Bacterial Flagella


Bacteria propel themselves through liquid or over semisolid media using rotation of a propeller‐like organelle, the flagellum. Flagellar rotation is energised by the membrane ion gradient and flagella enable bacteria to swim towards nutrients and away from harmful substances. The flagellum is a sophisticated, molecular nanomachine. To build a flagellum, more than two dozen proteins need to assemble in an ordered process. The accurate size and subunit composition of each substructure of this nanomachine is achieved by coupling gene expression to the assembly state. Major components of the flagellum assemble outside the cytoplasmic membrane. A specialised protein export system, termed ‘type‐III secretion’ transports flagellar substrates across the inner membrane. A flexible coupling structure, the hook, connects the membrane‐embedded basal body to the rigid, extracellular filament. The length of this flexible joint is tightly controlled in Salmonella enterica and hook length is determined by intermittent secretion of a molecular ruler.

Key Concepts:

  • Bacteria swim through their environment by rotating an extracellular appendage, the flagellum.

  • Rotation of the flagellum is energised by influx of protons through stator proteins that are attached to the cell body.

  • The bacterial flagellum consists of three major parts: (1) the basal body as the engine, (2) a flexible joint structure that connects the engine to (3) the long external filament.

  • Assembly of the flagellum involves dozens of proteins and the coordination of gene expression to the assembly state of the flagellum.

  • A specialised protein export system, a type‐III secretion apparatus, is responsible for the export of flagellar secretion substrates, for example, most external parts of the flagellum.

  • Protein export via the type‐III secretion system is dependent on the proton‐motive force.

  • The length of the flexible joint structure that connects the filament to the basal body is highly regulated to a final length of 55 nm in Salmonella. A molecular ruler determines the final length and catalyses a switch in secretion specificity from early to late‐substrate secretion mode.

Keywords: rotary motor; protein export; chemiosmotic device; supramolecular assembly; molecular ruler; proton‐motive force; type‐III secretion

Figure 1.

Morphology of motile bacteria. Bar, 10 μm. Modified from Khan with permission from Elsevier.

Figure 2.

Cell cycle of Caulobacter crescentus. Modified from Maddock et al. , with permission from American Society of Microbiology

Figure 3.

Schematic representation of the flagellum of Salmonella. The MS (membrane/supramembrane), L (lipopolysaccharide) and P (periplasma) rings anchor the structure into the cell envelope. The MotAB motor‐force generators, which drive flagellar rotation, are omitted in this scheme. The C (cytoplasmic) ring complex is involved in controlling the direction of flagellar rotation and in providing an affinity site for substrate docking. The flagellar‐specific type‐III secretion apparatus at the base exports flagellar proteins. Modified from Erhardt et al. .

Figure 4.

An infrequent molecular ruler determines hook length. The molecular ruler FliK is secreted throughout the hook–basal body assembly process (a). The N‐terminus of FliK interacts with the hook subunits (FlgE), as well as the hook cap (FlgD). During a measurement of hook length by FliK, hook polymerisation temporarily halts. If the hook is too short, FliK is secreted (b) and hook polymerisation continues until another FliK molecule measures hook length again (c). When the hook has reached its physiological length of 55 nm in Salmonella, the C‐terminus of FliK is in proximity with the FlhB component of the type‐III secretion apparatus for a productive interaction that catalyses the switch in secretion specificity from rod‐hook‐type (early) to filament‐type (late) substrate secretion mode (d). Modified from Erhardt et al. .



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How to Cite close
Hughes, Kelly T, and Erhardt, Marc(Oct 2011) Bacterial Flagella. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000301.pub2]