A Gram-negative intracellular pathogen causing bacillary dysentery (shigellosis), responsible for approximately 165 million illness episodes and over 1 million deaths annually, predominantly in children under 5 in developing countries. S. flexneri relies on nickel-dependent [NiFe] hydrogenases for a specific and critical virulence function: surviving the acid assault within macrophage phagolysosomes.
Nickel-Dependent Virulence
[NiFe] Hydrogenases -- Phagolysosomal Acid Combat
- Possesses H2-uptake hydrogenases Hya and Hyb that combat acid stress during intracellular survival in macrophages [1].
- Hya activity is three-fold activated within minutes of acid exposure — a rapid-response defense mechanism that provides immediate energy for maintaining cytoplasmic pH homeostasis.
- H2 oxidation generates PMF (proton motive force) that powers proton efflux pumps and ATP synthesis under the energy-limiting conditions of the phagolysosome.
- This represents a distinct use of hydrogenase compared to helicobacter pylori (gastric colonization energy) or campylobacter jejuni (intestinal mucosal competition): Shigella uses H2 metabolism specifically to survive intracellular killing.
Predicted Ni-GloI
- As a member of Enterobacteriaceae, S. flexneri is predicted to possess Ni-dependent glyoxalase for methylglyoxal detoxification during the high glycolytic flux of intracellular replication [1].
Iron Acquisition
- Produces aerobactin and enterobactin siderophores for iron scavenging.
- Iron acquisition is essential for intracellular replication within epithelial cells and macrophages.
- Host lactoferrin and transferrin restrict iron availability at the intestinal mucosal surface.
Pathogenesis
- Shigella invades colonic epithelial cells via a Type III secretion system (T3SS), then escapes the phagosome into the cytoplasm.
- Spreads cell-to-cell using actin-based motility (IcsA/VirG), avoiding extracellular immune surveillance.
- During initial uptake by macrophages, the bacterium must survive the phagolysosomal acid environment — this is where Ni-hydrogenase is critical.
- Induces massive inflammatory response with neutrophil infiltration, epithelial destruction, and bloody diarrhea.
Clinical Significance
- Bacillary dysentery: bloody diarrhea with mucus, fever, abdominal cramps. As few as 10 organisms can cause disease (extremely low infectious dose).
- Children under 5: bear the greatest burden; shigellosis is a leading cause of diarrheal death in this age group.
- Antibiotic resistance: multidrug-resistant Shigella is increasingly common; WHO lists fluoroquinolone-resistant Shigella as a priority pathogen.
- Reactive arthritis: post-infectious joint inflammation, similar to campylobacter jejuni.
- Metal exposure in endemic regions (contaminated water sources containing heavy metals) may promote Enterobacteriaceae enrichment in the gut, potentially favoring Shigella colonization [2].
Key Sources
Connections
- hydrogenase — [NiFe] Hya/Hyb for phagolysosomal acid survival
- glyoxalase — predicted Ni-GloI for metabolic stress detoxification
- nickel — essential cofactor for hydrogenase maturation
- iron — acquired via aerobactin and enterobactin
- metal dependent virulence — intracellular acid survival via Ni-hydrogenase
- nutritional immunity — host iron restriction at mucosal surfaces
- salmonella typhimurium — another intracellular Enterobacteriaceae using hydrogenase for macrophage survival
- escherichia coli — closely related; shares Ni-enzyme complement
- gut metal microbiome — metal-driven Enterobacteriaceae enrichment in endemic settings