Bacteroides fragilis

Structure & Biochemistry

Taxonomy	Domain Bacteria → Phylum Bacteroidota → Class Bacteroidia → Order Bacteroidales → Family Bacteroidaceae → Bacteroides fragilis
Cell morphology	Short rod, 0.8–1.3 µm × 1.6–8 µm; Gram-negative; non-motile; non-spore-forming; obligate anaerobe; encapsulated (polysaccharide capsule with 8 distinct loci, PSA–PSH)
Polysaccharide A (PSA)	Zwitterionic capsular polysaccharide; unique positively and negatively charged repeat motifs; processed by DCs and presented on MHC II to CD4+ T cells; activates TLR2 signalling; sole microbiome molecule known to rescue T-cell compartment development in germ-free mice
Fragilysin / BFT (ETBF only)	~20 kDa zinc-dependent metalloprotease encoded on a pathogenicity island (BfPAI); three isoforms (BFT-1, -2, -3); cleaves E-cadherin ectodomain (~80 kDa fragment shed), activating β-catenin/Wnt, NF-κB, MAPK, and STAT3; also cleaves fibronectin, collagen, and IL-8 precursor
Outer membrane vesicles (OMVs)	Constitutively shed; carry PSA; translocate to mesenteric lymph nodes and systemic circulation; mediate long-range immunomodulation; PSA-OMVs suppress intestinal inflammation in mouse colitis models
Polysaccharide utilisation loci (PULs)	>100 PULs in genome; enable degradation of diverse dietary polysaccharides and host glycans; major fermenters of complex dietary carbohydrates → acetate, propionate, succinate output
Antibiotic resistance	Intrinsically resistant to most penicillins (AmpC β-lactamase); metronidazole-susceptible; carbapenem-susceptible; MDRB. fragilis emerging with NDM-type carbapenemases (rare)
Genome	~5.2 Mb (NCTC 9343); ~4,578 ORFs; exceptionally large capsular polysaccharide biosynthesis gene clusters; GC content ~43%

Mechanisms of Benefit (NTBF) & Harm (ETBF)

• PSA → TLR2 → Treg induction (NTBF benefit) PSA is endocytosed by dendritic cells, processed in endosomes, and presented on MHC II. TLR2 signalling co-stimulates DC maturation. The resulting DCs preferentially induce Foxp3+ regulatory T cells (Tregs) and IL-10-producing Tr1 cells. Germ-free mice colonised with NTBF alone have fully corrected CD4+/CD8+ T-cell ratios vs uncorrected GF mice — demonstrating single-species immune system rescue.
• PSA → Th1/Th2 balance & protection from inflammatory disease PSA promotes systemic Th1 over Th2 skewing via IFN-γ induction; this protects against Th2-mediated conditions (asthma, allergic disease) in mouse models. Oral PSA administration (purified) protects against EAE (MS model), colitis (DSS model), and Helicobacter pylori-induced inflammation — positioning PSA as a drug candidate.
• Fragilysin (BFT) → E-cadherin cleavage → oncogenic signalling (ETBF harm) BFT secreted by ETBF cleaves the extracellular domain of E-cadherin on colonocytes; freed β-catenin translocates to the nucleus, activating Wnt target genes (c-Myc, cyclin D1). Concurrently, STAT3 phosphorylation (pSTAT3-Y705) drives NF-κB → IL-8, IL-6 inflammatory cascade and epithelial hyperproliferation. Both E-cadherin loss (pro-metastatic) and chronic STAT3 activation (anti-apoptotic) are hallmarks of CRC progression.
• ETBF-associated colorectal cancer initiation ETBF colonises the inner mucus layer (unlike NTBF which remains in outer layer). BFT triggers reactive oxygen species (ROS) and DNA double-strand breaks in colonocytes. In APC+/min mice, ETBF colonisation markedly accelerates polyp formation. Epidemiological studies: ETBF detected in 40–70% of CRC tissue vs 10–20% of healthy controls; ETBF-positive patients have higher Th17 infiltration and more advanced tumour stage.
• B. fragilis as opportunistic pathogen (post-surgical) B. fragilis (NTBF and ETBF) is the most common anaerobe in intra-abdominal infections (perforated appendix, colorectal surgery, peritonitis). Capsule inhibits phagocytosis; AmpC β-lactamase confers penicillin resistance. Bacteraemia from gut translocation carries ~25–30% mortality. Treated with metronidazole + β-lactam/β-lactamase inhibitor combinations.

Host–Microbiome Interactions

Clinical Associations

Condition	Strain type	Evidence	Role
Immune system development	NTBF	Germ-free mouse rescue; PSA supplementation studies	Beneficial (PSA)
Inflammatory Bowel Disease	NTBF (protective) / ETBF (harmful)	NTBF PSA OMVs suppress DSS colitis; ETBF exacerbates colitis via Th17	Strain-dependent
Colorectal Cancer	ETBF	ETBF enriched in CRC; BFT-driven β-catenin/STAT3 oncogenesis; polyp acceleration in mice	Harmful (ETBF)
Intra-abdominal infections	Both	Post-surgical bacteraemia; peritonitis; abscess (most common anaerobe isolated)	Opportunistic pathogen
Multiple Sclerosis / EAE (mouse)	NTBF (PSA)	Oral PSA suppresses EAE via IL-10+ Plasmacytoid DCs; Treg-mediated	Preclinical (beneficial)

Research & Therapeutic Status

• PSA as drug candidate for inflammatory disease Purified PSA (from NTBF) is under investigation as an immunomodulatory therapeutic. The Mazmanian lab demonstrated oral PSA cures murine colitis and EAE. Phase 1/2 safety/tolerability studies in progress for IBD. PSA-OMV nanoparticles offer alternative delivery platform with stable activity and no live-bacteria risk.
• ETBF as CRC biomarker & target ETBF detection in stool or rectal biopsy is under evaluation as CRC risk biomarker. Anti-BFT antibodies, metalloprotease inhibitors (hydroxamate class), and microbiome rebalancing strategies (antibiotic decolonisation) are early-stage approaches to reduce ETBF-driven oncogenesis.
• Antibiotic use — clinical note Metronidazole ± ampicillin-sulbactam or piperacillin-tazobactam for intra-abdominal B. fragilis infections; cefoxitin as monotherapy option. MDRB. fragilis (carbapenem-resistant) rare but increasing — surveillance required post-surgery.

Cross-Atlas Connections

References

• Mazmanian SK et al. (2005). An immunomodulatory molecule of symbiotic bacteria directs maturation of the host immune system. Cell 122(1):107–18.
• Mazmanian SK et al. (2008). A microbial symbiosis factor prevents intestinal inflammatory disease. Nature 453(7195):620–5.
• Wu S et al. (2004). A human colonic commensal promotes colon tumorigenesis via activation of T helper type 17 T cell responses. Nat Med 15(9):1016–22.
• Sears CL (2009). Enterotoxigenic Bacteroides fragilis: a rogue among symbiotes. Clin Microbiol Rev 22(2):349–69.
• Round JL, Mazmanian SK (2010). Inducible Foxp3+ regulatory T-cell development by a commensal bacterium of the intestinal microbiota. Proc Natl Acad Sci USA 107(27):12204–9.
• Bhatt AP, Redinbo MR, Bultman SJ (2017). The role of the microbiome in cancer development and therapy. CA Cancer J Clin 67(4):326–44.