6.4.3 Motility in Bacteria

Introduction

Motility is the ability of bacteria to move from one place to another. Many bacteria are motile and can move toward favorable conditions or away from unfavorable ones. This movement is essential for their survival, colonization, and interaction with their environment. Bacteria use various structures and mechanisms for motility, including flagella, gliding, twitching, and changes in buoyancy. Understanding bacterial motility is important for understanding bacterial behavior, pathogenesis, and ecology.

Definition

Motility in Bacteria: Motility in bacteria is the ability of bacteria to move independently using various structures and mechanisms.

In simple words: Motility is how bacteria move from one place to another.

Core Concept

• Motile bacteria can move using:
• A unique group of bacteria, the spirochetes, have structures similar to flagella.
• They have a distinctive helical body that twists about as it moves.
• In twitching motility, bacteria use their pili as a grappling hook, repeatedly extending it, anchoring it, and then retracting it with remarkable force.
• Bacterial species differ in the number and arrangement of flagella on their surface.

Detailed Explanation

Types of Bacterial Motility

1. Flagellar Motility

Definition: Motility using flagella, which are fine thread-like outgrowths.

• Flagella are the most common means of bacterial motility.
• They are fine thread-like outgrowths.
• They are composed of a single protein called flagellin.
• Flagella are about 20 nanometers in diameter and up to 20 micrometers in length.
• The 360° rotation of the flagellum causes the cell to spin and move forward.

Arrangement of Flagella:

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

• Monotrichous = single flagellum.
• Amphitrichous = flagellum at each end.
• Lophotrichous = clusters of flagella at the poles.
• Peritrichous = flagella distributed over the entire surface.

2. Bacterial Gliding

Definition: Movement along a surface without flagella.

• Gliding motility allows bacteria to move along solid surfaces.
• It does not require flagella.
• The mechanism is not fully understood.
• It is observed in cyanobacteria and myxobacteria.
• It may involve the secretion of slime and the use of surface proteins.

Example: Cyanobacteria, Myxobacteria

3. Twitching Motility

Definition: Movement using pili as a grappling hook.

• In twitching motility, bacteria use their pili as a grappling hook.
• They repeatedly extend the pilus, anchor it, and then retract it with remarkable force.
• This pulls the bacterium forward.
• It is observed in Pseudomonas, Neisseria, and other bacteria.

Example: Pseudomonas aeruginosa, Neisseria gonorrhoeae

4. Changes in Buoyancy

Definition: Movement by changing buoyancy.

• Some aquatic bacteria can move vertically by changing their buoyancy.
• They use gas vesicles to adjust their buoyancy.
• This allows them to move up or down in the water column.
• It helps them find optimal light and nutrient conditions.

Example: Some aquatic bacteria (e.g., cyanobacteria)

5. Spirochete Motility

Definition: Movement using axial filaments.

• A unique group of bacteria, the spirochetes, have structures similar to flagella.
• They have a distinctive helical body that twists about as it moves.
• The axial filaments (endoflagella) are located between the inner and outer membranes.
• The rotation of the axial filaments causes the helical body to rotate.
• This produces a corkscrew-like movement.
• It allows spirochetes to move through viscous environments.

Example: Treponema pallidum, Borrelia burgdorferi

Taxis in Bacteria

Motile bacteria are attracted or repelled by certain stimuli in behaviors called taxes.

• Chemotaxis = movement toward or away from chemicals.
• Phototaxis = movement toward or away from light.
• Magnetotaxis = movement along magnetic fields.

Movement Inside Host Cells

• Several species move inside host cells by usurping the cytoskeleton.
• The cytoskeleton is normally used to move organelles inside the cell.
• Bacteria like Listeria and Shigella use actin polymerization to move within host cells.

Example: Listeria monocytogenes, Shigella flexneri

Summary Table of Bacterial Motility

Intermediate Level

Flagellar Arrangements in Detail

Types of Motility in Detail

Taxis in Detail

Intracellular Movement in Detail

Important Points

• 1. Motile bacteria can move using flagella, bacterial gliding, twitching motility or changes of buoyancy.
• 2. A unique group of bacteria, the spirochetes, have structures similar to flagella.
• 3. They have a distinctive helical body that twists about as it moves.
• 4. In twitching motility, bacteria use their pili as a grappling hook, repeatedly extending it, anchoring it and then retracting it with remarkable force.
• 5. Bacterial species differ in the number and arrangement of flagella on their surface; some have:
• - A single flagellum i.e., monothricous
• - A flagellum at each end i.e., amphitrichous
• - Clusters of flagella at the poles of the cell i.e., lophotrichous
• - Flagella distributed over the entire surface of the cell i.e., peritrichous
• 6. Many bacteria (such as E. coli) have two distinct modes of movement:
• - Forward movement (swimming)
• - Tumbling
• 7. The tumbling allows them to reorient and make their movement a three-dimensional random walk.
• 8. Motile bacteria are attracted or repelled by certain stimuli in behaviors called taxes:
• - Chemotaxis
• - Phototaxis
• - Magnetotaxis
• 9. Several species move inside host cells by usurping the cytoskeleton, which is normally used to move organelles inside the cell.

Remember

• - Motility = ability to move
• - Flagella = most common means
• - Arrangements = monotrichous, amphitrichous, lophotrichous, peritrichous
• - Gliding = surface movement without flagella
• - **Twitching** = pili
• - **Buoyancy** = gas vesicles
• - Spirochetes = axial filaments
• - Taxis = directed movement
• - **Chemotaxis** = chemicals
• - **Phototaxis** = light
• - Magnetotaxis = magnetic fields
• - Intracellular movement = cytoskeleton usurpation

Common Mistakes

• 1. Thinking all bacteria are motile – Not all bacteria are motile.
• 2. Confusing flagella and pili – Flagella = motility; Pili = attachment and conjugation.
• 3. Confusing the arrangements of flagella – Monotrichous = single; Amphitrichous = at each end; Lophotrichous = clusters at poles; Peritrichous = all over.
• 4. Thinking gliding requires flagella – Gliding does not require flagella.
• 5. Confusing twitching and gliding – Twitching = pili; Gliding = surface movement without flagella.
• 6. Thinking spirochetes have flagella – Spirochetes have axial filaments (endoflagella).
• 7. Confusing taxis types – Chemotaxis = chemicals; Phototaxis = light; Magnetotaxis = magnetic fields.
• 8. Thinking bacteria move randomly – Bacteria move in a directed manner in response to stimuli.
• 9. Forgetting the examples – E. coli (peritrichous), Vibrio cholerae (monotrichous), Treponema pallidum (spirochete).
• 10. Thinking all bacteria use the same motility mechanism – Different bacteria use different mechanisms.
• 11. Confusing runs and tumbles – Run = forward movement; Tumble = reorientation.
• 12. Forgetting intracellular movement – Some bacteria move inside host cells by usurping the cytoskeleton.

Quick Revision

• Motile bacteria can move using flagella, bacterial gliding, twitching motility or changes of buoyancy.
• A unique group of bacteria, the spirochetes, have structures similar to flagella.
• They have a distinctive helical body that twists about as it moves.
• In twitching motility, bacteria use their pili as a grappling hook, repeatedly extending it, anchoring it and then retracting it with remarkable force.
• Bacterial species differ in the number and arrangement of flagella on their surface; some have a single flagellum i.e., monothricous, a flagellum at each end i.e., amphitrichous, clusters of flagella at the poles of the cell i.e., lophotrichous, while others have flagella distributed over the entire surface of the cell i.e., peritrichous.
• Many bacteria (such as E. coli) have two distinct modes of movement: forward movement (swimming) and tumbling.
• The tumbling allows them to reorient and make their movement a three-dimensional random walk.
• Motile bacteria are attracted or repelled by certain stimuli in behaviors called taxes: these include chemotaxis, phototaxis and magnetotaxis.
• Several species move inside host cells by usurping the cytoskeleton, which is normally used to move organelles inside the cell.

60-Second Revision

• Motility = ability to move
• Flagella = most common means
• Arrangements = monotrichous, amphitrichous, lophotrichous, peritrichous
• Gliding = surface movement without flagella
• **Twitching** = pili
• **Buoyancy** = gas vesicles
• Spirochetes = axial filaments
• Taxis = directed movement
• **Chemotaxis** = chemicals
• **Phototaxis** = light
• Magnetotaxis = magnetic fields
• Intracellular movement = cytoskeleton usurpation
• Warning: Not all bacteria are motile. Flagella are for motility; pili are for attachment. Spirochetes have axial filaments, not flagella. Gliding does not require flagella. Twitching uses pili. Taxis is directed movement. Runs and tumbles are different. Some bacteria move inside host cells by usurping the cytoskeleton. Do not confuse the arrangements of flagella.