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Bacterial Flagella: Structure, importance and examples of flagellated bacteria

Bacterial flagella are long, thin (about 20 nm), whip like appendages that move the bacteria towards nutrients and other attractants.  Flagella are free at one end and attached to the cell at the other end.  Flagellum can never be seen directly with the light microscope but only after staining with special flagella stains that increase their diameter. 

The long filament of flagella is composed of many subunits of a single protein, flagellin, arranged in several intertwined chains. The energy for movement, the proton motive force, is provided by ATP.

Flagellar motion in Bacterial Cells

Key Information: Most of the cocci (e.g. Staphylococci, Streptococci etc) don’t have flagella so they are non-motile.

Flagella are helical shaped structure which is composed of subunits of a protein called flagellin. The wider region at the base of the flagellum is called hook. It is different in structure than that of the filament. Hook connects filament to the motor portion of the flagellum called basal body.

Structure of the prokaryotic flagellumStructure of Bacterial Flagella

The basal body is anchored in the cytoplasmic membrane and cell wall. There are presence of rings which are surrounded by a pair of proteins called Mot. These proteins actually drive the flagellar motor causing rotation of the filament . Another set of proteins called Fli proteins function as the motor switch, reversing rotation of the flagella in response to intra-cellular signals.

Arrangement and Types of Bacterial Flagella The number and location of flagella are distinctive for each genus. There are four types of flagellar arrangement.

  1. Monotrichous (Mono means one): Single polar flagellum e.g. Vibrio cholerae, Campylobacter spp. (polar flagella often in pairs to give a “seagull” appearance).
  2. Amphitrichous: Single flagellum at both ends e.g. Alcaligenes faecalis  (Mneomonics: Remember: the characteristics of Amphibians: live both in land and water)
  3. Lophotrichous: Tuft of flagella at one or both ends e.g. Spirilla spp
  4. Peritrichous (flagella in periphery): Flagella surrounding the bacterial cell. All the members of family Enterobacteriaceae, if motile has peritrichous flagella. e.g. Salmonella typhi,  Escherichia coli, Proteus spp (highly motile organism; shows swarming motility)
Spirillum and its flagellar arrangement

Functions of Bacterial Flagella

Many prokaryotes are motile, and the majority of motile prokaryotes moves by means of flagella.

Medical Importance of Flagella