Bacterial Flagella

Abstract 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.