• Flagellum (singular) is hair like helical structure emerges from cell wall and cell membrane
• It is responsible for motility of the bacteria
• Size: thin 15-20nm in diameter.
• Single flagella can be seen with light microscope only after staining with special stain which increase the diameter of flagella.

Structure of flagella:

• Flagella is not straight but is helical.
• It is composed of flagellin protein (globular protein) and known as H antigen.
• Flagella has three parts. Basal body, Hook and filament

Basal body:

• it is composed of central rod inserted into series of rings which is attached to cytoplasmic memvbrane and cell wall.
• L-ring: it is the outer ring present only in Gram -ve bacteria, it anchored in lipopolysaccharide layer
• P-ring: it is second ring anchored in peptidoglycan layer of cell wall.
• M-S ring: anchored in cytoplasmic membrane
• C ring: anchored in cytoplasm

Hook:

• it is the wider region at the base of filament
• it connects filament to the motor protein in the base
• length of hook is longer in gram +ve bacteria than gram –ve bacteria

Filament:

• it is thin hair like structure arises from hook.

Types of flagella

On the basis of arrangement

1. Monotrichous:

• presence of single flagella in one end of cell.
• examples; Vibrio cholera, Pseudomonas aerogenosa

2. Lophotrichous:

• presence of bundle of flagella in one end of cell.
• example: Pseudomanas fluroscence

3. Amphitrichous:

• presence of single or cluster of flagella at both end of cell.
• example; Aquaspirillium

4. Peritrichous:

• presence of flagella all over the cell surface.
•   example; E.coli, Salmonella, Klebsiella

5. Atrichous:

• absent of flagella.
• example; Shigella

Function:

Flagellar motility:

• At the base surrounding the inner ring (M-S and C ring) there is a series of protein called Mot protein.
• A final set of protein called Fli protein function as motor switch. The flagella motor rotates the filament as a turbine causing movement of the cell in the medium.
• The movement of flagella results from rotation of basal body which is similar to the movement of the shaft of an electric motor.
• A turning motion is generated between S-ring and M ring. S-ring acts as starter while M ring acts as roter.
• The basal body as a whole give a universal joint to the cell and allows complete rotation of hook and filament.
• Flagella moves the cell by rotating the flagella about the basal body. Rotation of flagella is either clockwise or anticlockwise.

Bacterial Flagella: structure, types and function

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