Mediterraneibacter

A Gram-positive, obligate anaerobic genus within the family lachnospiraceae (phylum Firmicutes) that was *recently reclassified from Ruminococcus** based on phylogenomic evidence. The key species Mediterraneibacter faecis (formerly Ruminococcus gnavus* clade relatives) is a common gut commensal, but the genus has gained clinical attention through two distinct findings: its enrichment in PCOS fecal microbiota that causally transfers the PCOS phenotype via FMT, and its rapid cycling with dietary changes that mirrors LDL cholesterol fluctuations.

Taxonomy

  • Genus Mediterraneibacter, family Lachnospiraceae, order Lachnospirales (or Clostridiales in traditional schemes), class Clostridia, phylum Firmicutes.
  • Key species: M. faecis, M. gnavus (the relationship with ruminococcus gnavus requires careful disambiguation depending on the classification scheme used).
  • Named for the Mediterranean region; first characterized from Mediterranean population gut microbiomes.
  • The reclassification underscores the ongoing taxonomic revision within Lachnospiraceae, where many "Ruminococcus" species are being reassigned to new genera based on whole-genome phylogeny.

Metal Dependencies

Iron:

  • Standard iron-sulfur cluster requirements for anaerobic Lachnospiraceae metabolism.
  • Ferredoxin-dependent oxidoreductases support the fermentative energy generation characteristic of the family.

Key Enzymes and Metabolic Features

  • Bile salt hydrolase (BSH): Mediterraneibacter species can deconjugate bile salts, influencing bile acid composition and cholesterol metabolism. This function connects the genus to the gut-liver axis and cardiovascular health.
  • SCFA synthesis enzymes: As a Lachnospiraceae member, Mediterraneibacter participates in short-chain fatty acid production, though its contribution relative to other family members (Roseburia, Blautia, Coprococcus) remains to be quantified.

Ecological Role

In the Healthy Gut

Mediterraneibacter is a moderate-abundance member of the gut microbiota in individuals consuming varied diets. Its Lachnospiraceae family membership places it among the health-associated SCFA producers, though its specific functional contributions are less well characterized than those of roseburia or blautia.

Dietary Responsiveness

One of the most striking findings about Mediterraneibacter is its rapid response to dietary changes. Repeatedly adopting and abandoning a healthy dietary pattern leads to cycling in Mediterraneibacter (and collinsella) abundance, which is directly mirrored by cycling in LDL-C and total cholesterol zhang 2025 gut microbiota cvd mini review. This suggests:

  • Mediterraneibacter abundance is acutely sensitive to diet composition
  • The genus's bile salt hydrolase activity may mediate the cholesterol-microbiome link
  • The molecular benefits of a healthy diet are transient and dependent on sustained dietary habits, partly because microbial mediators like Mediterraneibacter revert quickly

Conditions Associated

Polycystic Ovary Syndrome (Enriched -- Causal Evidence)

Mediterraneibacter is enriched in PCOS fecal microbiota, and FMT from PCOS patients to germ-free mice recapitulates the full PCOS phenotype (insulin resistance, ovarian dysfunction, obese-like phenotype) huang 2024 fmt pcos metabolic ovarian dysfunction. The transferred microbiome was enriched in Mediterraneibacter, phocaeicola, oscillospiraceae, and Lawsonibacter. Whether Mediterraneibacter is a driver or passenger in PCOS pathogenesis remains to be determined.

Cardiovascular Risk

The dietary cycling data linking Mediterraneibacter to LDL-C fluctuations zhang 2025 gut microbiota cvd mini review suggests a role in cardiovascular risk modulation, though this is observational and mechanism-level evidence is limited.

Key Studies

Open Questions

  1. Is Mediterraneibacter a driver or passenger in PCOS? The FMT evidence demonstrates that the PCOS microbiota (including Mediterraneibacter) transfers the phenotype, but single-species colonization experiments would be needed to establish a direct causal role.
  2. Does the bile salt hydrolase activity of Mediterraneibacter mediate the LDL-C cycling? BSH activity alters the bile acid pool, which regulates cholesterol absorption and hepatic cholesterol synthesis.
  3. How does the reclassification affect interpretation of existing Ruminococcus literature? Many studies reporting "Ruminococcus" in disease associations may actually describe Mediterraneibacter species.

Cross-References

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