Bacteria: Flagella


Most prokaryotes are motile by swimming, and this function is typically due to a structure called the flagellum. The plural form of this word is flagella which means whip. Bacterial flagellum is a threadlike locomotor appendages extending outgrowth from the plasma membrane and cell wall.

Bacterial flagella are thin, rigid structures, about 20 nm across and up to 15-20 μm long.Flagella are so thin that a single flagellum can be seen with the light microscope only after being stained with special stains that increase their diameter. However, flagella are easily seen with the electron microscope.

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Arrangement of flagella

Bacterial cells have four arrangements of flagella-
  • Atrichous: Bacteria have no flagellum. e.g. Micrococcus corchorii.
  • Monotrichous: (mono means one; trichous means hair)Bacteria have one flagellum; if it is located at an end, It is said to be a polar flagellum. e.g. Pseudomonas aeruginosa.
  • Amphitrichous: (amphi means on both sides) Bacteria have a single flagellum or clusters at each pole. e.g. Aquaspirillum serpens.
  • Lophotrichous: (lopho means tuft) Bacteria have a cluster of flagella at one end. e.g. Rhodospirillum photometricum.  
  • Peritrichous: (peri means around) Flagella are spread fairly evenly over the whole surface of bacterial cell. e.g. Proteus vulgaris.
The type of flagellation, polar or peritrichous, is a charecteristic used in the classification of bacteria.
Flagella Images, Stock Photos & Vectors | Shutterstock
Arrangement of flagella in bacterial cells. Image source: Here


Flagella are not straight but are helical. When flattened, flagella show a constant distance between adjacent curves called the wave and this wave length is charecteristics for the flagella of any given species. A flagella consists of several components an moves by rotation, much like a propellar of a boat motor. The base of the flagellum is structurally different from the fillament. There is a wide region at the base of the fillament called the hook. The hook consist of a single type of proteins and connects the fillament to the motor portion of the base.

Structure and arrangement of flagella in bacterial cell and types of  flagellar motility - Online Science Notes
Structure and function of the flagellum in gram-negative bacteria. Image source: Here.

Transmission electon microscope(TEM) studies have shown that the bacterial flagellum is composed of three parts.

  • Filament: The longest and most obvious portion is the flagellar filament, which extends from the cell surface to the tip.
  • Basal body: The basal body anchors the flagellum to the cell wall and plasma membrane.
  • Hook: A short, curved segment consists of different protein, the flagellar hook links the filament to its basal body and acts as a flexible coupling.The motor is anchored in the cytoplasmic membrane and cell wall. The motor consists of a central rod that passes through a series of rings.


The L ring is embedded in the LPS and the P ring in peptidoglycan. The MS ring is embedded in the cytoplasmic membrane and the C ring in the cytoplasm.
A narrow channel exists in the rod and filament through which flagellin molecuIes diffuse to reach the site of flagellar synthesis.
The Mot proteins function as the flagellar motor, where as the Fli proteins function as the motor switch.
The flagellar motor rotates the filament to propel the cell through the medium.




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Abdul Bari Chowdhury
2 years ago

So much informative. Nice article.

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