Mycobacterium tuberculosis

[NiFe] Hydrogenase (Hyc)#

M. tuberculosis possesses a Hyc-type [NiFe] hydrogenase that is upregulated during macrophage infection.^{[1]Role of Nickel in Microbial PathogenesisRobert J. Maier, Stéphane L. Benoit · 2019Open reference 1 ↓}

Within the phagosome, conventional carbon sources are scarce. H2 oxidation provides an alternative energy source (PMF generation) for survival in this nutrient-limited intracellular niche.

The upregulation specifically during macrophage residence suggests the bacterium shifts to H2-dependent energy metabolism as part of its intracellular persistence program. Connects to the broader theme of hydrogenase-dependent intracellular survival seen in Salmonella enterica serovar Typhimurium and Shigella flexneri.

Ni-Urease#

Urease supports survival under nitrogen-limited conditions encountered during chronic infection and latency.^{[1]Role of Nickel in Microbial PathogenesisRobert J. Maier, Stéphane L. Benoit · 2019Open reference 1 ↓} Ammonia from urease provides a nitrogen source when amino acid availability is restricted within the granuloma.

Urease may also modulate phagosomal pH, though this is less well characterized than in Helicobacter pylori or Cryptococcus neoformans.

Mycobactins and Carboxymycobactins#

M. tuberculosis produces two structurally related siderophores.^{[2]Infection metallomics for critical care in the post-COVID eraPatil RH, Luptakova D, Havlicek V · 2021Open reference 2 ↓} Mycobactins: hydrophobic, cell-associated. Retain iron at the cell envelope for membrane transport.

Carboxymycobactins: hydrophilic, secreted into the extracellular environment to scavenge iron from host proteins.

Species-specific side chain variations in mycobactins enable diagnostic identification—each mycobacterial species produces characteristic mycobactin structures. Iron acquisition via mycobactins is essential for virulence; mutants lacking mycobactin biosynthesis are severely attenuated in animal models.

Host Iron Restriction#

Macrophages restrict iron availability within the phagosome via NRAMP1 (SLC11A1)-mediated iron export. NRAMP1 polymorphisms in humans are associated with susceptibility to TB—directly linking host metal transport to TB outcome. Hepcidin-mediated hypoferremia during TB infection reduces circulating iron, though this also contributes to the anemia of chronic infection.

Copper Toxicity Defense#

Macrophages deliver toxic copper concentrations into phagosomes as an antimicrobial strategy. M. tuberculosis possesses CtpV (P-type ATPase copper exporter) and MctB (outer membrane channel) for copper efflux. MymT: a copper metallothionein that sequesters cytoplasmic copper.

Copper resistance is essential for macrophage survival; CtpV mutants show increased copper sensitivity and reduced virulence.

Zinc Biology#

Host macrophages may deploy zinc intoxication as an antimicrobial strategy against intracellular mycobacteria, paralleling the zinc poisoning of streptococci.^{[3]Metal Homeostasis in Pathogenic StreptococciAkbari MS, Doran KS, Burcham LR · 2022Open reference 3 ↓}

M. tuberculosis zinc homeostasis involves multiple exporters and importers to navigate between zinc starvation and zinc toxicity within the phagosome.

Clinical Significance#

Tuberculosis: ~10 million new cases annually. Primarily pulmonary but can affect any organ (miliary TB, TB meningitis, bone TB).

Latent TB: one-quarter of the global population carries latent M. tuberculosis infection; metal homeostasis (particularly iron and nickel) likely determines the balance between latency and reactivation.

Drug-resistant TB: MDR-TB and XDR-TB are growing crises; novel drug targets including nickel (Ni)-dependent enzymes are urgently needed.

HIV co-infection: TB is the leading killer of people living with HIV. Altered metal homeostasis in HIV infection (reduced zinc, altered iron) may favor TB reactivation.

Mycobactin-based diagnostics: siderophore detection in sputum or urine could provide rapid, non-culture-based TB diagnosis.^{[2]Infection metallomics for critical care in the post-COVID eraPatil RH, Luptakova D, Havlicek V · 2021Open reference 2 ↓}