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 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 [maier 2019 nickel microbial pathogenesis]:
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 [maier 2019 nickel microbial pathogenesis].
- 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