Bacteroidota

Bacteroidota (formerly Bacteroidetes) is a major phylum of rod-shaped, non-spore-forming Gram-negative (diderm) bacteria grouped with Chlorobiota (green sulfur bacteria) and Fibrobacterota inside the FCB superphylum. Comprising more than 7,000 described species across anaerobic, aerobic, and halophilic lineages, Bacteroidota serves as the premier complex-carbohydrate degradation engine across two global habitats:

1. The Mammalian Distal Gut Microbiome (Order Bacteroidales): Obligate anaerobic genera—principally Bacteroides (B. thetaiotaomicron, B. fragilis, B. vulgatus), Phocaeicola, Prevotella (Segatella copri), Parabacteroides, and Alistipes—constitute 20–50% of the healthy adult human colonic microbiota, fermenting dietary plant cell-wall polysaccharides and host mucosal O-glycans that human digestive enzymes cannot cleave.
2. Marine, Freshwater, and Soil Biopolymer Cycling (Classes Flavobacteriia, Cytophagia, and Chitinophagia): Aerobic marine and terrestrial degraders (Flavobacterium, Cytophaga hutchinsonii, Zobellia galactanivorans) that mineralize refractory algal laminarin, alginate, fucoidan, carrageenan, cellulose, and arthropod chitin following marine phytoplankton blooms.

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Phylum Bacteroidota (FCB Superphylum) ├── Class Bacteroidia (Strictly anaerobic gut & oral symbionts/pathogens) │ └── Order Bacteroidales │ ├── Bacteroidaceae (Bacteroides thetaiotaomicron, B. fragilis, B. ovatus, Phocaeicola vulgatus) │ ├── Prevotellaceae (Prevotella / Segatella copri, Prevotella melaninogenica: plant-rich diet & rumen fermenters) │ ├── Porphyromonadaceae (Porphyromonas gingivalis: keystone periodontal pathogen) │ ├── Tannerellaceae (Parabacteroides distasonis, Tannerella forsythia) │ └── Rikenellaceae & Odoribacteraceae (Alistipes, Odoribacter: bile-acid & succinate metabolizers) │ ├── Class Flavobacteriia (Predominantly aerobic marine, freshwater, and fish-associated rods) │ └── Order Flavobacteriales │ ├── Flavobacteriaceae (Flavobacterium psychrophilum, Zobellia galactanivorans, Polaribacter, Capnocytophaga) │ └── Blattabacteriaceae (Blattabacterium cuenoti: obligate uric-acid-recycling cockroach endosymbiont) │ ├── Class Cytophagia (Aerobic gliding soil and marine cellulose/biopolymer degraders) │ └── Order Cytophagales (Cytophaga hutchinsonii: contact-dependent crystalline cellulose digestion; Hymenobacter) │ └── Class Chitinophagia & Sphingobacteriia (Chitinophaga, Sphingobacterium, Pedobacter: soil chitin & sphingolipid producers) *`

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While the human genome encodes only 17 digestive glycoside hydrolases capable of cleaving starch, sucrose, and lactose (leaving dietary pectins, xylans, arabinogalactans, β-glucans, and resistant starches completely intact), the 6.26-Mb genome of the human gut symbiont Bacteroides thetaiotaomicron encodes more than 260 glycoside hydrolases, polysaccharide lyases, and carbohydrate esterases organized into 88 physically linked gene clusters called Polysaccharide Utilization Loci (PULs).

Rather than secreting soluble enzymes into the colonic lumen—where liberated simple sugars would be stolen by competing bacteria ("public goods" cheating)—Bacteroidota capture and import oligosaccharides selectively into their own periplasm using the Starch Utilization System (Sus) paradigm:

1. Surface Glycan Binding (SusD, SusE, SusF): Outer-membrane lipoproteins anchored to the extracellular surface (SusD homologs and accessory surface glycan-binding proteins) bind specific helical or branched conformations of a target dietary polysaccharide (such as amylose, xyloglucan, or rhamnogalacturonan-II—the most structurally complex polysaccharide in nature, containing 21 distinct glycosidic linkages, all cleaved by a single B. thetaiotaomicron PUL).
2. Outer-Membrane Endo-Cleavage (SusG): A surface-exposed endo-acting glycoside hydrolase (SusG) snips the long polymer into mid-sized oligosaccharides (4 to 15 sugar units).
3. TonB-Dependent "Pedal-Bin / Mousetrap" Import (SusC): Each SusD lipoprotein sits like a hinged lid atop a 22-stranded β-barrel TonB-dependent transporter (SusC). When an oligosaccharide binds at the SusC–SusD interface, the SusD lid closes over the barrel and TonB-transduced proton-motive force from the inner membrane pulls the oligosaccharide into the periplasmic space.
4. Periplasmic Depolymerization and Hybrid Two-Component Sensing (SusA, SusB, SusR): Inside the sheltered periplasm, exo-acting glycosidases (SusA, SusB) hydrolyze the oligosaccharide to monosaccharides while a minimal diagnostic disaccharide or trisaccharide binds the periplasmic sensor domain of a Hybrid Two-Component System (HTCS) or ECF-σ / anti-σ factor transmembrane regulator, upregulating transcription of that specific PUL by 100- to 1,000-fold within minutes.
5. Dietary-to-Mucin Foraging Switch: When dietary plant fiber is absent (as during fasting or low-fiber Western diets), B. thetaiotaomicron represses its plant-glycan PULs and activates host-mucin O-glycan PULs (sulfatases, α-fucosidases, sialidases), grazing on the outer mucus layer (MUC2 glycoprotein) of the colonic epithelium. Liberated L-fucose also serves as an inter-kingdom signaling molecule and cross-feeds adjacent gut commensals.

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Bacteroidota exhibit three distinctive biochemical adaptations for lifelong persistence inside the mammalian immune environment:

• De Novo Bacterial Sphingolipid Synthesis: Unlike almost all other bacteria (which lack sphingolipids), Bacteroides, Prevotella, Porphyromonas, and Sphingobacterium express serine palmitoyltransferase (SPT) to synthesize odd-chain, branched ceramides, dihydroceramide phosphoethanolamines, and α-galactosylceramides (BfaGCs). Bacterial sphingolipids Delivered via outer-membrane vesicles (OMVs) cross the intestinal epithelium to modulate host hepatic lipid metabolism and tune colonic invariant natural killer T (iNKT) cell proliferation early in neonatal life.
• Invertible DNA Promoters and Eight Phase-Variable Capsules: Bacteroides fragilis encodes eight distinct capsular polysaccharide biosynthesis loci (PSA through PSH). Seven of these loci are controlled by reversible DNA inversions of ~200-bp promoter segments catalyzed by a site-specific serine recombinase (Mpi), continuously shuffling surface antigenic combinations to evade secretory IgA and predatory crAssphage bacteriophages.
• Zwitterionic Polysaccharide A (PSA) and Regulatory T-Cell Induction: Polysaccharide A (PSA) of B. fragilis is a unique zwitterionic polysaccharide carrying alternating positively charged free amino groups and negatively charged carboxyl/phosphate groups in every repeating tetrasaccharide unit. Processed via MHC Class II on intestinal dendritic cells and recognized by TLR2, PSA stimulates naive CD4⁺ T cells to differentiate into interleukin-10 (IL-10)-producing Foxp3⁺ regulatory T cells (T_regs), suppressing pathogenic Th17 inflammation.
• Endotoxin Stealth (Penta-Acylated, Monophosphorylated Lipid A): Whereas the hexa-acylated, diphosphorylated Lipid A of Pseudomonadota (E. coli) is a potent agonist of human TLR4, Bacteroides synthesizes an under-acylated (tetra- or penta-acylated), monophosphorylated Lipid A (generated by the Lipid A 1-phosphatase LpxF, which also confers extreme resistance to cationic antimicrobial peptides such as polymyxin B). This modified LPS is 1,000-fold less inflammatory at TLR4, permitting trillions of Bacteroides cells to reside peacefully in the human colon without triggering chronic sepsis.

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Members of Bacteroidota completely lack bacterial flagella and Type III/IV pili; instead, classes Flavobacteriia, Cytophagia, and Bacteroidia evolved a phylum-specific protein export and motility machine—the Type IX Secretion System (T9SS):

1. Rotary SprB Adhesin Gliding (Flavobacterium johnsoniae): In aerobic Flavobacterium and Cytophaga, the T9SS works in concert with a proton-motive-force-driven rotary motor (GldLM) that propels cell-surface adhesins (SprB) along a closed helical track on the outer surface of the outer membrane at speeds of 2 to 4 μm/s, allowing the rod-shaped cell to glide smoothly and rotate over solid chitin and cellulose fibers.
2. Gingipain Secretion in Porphyromonas gingivalis: In the oral cavity, the anaerobic pathogen Porphyromonas gingivalis uses its T9SS to secrete and covalently attach (via an anionic lipopolysaccharide anchor, A-LPS) massive quantities of cysteine proteases called gingipains (RgpA, RgpB [arginine-specific], and Kgp [lysine-specific]). Gingipains cleave host complement components (C3 and C5, generating C5a to subvert neutrophil clearance while destroying bactericidal activity), degrade gingival collagen, and scavenge heme/iron from hemoglobin, driving dysbiotic chronic periodontitis.

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1. Xu, J., et al. (2003). A genomic view of the human-Bacteroides thetaiotaomicron symbiosis. Science, 299(5615), 2074–2079. Google Scholar
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• Microbiome
• Bacillota
• Pseudomonadota
• Fermentation
• Innate Immunity