Helicobacter Pylori

A gastric pathogen that is arguably the most nickel-dependent human pathogen known. Two of its key virulence factors — urease and [NiFe] hydrogenase — require nickel, and the bacterium has evolved an elaborate nickel trafficking, storage, and regulation system to support them.

Nickel-Dependent Virulence Factors

Urease

  • Up to 10% of total proteome.
  • Essential for in vivo survival: hydrolyzes urea → ammonia + bicarbonate, buffering cytoplasmic pH to near-neutral in the acidic microenvironment of the stomach.
  • Roles beyond acid neutralization [1]:
  • Required for persistence even at neutral gastric pH.
  • Promotes angiogenesis.
  • Stimulates pro-inflammatory cytokines (neutrophil/monocyte chemotaxis).
  • Binds Class II MHC on gastric epithelial cells → induces apoptosis.
  • Disrupts epithelial tight junctions (via ammonia production → myosin activation).
  • Activates blood platelets (lipoxygenase-mediated pathway).
  • Alters mucin gene expression.
  • Holo-urease (Ni-bound): catalytic urea hydrolysis + non-catalytic oxidant quenching (Met/Met-sulfoxide cycle with MSR repair).
  • Apo-urease (Ni-free): retains only oxidant-quenching activity.
  • Only 2-25% of urease is actually nickel-activated; the rest may serve the antioxidant role.

[NiFe] Hydrogenase

  • Single H₂-uptake type (hydABCDE operon).
  • H₂ is chronically available in the stomach (dissolved H₂ ~80 μM; enzyme Km ~1.8 μM — always saturated).
  • Powers CagA translocation: the carcinogenic effector. Hydrogenase deletion mutants cannot translocate CagA and do not induce gastric cancer in gerbils.
  • Enables H₂-stimulated CO₂ fixation (mixotrophy) — a growth mode never before described in a human pathogen.
  • Strains from cancer patients have higher hydrogenase activity than gastritis-only strains.

Nickel Trafficking System

Transport

  • NiuBDE (ABC-type): the only transporter operating at both acidic and neutral pH. Can also transport cobalt/bismuth (relevant to bismuth-based eradication therapy).
  • NixA (NiCoT-type): secondary, Ni-only. Required in vivo (nixA mutants cannot colonize mouse stomachs) but nixA mutants retain some colonization in other models.
  • TonB-dependent FrpB4 for outer membrane transport.

Storage

  • Hpn: 47% histidine, 20-mer binding 5 Ni(II)/monomer. Present in all gastric Helicobacter. Primary nickel reservoir.
  • HpnI (Hpn-like): 25% histidine. Restricted to H. pylori and H. acinonychis.
  • Both compete for nickel under limiting conditions.
  • Recent discovery: storage proteins interact with a wide array of metabolic proteins — AmiE (aliphatic amidase), PepA (aminopeptidase), and maturation proteins. Suggests a central hub role in nickel metabolism far beyond simple storage.

Regulation

  • NikR: nickel-responsive transcriptional regulator controlling urease and hydrogenase expression.
  • Fur: iron-responsive regulator that also influences hyd gene expression.

Export

  • CznABC: cobalt-zinc-nickel efflux pump. Critical for nickel homeostasis and in vivo colonization.

Clinical Significance

  • Infects ~50% of the global population.
  • Causes gastritis, peptic ulcers, gastric adenocarcinoma, MALT lymphoma.
  • Eradication therapy often includes bismuth — which competes with nickel transport via NiuBDE [1].
  • HspA (GroES homolog with His-rich Ni-binding C-terminus) has been explored as a vaccine candidate — partial protection in mice via intranasal administration.

Connections

  • metal dependent virulence — Ni-urease and [NiFe]-hydrogenase are the paradigmatic metal-dependent virulence factors
  • nickel — essential cofactor for its two main virulence factors
  • nutritional immunity — host calprotectin/lactoferrin may restrict nickel availability
  • metal carcinogenesis — H. pylori-mediated gastric cancer is linked to hydrogenase-powered CagA translocation
  • gastric cancer — H. pylori is the primary causative organism; nickel-dependent metalloenzymes power the cancer cascade
  • gerd — controversial relationship; H. pylori eradication may worsen reflux in some patients [2] [3] [4]
  • Contrast with [5]: nickel causes cancer in host cells via epigenetics, while in H. pylori it enables cancer via CagA

References (7)

  1. ★Robert J. Maier, Stéphane L. Benoit (2019). Role of Nickel in Microbial Pathogenesis. Inorganics. doi:10.3390/inorganics7070080
  2. Chen J, Zhang J, Ma X et al. (2023). Causal relationship between Helicobacter pylori antibodies and gastroesophageal reflux disease (GERD): A mendelian study. PLoS ONE. doi:10.1371/journal.pone.0294771
  3. Sugihartono T, Fauzia KA, Miftahussurur M et al. (2022). Analysis of gastric microbiota and Helicobacter pylori infection in gastroesophageal reflux disease. Gut Pathogens. doi:10.1186/s13099-022-00510-3
  4. Liang T, Liu F, Liu L et al. (2021). Effects of Helicobacter pylori Infection on the Oral Microbiota of Reflux Esophagitis Patients. Frontiers in Cellular and Infection Microbiology. doi:10.3389/fcimb.2021.732613
  5. Konstantin Salnikov, Anatoly Zhitkovich (2008). Genetic and Epigenetic Mechanisms in Metal Carcinogenesis and Cocarcinogenesis: Nickel, Arsenic, and Chromium. Chemical Research in Toxicology. doi:10.1021/tx700198a
  6. Maurya AP, Rajkumari J, Bhattacharjee A et al. (2020). Development, spread and persistence of antibiotic resistance genes (ARGs) in the soil microbiomes through co-selection. Reviews on Environmental Health. doi:10.1515/reveh-2020-0035
  7. Campanale M, Nucera E, Ojetti V et al. (2014). Nickel Free-Diet Enhances the Helicobacter pylori Eradication Rate: A Pilot Study. Digestive Diseases and Sciences. doi:10.1007/s10620-014-3060-3

Related Articles