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 ([1], 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 ([2], 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 ([3], 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

  • Colorectal cancer — Enriched specifically in proximal tumors; part of the "driver-passenger" model of CRC microbial ecology ([3], cross-sectional)
  • Schizophrenia — Enriched in first-episode psychosis oral microbiome as an H2S-producing bacterium; correlates with neuroinflammation ([1], cross-sectional)
  • Chronic kidney disease — Increased with synbiotic treatment, suggesting a role in saccharolytic fermentation ([2], RCT, n=50)

Key Studies

  • [1] (cross-sectional, n=208) — Identifies Selenomonas among H2S-producing bacteria enriched in first-episode schizophrenia; correlates with neuroinflammation markers.
  • [3] (cross-sectional, n=61) — Maps Selenomonas enrichment in proximal CRC tumors with PCA separation from distal tumors.
  • [2] (RCT, n=50) — Documents unexpected Selenomonas enrichment during synbiotic-mediated metabolic shift in CKD.

Cross-References

References (7)

  1. Ying Qing, Lihua Xu, Ganqing Cui et al. (2021). Qing 2021 -- Salivary microbiome profiling reveals a dysbiotic schizophrenia-associated microbiota. npj Schizophrenia. doi:10.1038/s41537-021-00180-1
  2. Vacca, Calabrese, Nesti et al. (2023). Vacca et al. 2023 — Synbiotic Intervention in CKD Stage IIIb-IV. Frontiers in Nutrition. doi:10.3389/fnut.2023.1215836
  3. Zhiguang Gao, Bomin Guo, Renyuan Gao et al. (2015). Microbiota disbiosis is associated with colorectal cancer. Frontiers in Microbiology. doi:10.3389/fmicb.2015.00020
  4. Gong W, Jin G, Bao Y et al. (2025). Characteristics and potential diagnostic value of gut microbiota in ovarian tumor patients. Scientific Reports. doi:10.1038/s41598-025-99912-x
  5. Choi S, Chung J, Cho ML et al. (2021). Analysis of Changes in Microbiome Compositions Related to the Prognosis of Colorectal Cancer Patients Based on Tissue-Derived 16S rRNA Sequences. Journal of Translational Medicine. doi:10.1186/s12967-021-03154-0
  6. Liang T, Liu F, Liu L et al. (2021). Effects of Helicobacter pylori Infection on the Oral Microbiota of Reflux Esophagitis Patients. Frontiers in Cellular and Infection Microbiology. doi:10.3389/fcimb.2021.732613
  7. Jessica Roelands, Peter J. K. Kuppen, Eiman I. Ahmed et al. (2023). An integrated tumor, immune and microbiome atlas of colon cancer. Nature Medicine. doi:10.1038/s41591-023-02324-5

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