Mechanism of bacterial flagellar motility By what physical mechanism do bacterial flagella move the cell through a liquid environment?

Introduction / Context:
Bacterial flagella differ fundamentally from eukaryotic cilia and flagella in structure and mechanism. Understanding this difference is essential for microbiology, as it relates to motility, chemotaxis signaling, and the action of certain antibiotics that target motility-related structures.

Given Data / Assumptions:

• Bacterial flagella are helical filaments made of flagellin.
• A rotary motor at the base is powered by ion gradients (proton or sodium motive force).
• Eukaryotic flagella use a different mechanism involving dynein motors and microtubules.

Concept / Approach:

Bacterial flagella rotate like a propeller. Directional changes (clockwise vs counterclockwise) modulate running and tumbling behavior during chemotaxis. This is distinct from the whip-like bending motion of eukaryotic flagella and cilia. Therefore, the correct choice is the propeller-like rotation mechanism.

Step-by-Step Solution:

Verification / Alternative check:

Experimental observations show rotary behavior, and motor reversal correlates with tumbling in peritrichous bacteria such as E. coli.

Why Other Options Are Wrong:

• Synchronous or single whip-like beating: Describes eukaryotic motility, not bacterial.
• Attachment and contraction: Not a mechanism for flagella; some cells show twitching via pili, but that is different.
• Actin–myosin sliding: A eukaryotic muscle mechanism, irrelevant to bacterial flagella.

Common Pitfalls:

• Projecting eukaryotic ciliary motion onto bacteria.
• Confusing type IV pilus twitching with flagellar swimming.

Final Answer:

Flagellar filaments rotate and spin like a propeller