bacterial-type flagellum

GO:0009288 · APPENDAGE · APPENDAGE · PROPOSED

The rotary motility organelle of bacteria: a long helical filament of flagellin joined by a flexible hook to a basal body that spans the cell envelope. The basal body houses a rotor (MS and C rings) driven by membrane-embedded stator units that convert ion flow (H+ or Na+) into torque; the flagellar type III export apparatus at its base secretes the axial subunits. Not homologous to the archaeal archaellum or the eukaryotic cilium. [DOI:10.1146/annurev.biochem.72.121801.161737]

Transmission electron micrograph (pseudoreplica technique) of an Escherichia coli O157:H7 cell with peritrichous flagella. TEM · *Escherichia coli O157:H7* (`NCBITaxon:83334`) · E. H. White, Content Provider: Peggy S. Hayes; Public domain, via Wikimedia Commons · PUBLIC_DOMAIN · source: WIKIMEDIA_COMMONS File:Escherichia_coli_flagella_TEM.png (M4617695) ·

Synonyms: bacterial flagellum

Has part: GO:0009420 GO:0009424 GO:0009425

Xrefs: uniprot.location:SL-0307

Components

Component	Type	Genes	Stoichiometry	Essential	Role
flagellin (FliC)	PROTEIN	fliC, fljB	~20,000 subunits per filament; 11 protofilaments	ESSENTIAL	Polymerises into the helical propeller filament. `DOI:10.1146/annurev.micro.57.030502.090832` Macnab 2003 reviews filament assembly from FliC exported through the central channel and capped by FliD.
hook protein (FlgE)	PROTEIN	flgE		ESSENTIAL	Flexible universal joint transmitting torque from the basal body to the filament. `DOI:10.1146/annurev.micro.57.030502.090832` Hook length is controlled by FliK during assembly.
MS ring (FliF)	PROTEIN_COMPLEX	fliF		ESSENTIAL	Membrane-embedded rotor ring; the platform on which the basal body assembles. `DOI:10.1146/annurev.micro.57.030502.090832` The MS ring is the first substructure assembled.
C ring / switch complex (FliG, FliM, FliN)	PROTEIN_COMPLEX	fliG, fliM, fliN		ESSENTIAL	Cytoplasmic rotor ring; FliG engages the stator and the complex sets rotational direction in response to CheY-P. `DOI:10.1016/j.tim.2014.12.011` Minamino & Imada 2015 describe the rotor–stator interface at FliG.
stator unit (MotA5MotB2)	PROTEIN_COMPLEX	motA, motB		ESSENTIAL	Ion-driven torque generator anchored to peptidoglycan through MotB. `DOI:10.1016/j.cell.2020.08.016` Santiveri et al. 2020 resolve the 5:2 MotA:MotB stoichiometry and show the unit rotates. `DOI:10.1038/s41564-020-0788-8` Deme et al. 2020 report the same 5:2 architecture independently.
rod (FlgB, FlgC, FlgF, FlgG)	PROTEIN_COMPLEX	flgB, flgC, flgF, flgG		ESSENTIAL	Drive shaft spanning the periplasm from MS ring to hook. `DOI:10.1146/annurev.micro.57.030502.090832` Rod subunits are exported by the fT3SS and polymerise in order.
L and P rings (FlgH, FlgI)	PROTEIN_COMPLEX	flgH, flgI		CONDITIONAL	Bushing through the outer membrane and peptidoglycan in Gram-negative bacteria; absent in Gram-positives. `DOI:10.1146/annurev.micro.57.030502.090832` The L and P rings are not rotor components but bearings.
flagellar type III export apparatus (FlhA, FlhB, FliP, FliQ, FliR)	PROTEIN_COMPLEX	flhA, flhB, fliP, fliQ, fliR		ESSENTIAL	Secretes rod, hook and filament subunits through the growing structure. `DOI:10.1146/annurev.micro.57.030502.090832` Homologous to the injectisome export gate.

Taxonomic distribution

NCBITaxon:2 Bacteria — COMMON: Widespread but repeatedly lost; some lineages (spirochaetes) keep the flagella periplasmic. [DOI:10.1038/nrmicro1887]
NCBITaxon:2157 Archaea — ABSENT: Archaea swim with the archaellum, a type-IV-pilus-related structure with no homology to the bacterial flagellum. [DOI:10.3389/fmicb.2015.00023]
NCBITaxon:83334 Escherichia coli O157:H7 — UNIVERSAL: Peritrichously flagellated; the strain in the hosted micrograph. [DOI:10.1146/annurev.biochem.72.121801.161737]

Canonical examples

NCBITaxon:90371 Salmonella enterica subsp. enterica serovar Typhimurium — The genetic model for flagellar assembly and export. [DOI:10.1146/annurev.micro.57.030502.090832]
NCBITaxon:83333 Escherichia coli K-12 — Model for motor physics and chemotaxis. [DOI:10.1146/annurev.biochem.72.121801.161737]

Functions

bacterial-type flagellum-dependent cell motility GO:0071973
- DOI:10.1146/annurev.biochem.72.121801.161737 Berg 2003 reviews the rotary motor and swimming.

Associated traits

CONFERS METPO:1000704 flagellated
CONFERS METPO:1000702 motile — One of several motility machines; gliding and twitching motility do not use it.

Physical properties

DIAMETER: ~20 UO:0000018 (filament) [DOI:10.1146/annurev.micro.57.030502.090832]
LENGTH: 5-20 UO:0000017 (filament) [DOI:10.1146/annurev.micro.57.030502.090832]
ROTATION_SPEED: ~100-300 UO:0000106 (Escherichia coli motor, proton-driven) [DOI:10.1146/annurev.biochem.72.121801.161737]

Mechanism graphs

Ion flow through the stator turns the rotor and the filament (FUNCTION)

Subject	Predicate	Object	Evidence
proton motive force	powers	MotA5MotB2 stator unit	`DOI:10.1016/j.cell.2020.08.016` Santiveri et al. 2020 propose ion flow rotates MotA around the MotB dimer.
MotA5MotB2 stator unit	applies torque to	C ring (FliG)	`DOI:10.1038/s41564-020-0788-8` Deme et al. 2020 model the stator as a gear meshing with FliG.
C ring (FliG)	drives	flagellar rotation	`DOI:10.1016/j.tim.2014.12.011` The C ring, MS ring and rod rotate as one rigid rotor.
flagellar rotation	propels	flagellar filament	`DOI:10.1146/annurev.biochem.72.121801.161737` Rotation of the helical filament produces thrust.
flagellar filament	enables `RO:0002327`	bacterial-type flagellum-dependent cell motility	`DOI:10.1146/annurev.biochem.72.121801.161737` Berg 2003.

Discussions

CURATION_TODO (OPEN): Assign InterPro family groundings to each protein component (flagellin, FlgE, FliF, FliG/M/N, MotA/MotB, rod, FlgH/I, fT3SS). Groundings were deliberately left blank rather than guessed; verify each accession against InterPro before adding.

Evidence

DOI:10.1146/annurev.biochem.72.121801.161737 Berg 2003, "The rotary motor of bacterial flagella".
DOI:10.1146/annurev.micro.57.030502.090832 Macnab 2003, "How bacteria assemble flagella".
DOI:10.1038/nrmicro1887 Chevance & Hughes 2008, "Coordinating assembly of a bacterial macromolecular machine".

Curation history

2026-08-29T00:00:00Z claude · CREATED_RECORD — Drafted composition, distribution, trait links and torque-generation mechanism graph from the cited reviews and stator structures. Not yet human-reviewed.
2026-08-29T03:00:00Z claude · ADD_IMAGE — Added Commons image File:Escherichia_coli_flagella_TEM.png (public domain, CDC PHIL, NCBITaxon:83334); no publication, per decision #30. Added E. coli O157:H7 to taxonomic_distribution as the imaged strain.
2026-08-30T02:14:17Z uniprot_sl · ADD_UNIPROT_SL_XREF — Added uniprot.location:SL-0307 (Bacterial flagellum); mapping read from the GO line of UniProt subcell.txt.
2026-08-30T19:08:55Z fetch_commons_image · ADD_IMAGE — Updated Commons image File:Escherichia_coli_flagella_TEM.png (PUBLIC_DOMAIN, Escherichia coli O157:H7); licence, attribution and taxon read from the Commons and Wikidata APIs at retrieval.
2026-08-30T19:11:12Z fetch_commons_image · ADD_IMAGE — Updated Commons image File:Escherichia_coli_flagella_TEM.png (PUBLIC_DOMAIN, Escherichia coli O157:H7); licence, attribution and taxon read from the Commons and Wikidata APIs at retrieval.

Source: data/structures/appendage/bacterial_type_flagellum.yaml