Questions: Bacterial Flagella, Pili, and Cell-Surface Structures

Question 1

A researcher hypothesizes that blocking ATP synthase will immobilize bacteria by starving the flagellar motor of ATP. Based on how the flagellar motor actually works, this hypothesis is:

• A. Correct — ATP is the direct fuel for flagellar rotation via the stator proteins
• B. Incorrect — the flagellar motor is powered by proton flow through the stator proteins (the proton motive force), not by ATP hydrolysis; blocking ATP synthase would collapse the PMF and stop the motor, but not because ATP was removed
• C. Incorrect — the flagellar motor uses GTP, not ATP or PMF
• D. Correct — ATP powers the MotA/MotB stator proteins that rotate the basal body

Answer: (B)

The bacterial flagellar motor is powered by the proton motive force: protons flow down their electrochemical gradient through the stator proteins (MotA/MotB), driving rotation of the rotor at up to 1,000 rpm. ATP is not the direct energy source. ATP synthase uses PMF to make ATP — so blocking ATP synthase collapses PMF (and thus would stop the motor), but the mechanism is indirect and the reasoning in the hypothesis is wrong. The flagellar motor and ATP synthase are two different consumers of the same PMF currency.

Question 2

A bacterium possesses Type IV pili but no flagella. Which phenotype would you predict?

• A. The bacterium can swim through liquid but cannot attach to host cells
• B. The bacterium can move on solid surfaces via twitching motility and can take up environmental DNA, but cannot swim through liquid
• C. The bacterium is completely immotile and cannot adhere to surfaces
• D. The bacterium can swim through liquid because Type IV pili can bundle and rotate like flagellar filaments

Answer: (B)

Type IV pili extend, adhere, then retract (via PilT) to generate twitching motility on surfaces — but this does not propel bacteria through liquid. Swimming requires the helical rotation of flagellar filaments. Type IV pili also mediate natural transformation (DNA uptake uses the same extension-retraction mechanism). Option D is wrong: twitching is jerky surface movement, not swimming, and pili cannot rotate like flagella.

Question 3

Both flagella and pili are used by bacteria for locomotion — the difference is mainly whether they move in liquid or on surfaces.

• T. True
• F. False

Answer: False

Most pili and fimbriae are primarily adhesion structures, not motility structures — they anchor bacteria to surfaces and host cells. Type I pili (with FimH adhesins), for example, mediate adhesion to bladder epithelium and do not generate movement. Only Type IV pili produce motility (twitching), and that is a special case. The primary functional distinction between flagella and most pili is motility versus adhesion, not liquid versus surface — conflating the two misrepresents how these distinct molecular machines are used.

Question 4

In E. coli, when all flagellar motors spin counterclockwise, the helical filaments bundle together and propel the cell forward; when any motor switches to clockwise rotation, the bundle disperses and the cell tumbles.

• T. True
• F. False

Answer: True

This run-and-tumble mechanism is the basis of bacterial chemotaxis. Counterclockwise rotation causes filaments to form a coherent left-handed helical bundle — a propeller that drives smooth forward swimming. Clockwise rotation by any one motor breaks the bundle geometry; the filaments interfere with each other and the cell reorients randomly. Chemotaxis signaling modulates the probability of the clockwise switch, biasing runs toward attractants and tumbles away from repellents.

Question 5

Explain how the same molecular mechanism — Type IV pilus extension and retraction — serves two seemingly unrelated functions: twitching motility and natural transformation (DNA uptake).

Model answer: Both functions use extension of the Type IV pilus, followed by retraction driven by the PilT motor, which depolymerizes pilin subunits back into the membrane. For twitching: the pilus tip adheres to a surface, then retraction pulls the cell toward the attachment point, producing jerky movement. For natural transformation: the pilus tip binds extracellular DNA, then retraction pulls the DNA into the cell. The same force-generating retraction mechanism is applied to different substrates — a surface versus a DNA molecule.

This illustrates how evolution co-opts a single molecular machine for multiple functions. PilT-driven retraction generates some of the strongest forces known in biology relative to the cell's scale. The key insight is that the mechanism (extend, grip, retract) is substrate-agnostic — whether it is pulling the bacterium toward a surface or pulling DNA into the cell depends entirely on what the pilus tip happens to bind.