Overview
Bacteroidetes (recently reclassified as Bacteroidota) is one of the two dominant bacterial phyla in the human gut, together with firmicutes typically comprising >90% of the intestinal microbiota. Bacteroidetes are Gram-negative, obligately anaerobic, non-spore-forming rods characterized by their extraordinary capacity for complex polysaccharide degradation. They encode some of the largest repertoires of carbohydrate-active enzymes (CAZymes) in the gut microbiome, enabling them to break down dietary fiber, host mucins, and other glycans that the human genome cannot digest.
The phylum's abundance relative to Firmicutes — the Firmicutes/Bacteroidetes (F/B) ratio — has been one of the most widely reported microbiome metrics in disease research, though its utility is now understood to be limited by the functional diversity within each phylum.
Key Genera with WikiBiome Entity Pages
| Genus | Notable Species | Primary Function |
|---|---|---|
| bacteroides fragilis | B. fragilis | Polysaccharide metabolism; BFT toxin producer; Zn-dependent metalloprotease |
| bacteroides thetaiotaomicron | B. thetaiotaomicron | Premier glycan degrader; starch utilization system (Sus) |
| bacteroides vulgatus | B. vulgatus | Common gut commensal; immunomodulatory |
| prevotella copri | P. copri | Plant polysaccharide degradation; enriched in plant-based diets |
| prevotella | Multiple species | Fiber fermentation; oral and gut habitats |
| alistipes | Multiple species | Bile acid metabolism; tryptophan metabolism |
| porphyromonas gingivalis | P. gingivalis | Periodontal pathogen; Mn-SOD; gingipain proteases |
| odoribacter | O. splanchnicus | Fe-S dependent anaerobic fermentation |
| parabacteroides | P. distasonis | Bile acid deconjugation; anti-inflammatory |
| butyricimonas | Multiple species | Butyrate production (unusual for Bacteroidetes) |
| alloprevotella | Multiple species | Oral and gut commensal |
Metabolic Roles
Polysaccharide Degradation
Bacteroidetes are the primary degraders of complex carbohydrates in the gut. B. thetaiotaomicron alone encodes over 260 glycoside hydrolases — more than the entire human genome. This enzymatic arsenal enables:
- Dietary fiber fermentation (resistant starch, pectin, xylan, arabinoxylan)
- Host mucin degradation (when dietary fiber is scarce)
- Release of monosaccharides that cross-feed other community members
Propionate Production
Bacteroidetes are the dominant propionate producers in the gut, primarily via the succinate pathway. Propionate has systemic effects including appetite regulation, hepatic gluconeogenesis modulation, and anti-inflammatory signaling.
Bile Acid Metabolism
Several Bacteroidetes genera (particularly alistipes and parabacteroides) participate in bile acid deconjugation and biotransformation, linking this phylum to bile acid metabolism and its downstream effects on metabolic and immune signaling.
Metal Interactions
Heavy metal exposure differentially affects Bacteroidetes abundance, creating a phylum-level signature that varies by metal:
| Metal | Effect on Bacteroidetes | Evidence |
|---|---|---|
| Cadmium | Significantly decreased | Cd selects against Bacteroidetes while enriching proteobacteria [1] |
| Arsenic | Increased | As exposure increases Bacteroidetes at phylum level [1] |
| Mercury | Increased | Hg increases Bacteroidetes [2] |
| Nickel | F/B ratio disturbed | Ni disrupts the balance; direction varies by dose [3] |
| Zinc | Dose-dependent; B:F ratio negatively related to Zn dosage (short-term) | High-dose zinc may suppress Bacteroidetes relative to Firmicutes [4] |
| Iron | Increased (supplementation) | Iron supplementation increases Bacteroidetes in African children [2] |
| Lead | Increased (both phyla) | Pb increases both Firmicutes and Bacteroidetes |
The Firmicutes/Bacteroidetes Ratio
The F/B ratio was among the first microbiome metrics to gain widespread attention (Ley et al., 2006, linking elevated F/B to obesity). It remains widely reported but is now recognized as overly simplistic — phylum-level changes obscure functionally important genus-level shifts.
| F/B Direction | Conditions |
|---|---|
| Elevated F/B (Bacteroidetes relatively depleted) | obesity, endometriosis (stages 3/4), autism spectrum disorder (some cohorts), IBS, hypertension, hashimotos thyroiditis |
| Decreased F/B (Bacteroidetes relatively enriched) | IBD, graves disease, pancreatic cancer, antidepressant treatment |
| Both phyla decline | Severe dysbiosis where proteobacteria dominate |
The ratio's clinical utility is limited because a "high F/B" could mean loss of beneficial Bacteroidetes polysaccharide degraders OR gain of pathogenic Firmicutes — two very different ecological situations requiring different interventions.
Ecological Role
In the healthy gut, Bacteroidetes occupy the mucus-adjacent niche, specializing in complex glycan degradation at the mucus-epithelium interface. Their ecological role includes:
- Primary degraders of dietary fiber, producing substrates for cross-feeding networks
- Mucin foragers when dietary fiber is scarce — a double-edged sword that can thin the mucus barrier
- Competitive exclusion of pathogens through niche occupation and bacteriocin production
- Immune education through capsular polysaccharides (PSA from B. fragilis promotes Treg differentiation)
Cross-References
- firmicutes — Partner phylum in the F/B ratio
- proteobacteria — Phylum that expands when both Firmicutes and Bacteroidetes decline
- short chain fatty acids — Bacteroidetes are major propionate producers
- bile acid metabolism — Bacteroidetes genera participate in bile acid biotransformation
- gut microbiome — Bacteroidetes as one of two dominant phyla
- cadmium — Cd significantly depletes Bacteroidetes
- dysbiosis — F/B ratio as (imperfect) dysbiosis metric