Flagella are long, whip-like structures that protrude from the cell surface and enable motility by rotating or undulating.
Structure, types and motion
Bacterial flagella consist of three main parts: a basal body embedded in the cell envelope, a curved hook and an external filament composed of the protein flagellin. The basal body functions as a rotary motor powered by the proton motive force or, in some marine species, a sodium gradient. Rotation of the filament propels the cell forward or backward; by altering rotation direction and speed, bacteria perform runs and tumbles that underlie chemotaxis. Flagella can be arranged in different patterns: monotrichous cells have a single polar flagellum, lophotrichous cells carry tufts at one or both poles, amphitrichous cells have flagella at each end and peritrichous bacteria are covered with many flagella. Spirochetes possess periplasmic flagella that wrap around the cell body, producing a corkscrew motion. In eukaryotes, flagella and closely related cilia are composed of microtubules in a 9 + 2 arrangement and move by the bending action of dynein motors rather than rotation. These differences reflect distinct evolutionary origins but similar functional roles in locomotion and fluid movement.
Diverse organisms and roles
Peritrichous flagella allow Escherichia coli to swim toward nutrients or away from harmful substances, while polar flagella give Vibrio cholerae rapid mobility in aquatic habitats and during colonization of the intestinal mucosa. Salmonella enterica expresses phase-variable flagellins that help evade host immunity; the flagellar filament also acts as the H antigen used in serotyping. Spirochetes such as Treponema pallidum and Borrelia burgdorferi use endoflagella to penetrate viscous tissues, contributing to diseases like syphilis and Lyme disease. Among eukaryotes, sperm cells depend on flagella for propulsion, and single-celled algae like Chlamydomonas use flagella to swim and sense light. Flagella also influence biofilm formation, surface attachment and secretion of virulence factors in some bacteria. Because they are antigenic and energetically costly, flagella are often tightly regulated and can be downregulated during host infection or inside biofilms.
Flagella illustrate how cellular structures are adapted for movement and sensing across the tree of life. Their varied architectures and control mechanisms enable organisms to move through complex environments and colonize new niches.
Related Terms: Motility, Chemotaxis, Pili, Cilia, Spirochete