Perhaps the most recognizable
extracellular bacterial cell structures are flagella. Flagella are whip-like
structures protruding from the bacterial cell wall and are responsible for
bacterial motility (i.e. movement). The arrangement of flagella about the
bacterial cell is unique to the species observed. Common forms include:
Monotrichous - Single
flagellum
Lophotrichous - A tuft of
flagella found at one of the cell pole
Amphitrichous - Single
flagellum found at each of two opposite poles
Peritrichous - Multiple
flagella found at several locations about the cell
The bacterial flagellum
consists of three basic components: a whip-like filament, a motor complex, and
a hook that connects them. The filament is approximately 20 nm in diameter and
consists of several protofilaments, each made up of thousands of flagellin
subunits. The bundle is held together by a cap and may or may not be
encapsulated. The motor complex consists of a series of rings anchoring the
flagellum in the inner and outer membranes, followed by a proton-driven motor
that drives rotational movement in the filament.
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