Salmonella Enterica Serovar Typhimurium

A Gram-negative enteric pathogen that possesses four distinct [NiFe] hydrogenases — the most of any well-characterized human pathogen. These hydrogenases are critical for intracellular survival within macrophages, and a triple hydrogenase mutant is 100% avirulent in mouse typhoid models, making nickel-dependent hydrogen metabolism (via nife hydrogenase) a central pillar of Salmonella virulence.

Metal-Dependent Virulence Factors

[NiFe] Hydrogenases -- The Core of Ni-Dependent Virulence

S. Typhimurium encodes four distinct [NiFe] hydrogenases, each with different roles [1]:

  1. Hya (hydrogenase-1): membrane-bound, H2-uptake. Contributes to respiratory H2 oxidation.
  2. Hyb (hydrogenase-2): membrane-bound, H2-uptake. Most important for virulence. Primary contributor to macrophage survival and systemic infection.
  3. Hyc (hydrogenase-3): cytoplasmic, part of the formate hydrogenlyase (FHL) complex. Produces H2 under fermentative conditions. Likely important for anaerobic gut survival.
  4. Hyd (hydrogenase-4): second FHL-associated hydrogenase.

Avirulence of Hydrogenase Mutants

  • Triple mutant (DeltahyaDeltahybDeltahyd): 100% survival in a mouse typhoid fever model — completely avirulent [1].
  • This demonstrates that H2 metabolism is not merely advantageous but essential for Salmonella systemic virulence.
  • Hyb is the single most important hydrogenase: Hyb mutants alone show significant attenuation.

Hydrogenase Function in Macrophage Survival

  • After phagocytosis, S. Typhimurium resides in the Salmonella-containing vacuole (SCV).
  • H2 oxidation by Hyb provides electrons to the respiratory chain, generating PMF for ATP synthesis inside the nutrient-limited SCV.
  • This H2-powered energy source allows survival in the hostile intracellular environment where carbon sources are scarce.

Fe-Dependent Systems

  • Enterobactin: primary siderophore for iron acquisition in iron-limited host environments.
  • Salmochelin: glycosylated enterobactin derivative that evades host lipocalin-2 (which sequesters enterobactin).
  • SitABCD: Mn/Fe ABC transporter required for full virulence.
  • Fur regulon: iron-responsive regulation of virulence genes.

Metal Acquisition Systems

Nickel Import

  • NikABCDE: ABC-type nickel transporter homologous to the E. coli system.
  • Nickel import is essential for metalation of all four hydrogenase active sites.
  • Nickel import likely upregulated during intracellular infection to support hydrogenase-dependent energy generation.

Hydrogenase Maturation

  • HypABCDEF accessory proteins: required for [NiFe] active site assembly in all four hydrogenases.
  • The maturation machinery must supply nickel to four separate enzyme complexes — a significant metabolic investment reflecting the importance of H2 metabolism.

Iron Acquisition

  • Multiple redundant systems: enterobactin, salmochelin, SitABCD, FeoABC (ferrous iron).
  • Redundancy ensures iron access across diverse host niches (gut lumen, macrophage SCV, bloodstream).

Nutritional Immunity Evasion

  • Host calprotectin: released by neutrophils at gut infection sites; sequesters Zn and Mn (and likely Ni).
  • Lipocalin-2: sequesters enterobactin; S. Typhimurium evades this with salmochelin.
  • NRAMP1 (SLC11A1): macrophage phagosomal metal exporter. Restricts Fe, Mn, and Ni availability within the SCV. NRAMP1-deficient mice are highly susceptible to Salmonella, demonstrating the importance of metal restriction.
  • S. Typhimurium counters NRAMP1 by upregulating high-affinity metal transporters and relying on H2 as an alternative energy source.

Disease Associations

  • Gastroenteritis (non-typhoidal salmonellosis): most common manifestation in humans
  • Typhoid-like systemic disease in mice (model for S. Typhi typhoid fever)
  • Bacteremia in immunocompromised patients (HIV/AIDS, sickle cell)
  • Osteomyelitis (especially in sickle cell disease)
  • Reactive arthritis (post-infection complication)

Connection to Environmental Metal Exposure

  • Gut H2 is produced by commensal microbiota during fermentation of dietary fiber — dietary patterns that increase colonic H2 production could theoretically provide more energy substrate for Salmonella hydrogenases.
  • Environmental nickel in drinking water or food may increase nickel availability for hydrogenase metalation during gut colonization.
  • Agricultural use of heavy metals in livestock operations promotes metal-tolerant Salmonella populations.

Connections

  • metal dependent virulence — four [NiFe] hydrogenases; triple mutant 100% avirulent
  • nickel — essential cofactor for all four [NiFe] hydrogenases
  • iron — acquired via enterobactin/salmochelin; regulated by Fur
  • helicobacter pylori — both use [NiFe] hydrogenases for virulence, but H. pylori has one while Salmonella has four
  • nutritional immunity — NRAMP1-mediated metal export from SCV is critical host defense
  • escherichia coli — closely related; shares NikABCDE transporter architecture
  • staphylococcus aureus — both face calprotectin-mediated metal restriction

References (6)

  1. . maier 2019 nickel microbial pathogenesis
  2. . benoit 2019 nickel chelation therapy mdr enteric pathogens
  3. . mabrouk 2026 repurposing gut microbiota modulators
  4. . breton 2016 cadmium lead oral exposure colitis
  5. . mcewan 2024 metalloproteome plasticity pathogen adaptation
  6. . trecarten 2025 obesity diet microbiome prostate cancer

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