Bacterial Chemotaxis

How It Works — Step by Step

1. 1

   One Cell, Two Moves

   Start with the smallest part: one bacterium, the yellow cell, about 2 µm long. It has only two moves. It runs, swimming straight at about 20 µm per second while its flagella turn together as one bundle. Then it tumbles: the flagella fly apart for a tenth of a second and the cell flips to a new, roughly random direction. The 'Cell is' readout switches between Running and Tumbling, and each orange dot on the path marks a tumble.

2. 2

   Tumbling Alone Is a Random Walk

   Choose 'Off' in the 'Sensing' menu. Now tumbles come at a steady rate, about one per second, so an average run lasts 1 s and covers 20 µm, whichever way the cell is heading. Choose 'A crowd of 40' in the 'Show' menu and press Reset. The crowd spreads out like a drop of ink in water and wanders past the food. 'Crowd near the food' hovers around 10%, about the share of the dish the dashed circle covers.

3. 3

   Too Small to Compare Its Ends

   To find food the cell must know which way is better, but it is too short to compare its two ends: across 2 µm the food signal differs by only about 2%, lost in the noise of catching a few molecules. Instead it compares in time. Receptors in its membrane keep a memory of the last second, and on a straight run toward the food the 'Signal vs 1 s ago' readout climbs to about 20%, ten times the difference across its body.

4. 4

   Tumble Less When Things Improve

   Choose 'On' in the 'Sensing' menu. The rule is simple: while the signal is higher than a second ago, the cell holds back some of its tumbles, so runs that happen to point toward the food last longer. Runs pointing away are not punished; they end at the usual rate. The cell never steers. Watch the crowd drift toward the food and 'Crowd near the food' climb well above the 10% of the random walk.

5. 5

   Worked Example: Longer Runs Uphill

   Set 'Tumble suppression' to 50%. While the signal falls the cell tumbles once per second, so a run averages 1 s, or 20 µm. While the signal rises half the tumbles are held back, so runs average 2 s, or 40 µm. Picture runs pointing straight toward and straight away from the food in turn: 40 µm forward, 20 µm back, a net 20 µm in 3 s. That is about 7 µm per second, a third of the swimming speed, with no steering at all.

6. 6

   Stronger Bias, Tighter Crowd

   Move the 'Tumble suppression' slider to 0%: even with sensing on, runs are the same length either way and the drift vanishes. Slide it to 90% and runs toward the food average 10 s, so the crowd gathers more tightly inside the dashed circle. Now click somewhere else in the dish to move the food. Runs toward the new spot are the ones that last, and within half a minute the crowd follows it.

7. 7

   Running Away Uses the Same Machinery

   Choose 'Repellent' in the 'Sensing' menu. The chemical now marks something to avoid, and the rule flips: tumbles are held back while the signal falls. The same two moves now carry the crowd away from the source, and 'Crowd near the food' drops toward zero. Real bacteria handle attractants and repellents with the same receptors and the same tumble switch; the two kinds of chemical simply push the switch in opposite directions.