6.4.4 Structure of Bacterial Flagellum

Introduction

Many bacteria have fine thread-like outgrowths called flagella (singular: flagellum). These structures are responsible for bacterial motility. The bacterial flagellum is a complex structure composed of a single protein called flagellin, arranged in intertwined chains. Unlike eukaryotic flagella, bacterial flagella are non-contractile proteins and lack microtubules. Understanding the structure of the bacterial flagellum is essential for understanding bacterial motility and pathogenesis.

Definition

Bacterial Flagellum: A bacterial flagellum is a fine thread-like outgrowth composed of a single protein called flagellin, arranged in intertwined chains, which are non-contractile proteins and lack microtubules.

In simple words: A bacterial flagellum is a whip-like structure that bacteria use for movement.

Core Concept

• Many bacteria have fine thread-like outgrowths called flagella (singular: flagellum).
• It is composed of a single protein flagellin, arranged in intertwined chains.
• Flagella are non-contractile proteins and lack microtubules.
• Flagella are about 20 nanometers in diameter and up to 20 micrometers in length.
• Bacterial flagella consist of three parts:
• The 360° rotation of the flagellum causes the cell to spin and move forward.

Detailed Explanation

Structure of Bacterial Flagellum

The bacterial flagellum consists of three main parts:

1. Basal Body

• Location: Originates just beneath the cell membrane.
• Structure: Complex structure.
• Function: Produces rotatory motion.
• Components:
• Rings: In Gram-negative bacteria, the basal body has four rings (L, P, MS, C).
• Rods: Central rod that connects the rings.
• Motor complex: Proton motive force-driven motor.

• The basal body is the motor of the flagellum.
• It is embedded in the cell membrane and cell wall.
• It uses the proton motive force to rotate.

2. Hook

• Location: Connects the basal body to the filament.
• Structure: Short, curved structure.
• Function: Acts as a universal joint.
• Key Point: The hook allows the filament to rotate freely.

• The hook is a flexible coupling.
• It connects the motor to the propeller.

3. Filament

• Location: Extends outward from the hook.
• Structure: Hollow structure.
• Composition: Consists of several protein chains twisted into a helical structure.
• Protein: Flagellin.
• Function: Acts as a propeller.
• Key Point: The filament is the longest part of the flagellum.

• The filament is a hollow tube.
• It is made of flagellin subunits.
• It rotates like a propeller.

Detailed Structure of the Filament

Mechanism of Rotation

• The basal body uses the proton motive force to rotate.
• The hook acts as a universal joint.
• The filament rotates like a propeller.
• The 360° rotation causes the cell to spin and move forward.

• The rotation is driven by the proton motive force.
• The rotation is counterclockwise for forward movement (run).
• The rotation is clockwise for tumbling (reorientation).

Arrangement of Flagella

Bacterial species differ in the number and arrangement of flagella on their surface.

Comparison of Bacterial and Eukaryotic Flagella

Summary Table of Bacterial Flagellum

Intermediate Level

Structure of Bacterial Flagellum in Detail

Detailed Structure of the Filament

Mechanism of Rotation

Arrangement of Flagella

Comparison of Bacterial and Eukaryotic Flagella

Important Points

• 1. Many bacteria have fine thread like outgrowth called flagella (singular: flagellum).
• 2. It is composed of a single protein flagellin, arranged in intertwined chains, which are non-contractile proteins, and lack microtubules.
• 3. Flagella are about twenty nanometers diameter and up to 20 micrometers in length.
• 4. Bacterial flagella consist of three parts: a basal body, a hook and a filament.
• 5. The basal body originates just beneath the cell membrane.
• 6. It is a complex structure that produces rotatory motion.
• 7. The hook connects the basal body to the filament.
• 8. The filament is a hollow structure which consists of several protein chains twisted into a helical structure.
• 9. The 360° rotation of the flagellum causes the cell to spin and move forward.

Remember

• - Flagellum = structure for movement
• - Composition = flagellin protein
• - Parts = basal body, hook, filament
• - Basal body = motor
• - Hook = universal joint
• - **Filament** = propeller
• - Rotation = 360°; causes cell to spin and move
• - Arrangements = monotrichous, amphitrichous, lophotrichous, peritrichous
• - Energy = proton motive force
• - No microtubules = unlike eukaryotic flagella

Common Mistakes

• 1. Confusing bacterial and eukaryotic flagella – Bacterial = flagellin, rotation; Eukaryotic = tubulin, whip-like beating.
• 2. Thinking the basal body is the propeller – The basal body is the motor; the filament is the propeller.
• 3. Confusing the hook and filament – The hook is the universal joint; the filament is the propeller.
• 4. Thinking flagella are made of microtubules – Bacterial flagella are made of flagellin and lack microtubules.
• 5. Forgetting the energy source – The energy source is the proton motive force, not ATP.
• 6. Confusing the arrangements – Monotrichous = single; Amphitrichous = at each end; Lophotrichous = clusters; Peritrichous = all over.
• 7. Thinking the filament is solid – The filament is hollow.
• 8. Confusing runs and tumbles – Run = forward movement; Tumble = reorientation.
• 9. Thinking all bacteria have flagella – Not all bacteria have flagella.
• 10. Forgetting the examples – Vibrio cholerae (monotrichous), E. coli (peritrichous).
• 11. Thinking the flagellum is covered by a membrane – Bacterial flagella are not covered by a membrane.
• 12. Confusing the diameter and length – Diameter = ~20 nm; Length = up to 20 μm.

Quick Revision

• Many bacteria have fine thread like outgrowth called flagella (singular: flagellum).
• It is composed of a single protein flagellin, arranged in intertwined chains, which are noncontractile proteins, and lack microtubules.
• Flagella are about twenty nanometers diameter and up to 20 micrometers in length.
• Bacterial flagella consist of three parts: a basal body, a hook and a filament.
• The basal body originates just beneath the cell membrane.
• It is a complex structure that produces rotatory motion.
• The hook connects the basal body to the filament.
• The filament is a hollow structure which consists of several protein chains twisted into a helical structure.
• The 360° rotation of the flagellum causes the cell to spin and move forward.

60-Second Revision

• Flagellum = structure for movement
• Composition = flagellin protein
• Parts = basal body, hook, filament
• Basal body = motor
• Hook = universal joint
• **Filament** = propeller
• Rotation = 360°; causes cell to spin and move
• Arrangements = monotrichous, amphitrichous, lophotrichous, peritrichous
• Energy = proton motive force
• No microtubules = unlike eukaryotic flagella
• Warning: Bacterial flagella are made of flagellin, not tubulin. They rotate, they do not beat. The energy source is the proton motive force, not ATP. The basal body is the motor; the filament is the propeller. The hook is the universal joint. Bacterial flagella lack microtubules. Not all bacteria have flagella. Do not confuse bacterial and eukaryotic flagella.