Questions: Bacterial Flagella, Motility, and Chemotaxis

Question 1

A single E. coli cell is placed in a solution where glucose concentration is uniform but rising steadily everywhere. Despite swimming in any random direction, the cell runs longer and tumbles less. What explains this?

• A. The cell senses a spatial concentration gradient across its body length and swims toward the higher-concentration end
• B. The cell detects a temporal increase in glucose concentration and suppresses tumbling regardless of swimming direction
• C. Higher glucose provides more energy to the flagellar motor, increasing rotation speed and reducing tumble frequency
• D. Uniform glucose saturates all chemoreceptors symmetrically, locking the flagellar motor in the CCW (run) state

Answer: (B)

Bacteria are too small to detect spatial gradients — the concentration difference across a single cell is far below noise threshold. Instead, chemotaxis is a temporal comparison: the cell compares current attractant concentration to what it was a few seconds ago. A rising concentration signal (regardless of direction) is interpreted as 'swimming productively' and suppresses tumbling. The adaptation system (CheR/CheB) resets the baseline after a few seconds, which is why this is a comparison to the recent past rather than an absolute concentration measurement.

Question 2

In E. coli chemotaxis, when an attractant binds to membrane chemoreceptors, the downstream signaling sequence is:

• A. CheA activity is inhibited → less phospho-CheY → less clockwise (CW) flagellar rotation → longer runs
• B. CheA activity is increased → more phospho-CheY → less CW flagellar rotation → longer runs
• C. CheA activity is inhibited → less phospho-CheY → more CW flagellar rotation → more tumbles
• D. CheY is directly activated by the attractant → flagellar bundle dissociates → tumble

Answer: (A)

The signaling logic is: attractant binding → inhibit CheA kinase → reduce phosphorylation of CheY → less phospho-CheY at the motor switch → less CW rotation → less tumbling → longer runs. Phospho-CheY is the 'tumble signal' — when it binds the flagellar motor switch, it promotes CW rotation and bundle dispersal. Attractants suppress CheA, drain the phospho-CheY pool, and the motor defaults to CCW (run). This inverted logic (less signal = more running) ensures that the cell runs *toward* attractants and tumbles *away* from repellents.

Question 3

The bacterial flagellum generates thrust using the same bending and undulating mechanism as eukaryotic flagella, differing mainly in its energy source (proton motive force instead of ATP).

• T. True
• F. False

Answer: False

Bacterial and eukaryotic flagella are completely unrelated in structure and mechanism — a classic example of convergent evolution for a similar function. Bacterial flagella are rigid, helical filaments that rotate like propellers, driven by a rotary motor powered by proton flow. Eukaryotic flagella (and cilia) contain a '9+2' microtubule axoneme and generate movement through dynein-powered sliding of microtubule doublets — a bending/undulating mechanism driven by ATP hydrolysis. They share only the word 'flagellum' and the broad function of motility.

Question 4

The CheR/CheB receptor methylation cycle allows bacteria to respond to changes in attractant concentration rather than absolute concentration levels, enabling navigation of gradients across a wide dynamic range.

• T. True
• F. False

Answer: True

This adaptation mechanism is essential for robust chemotaxis. CheR continuously methylates MCPs (increasing their activity), while CheB (activated by phospho-CheA) demethylates them (reducing activity). After a step-change in attractant concentration, the system responds transiently and then adapts back to baseline behavior through this methylation feedback — regardless of the new absolute concentration. This means bacteria can continue responding to further increases even after adapting to a high background, allowing navigation across concentration ranges spanning many orders of magnitude.

Question 5

Why can't bacteria sense chemical gradients by comparing concentrations at their front versus their back, and how do they achieve directed movement toward attractants instead?

Model answer: A typical bacterium is only 1–2 micrometers long. The difference in attractant concentration between the front and back of the cell is far too small to detect against the noise of molecular diffusion — there is no meaningful spatial gradient signal across that distance. Instead, bacteria use temporal sensing: as they swim, they compare the current concentration to what it was a few seconds ago using the receptor methylation adaptation system as a 'molecular memory.' If concentration has increased, they suppress tumbling and keep running; if it has decreased, they tumble and randomly reorient. This biased random walk effectively navigates gradients using time rather than space.

This solution is elegant precisely because it converts a spatial problem (where is the food?) into a temporal problem (is my situation improving?). The adaptation time constant (~1–4 seconds) matches the timescale of a run, ensuring the comparison is 'recent enough' to reflect swimming direction but 'delayed enough' to span a meaningful distance. The molecular implementation — a reversible covalent modification (methylation) as memory — is one of the simplest and best-understood signal processing circuits in biology.