux as Counter to Host Toxicity
- The host deliberately floods phagolysosomes with copper and zinc to kill engulfed bacteria. Pathogen copper/zinc efflux systems (CopA, CzcD, CznABC) are therefore virulence factors -- enabling survival of the host's metal intoxication strategy [akbari 2022 metal homeostasis streptococci], [cassat 2012 metal acquisition staphylococcus aureus].
- PMI1518 (P. mirabilis): Nickel efflux system essential for catheter-associated UTI, preventing nickel toxicity in the urinary niche [maier 2019 nickel microbial pathogenesis].
The Arms Race: Host vs. Pathogen
The interplay between host metal restriction and pathogen metal acquisition determines infection outcome:
| Host Strategy | Mechanism | Pathogen Counter-Strategy |
|---|---|---|
| Calprotectin | Sequesters Mn, Zn, Ni at infection sites | Redundant high-affinity ABC transporters (Adc, Mnt, Nik) |
| Lactoferrin | Binds Fe (and possibly Ni) in mucosal secretions | Siderophores, heme uptake systems (Isd, Shp/Shr) |
| Hepcidin | Degrades ferroportin; reduces iron export to plasma | Heme uptake from hemoglobin; siderophores bypass transferrin |
| NRAMP1 | Exports Fe, Mn, Ni from phagolysosomes | MntH (NRAMP homolog); intracellular metal storage |
| Transferrin/hemopexin | Bind free Fe/heme in circulation | Surface receptors for Hb/heme (IsdB, 22/37 kDa proteins) |
| Cu/Zn intoxication | Flood phagolysosomes with toxic Cu/Zn | CopA, CzcD, CznABC efflux systems |
The environmental dimension: When dietary or environmental metal exposure exceeds the host's sequestration capacity (e.g., nickel from soy formula in preterm infants [pendergrass 2026 nickel nec preterm gut]), the pathogen's acquisition systems become less important -- metals are freely available. The arms race shifts decisively in the pathogen's favor.
Connections
- metal dependent virulence -- the virulence factors these acquisition systems supply metals to
- siderophores metallophores -- the extracellular chelators that feed into these import systems
- inter kingdom metal shielding -- community-level metal dynamics modulating individual acquisition
- nutritional immunity -- the host defense these systems are designed to overcome
- iron -- the most contested metal; largest diversity of acquisition systems
- nickel -- unique pathogen-specific requirement; mammals have no Ni-enzymes
- zinc -- both a nutrient to acquire and a host weapon to resist
- gut metal microbiome -- metal acquisition determines competitive outcomes in the gut
- environmental metal exposure -- when environmental input overwhelms the arms race