Selenomonas

A genus of Gram-negative, crescent-shaped, strictly anaerobic bacteria found in both the oral cavity and the gastrointestinal tract. Selenomonas species are characterized by their distinctive morphology — a curved rod with a tuft of flagella arising from the concave side — and their metabolic versatility, producing propionate and hydrogen sulfide (H2S) among other metabolites. The genus occupies an unusual ecological position: it appears as a commensal in healthy gut and oral ecosystems but is consistently enriched in several disease states, suggesting context-dependent pathogenic behavior that may be driven by altered metal and metabolite environments.

Metal Dependencies

Selenomonas species require iron for their anaerobic metabolic machinery, including iron-sulfur cluster-dependent enzymes involved in propionate production. The genus is poorly characterized at the metallomic level compared to major pathogens — an important knowledge gap given its consistent appearance in disease-associated microbiome signatures. <!— NEEDS VERIFICATION: iron-dependent enzyme characterization in Selenomonas is extrapolated from anaerobic metabolic requirements rather than direct experimental data —>

Key Enzymes and Virulence Factors

  • Propionate production enzymes — Selenomonas contributes to the succinate/propionate pathway, producing propionic acid as a major fermentation end product
  • H2S-producing enzymes — Generation of hydrogen sulfide, a gasotransmitter that at high concentrations damages colonocytes and binds metal cofactors in host enzymes
  • Flagellar motility — Unusual lateral flagellation pattern enables movement through viscous mucosal environments

Ecological Role

In the Oral Cavity

Selenomonas is a normal resident of the human oral cavity, particularly in subgingival plaque. In schizophrenia, oral Selenomonas is among the H2S-producing bacteria enriched in patients with first-episode psychosis, correlating with neuroinflammation markers including CRP, IFN-gamma, TNF-alpha, IL-8, IL-1beta, and S100B (qing 2021 salivary microbiome dysbiotic schizophrenia, cross-sectional). H2S can bind iron, copper, and zinc in metalloenzymes, potentially contributing to mis metallation — a mechanism that links oral microbial metabolites to systemic metal homeostasis disruption.

In the Gut

In the gut, Selenomonas participates in saccharolytic fermentation. In a synbiotic intervention trial for chronic kidney disease (CKD stages IIIb-IV), Selenomonas unexpectedly increased with synbiotic treatment alongside the intended restoration of lachnospiraceae (vacca 2023 synbiotic ckd stage iiib iv, randomized-controlled-trial, n=50). This suggests Selenomonas may benefit from the saccharolytic environment created by prebiotic supplementation (FOS + inulin).

In Colorectal Cancer

Selenomonas is consistently enriched in proximal (right-sided) colorectal tumors, distinguishing them from distal tumors where Fusobacterium and Escherichia-Shigella predominate (gao 2015 microbiota disbiosis colorectal cancer, cross-sectional, n=61). This anatomical specificity suggests Selenomonas colonization is influenced by the distinct metabolic and oxygen gradients of the proximal colon.

Conditions Associated

Key Studies

Cross-References

Related Articles