Bacterial Flagella - Arrangement of Flagella -How Flagellum works?

Bacterial Flagella
Bacterial Flagella
Flagellum is an extra cellular appendage for motility in bacteria. (Flagellum S:  a whip, flagella plural). 

In this post we will discuss the ultra-structure of bacterial flagella and how it works?.

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.

Gram negative and Gram Positive Flagella
Gram negative and Gram Positive Flagella
Arrangement of Flagella
Monotrichous: a single flagellum at one end of the cell eg: Psuedomonas aeruginosa
Vibrio chloerae

Lophotrichous: Tuft or group of flagella at one end 
eg: Spirillum volutans
Pseudomonas fluorescens

Amphitrichous: one or  group of flagella at both ends. 
eg: Aquaspirillum sepens

Peritrichous: Here flagella are distributed randomly all over the cell surface or cell perimeter. 
eg: Salmonella typhii 
Proteus mirabilis

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 PiliThese 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.

   Major functions:
  • 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.

Practice Questions
1. Which of the following is having rings and shaft arrangement?

A) Pili 

B) Fimbriae 

C) Axial filament 

D) Flagellum

Ans: D) Flagellum

Learn more: MCQ on Cilia and Flagella

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