First of all let us have a look at the figure.
Electron microscopic studies revealed that bacterial flagellum consists of three parts. A long filament, hook and basal body (flagellar motor).
Part 1: Filament: is a very long hollow cylindrical structure made up of protein flagellin. There is a slender non-protein core which is surrounded by protein fibrils often three or more in number.
Part II: Hook: is the short curved segment that connects filament to the basal body. The hook is slightly wider than the filament.
Part III: basal body or blepharoplast: This is the “flagellar motor” lies within the plasma membrane.
In Gram negative bacteria, the basal body consists of 4 rings. 2 attached to the plasma membrane, one to the peptiodoglycan layer and other to the outer lipopolysaccharide (LPS) layer.
In Gram positive bacteria, outer LPS layer is absent and basal body has two rings one attached to the plasma membrane and one attached to the peptidoglycan wall.
Arrangement of flagella: Two patterns are observed
a) Polar arrangement: Here flagellum is attached at one or both ends of the bacterial cell.
Lophotrichous: Tuft or group of flagella at one end eg: Spirillum volutans
Amphitrichous: one or group of fkagella at both ends. Eg: Aquaspirillum
Second type is the peritrichous type of arrangement; here flagella are distributed randomly all over the cell surface or cell perimeter. Eg: E.coli
How flagellum works? Mechanism of flagellar movement.
Flagella can rotate at ~100 revolutions per second. The direction of the flagellar rotation determines the nature of bacterial movement. Look at the figure where different movement of filament causes movement of bacteria in different directions. The bacterial flagellar movement is driven by flow of protons through an outer ring of proteins. The basal body is the ‘motor’ that causes the rotation of the hook just like a propeller of ship. The flagellar motor consists of 4 parts: rotor (M ring), stator, bearing (S ring) and rod. The ‘rotor’ is a protein attached to the plasma membrane (inner ring) and the movement is proton driven. It rotates ~100 revolutions /second. Just above the rotor is another protein disc called stator. A small rod links rotor to the hook and filament and movement of rotor induces movement of hook and filament. Below the hook region is another protein disc called ‘bearing’ that fix the whole stricture to the outer membrane. Both ‘stator’ and ‘bearing’ are stationary.
Other extra cellular appendages in Bacteria
Fimbrae or Pili: These are extremely fine structures present in bacteria for cell –cell contact. Pili involved in bacterial conjugation are called sex-pili. It is made up of a protein called pilin. These structures are not involved in locomotion.
- Involved in bacterial conjugation, then called as sex-pili
- Helps in attachment of pathogenic bacteria to the host.
- Specific site of attachment for bacteriophages
- Mostly seen in Gram negative bacilli
Spinae: are tubular rigid appendages present in Gram positive bacteria.
It is made up of protein called spinin.
Exact function is not known. It is believed to play an important role in helping bacteria to tolerate extreme environmental conditions such as temperature, salinity, pH etc.