Structure and arrangement of flagella in bacterial cell and types of flagellar motility

Bacterial flagella are hairlike, helical appendages that protrude through the cell wall.
They are responsible for motility in bacteria and are much thinner than the flagella or cilia of eukaryotes.
They are much simpler in structure and are 0.01-0.02µm in diameter.
A flagellum is composed of three parts:
- Basal body:
  - It is associated with the cytoplasmic membrane and cell wall of bacteria.
  - Basal body of Gram negative bacteria consists of 4 rings; L, P, S and M rings.
  - L- ring is associated with the outer membrane and P- ring is with the peptidoglycan layer.
  - Both S and M rings are associated with the cell membrane.
  - In case of Gram positive bacteria, the outer L-ring is absent.
  - Basal body acts as a motor for the rotation of flagella.
- A short hook
- Filament: a thin helical filament which is usually several times as long as the cell

Some Gram negative bacteria have a sheath surrounding the flagellum which is continuous with the outer membrane of the Gram negative cell wall.
The filament of the flagellum is made up of single fibril, composed of a protein called flagellin.
Unlike a hair, a flagellum grows at its tip rather than at the base.

Arrangement of flagella in bacteria:

On the basis of arrangement in the bacterial cell, flagella are of following types:
- Monotrichous: it is a single polar(at one or both ends of the cell) flagellum e.g. Pseudomonas aeruginosa
- Lophotrichous: a cluster of flagella lie at one end or pole of the cell. e.g. Pseudomonas florescence
- Amphitrichous: either single or cluster of flagella lies at both the ends of the cell. e.g. Aquaspirillum serpens
- Peritrichous: flagella are present all over the cell surface. e.g. Escherichia coli

Mechanism of flagellar rotation:

Flagella are the organelles of motility in bacteria and are responsible for swimming motility.
Mot protein acts as a starter to rotate flagellum and helical filament of flagellum rotates in screw type motion either in clockwise or anti-clockwise direction.
This rotation of flagellum either pushes or pulls the bacterial cell.
Proton gradient provides the necessary energy for the rotation of flagella.
When proton flows through the Mot protein, it attracts the opposite charge on M and S rings of flagellar basal body, thus rotating the flagella in screw type motion.

Types of flagellar motility:

Reversible flagellar motility:
- Bacteria with polar flagella move by two mechanisms.
- Flagella of some bacteria rotate in both directions, such flagella are called reversible flagella.
- In such bacteria, at first flagella rotate in one direction and make the bacteria move forward and to move in reverse direction, bacteria change the direction of rotation of flagella.
Unidirectional flagellar motility:
- Flagella of some bacteria rotate only in one direction.
- In such bacteria, to change the direction of movement, bacteria first should stop and orient to change the direction again.
- Such motility is unidirectional motility of flagella.
Peritrichous flagellar motility:
- In bacteria with peritrichous flagella, flagellar motility is different.
- At first, peritrichous flagella assemble at one pole forming a bundle which then rotates in anti-clockwise direction and pushes the bacteria forward.
- To change the direction of movement, flagella rotate in clockwise direction, by which flagella unwind or release from the bundle and bacteria tumble.
- Flagellar bundle is formed again on the other pole and bundle rotates in anti-clockwise direction and bacteria move in a new direction.

Types of motility in bacterial cells:

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