Oddly Enough / Episode 05
Dinner Without a Map
A bacterium’s route can look like a string of wrong turns. How does it end up where the food is?
It darts. It tumbles. It sets off again. One part of this wandering turns out to matter more than it first appears.
Follow a swimmer ↓Not an obvious route to lunch.
Watch a swimming Escherichia coli cell and you can see relatively straight dashes interrupted by brief tumbles. After a tumble, it heads off in a different direction.
There is no neat line pointing to a destination. Yet these wandering swimmers can gather where chemical conditions are more favourable.
So what changes along the way? The answer became clearer when researchers followed the movement closely enough.
The turn that didn’t happen.
In 1972, Howard Berg and Douglas Brown tracked cells in three dimensions. Movement towards an attractant was linked to fewer changes of direction.
A run through improving conditions could last longer. A direction that happened to be useful got more time to carry the cell along.
The next tumble still brings uncertainty. But over many runs, spending longer on the promising stretches can change where the cells end up.
You can follow a clue without a map.
Imagine looking for a bakery by smell. You do not know where it is, but the smell gets stronger as you walk. Continuing that way for a little longer may help.
That comparison needs a recent past: how things are now, compared with how they were a moment ago.
A bacterium encounters changing chemical conditions as it swims. A trace of recent conditions lingers in its signalling machinery. When the signal improves, the response can make a tumble less likely.
The bakery is an analogy. The cell’s response is chemistry, with no destination plotted in advance.
The wandering was part of the method.
The useful trick is small: keep a favourable direction for longer. Repeated many times, that bias can turn wandering into progress.
It will not make every individual journey successful. But the route does not have to look purposeful for the behaviour to be useful.
What first looked like a string of wrong turns was missing its most important detail: the timing between them.
A little further into the story
Does a bacterium actually remember?
Here, “memory” means a lingering biochemical state. The chemical-sensing and signalling machinery responds to recent conditions as well as the current signal. This is not conscious recollection.
Studies by Macnab and Koshland (1972) and Segall, Block and Berg (1986) investigated responses to changing chemical conditions. The latter provides evidence for short-term comparisons over the recent past.
Is the chemical always food?
A chemical that draws a cell towards it is called an attractant. It may be a nutrient, or another signal associated with favourable conditions. “Finding dinner” is an everyday way into the story, not a claim that every attractant is eaten.
Movement biased by a chemical environment is called chemotaxis.
Does the method always work?
An individual cell can wander away from favourable conditions. A bias that changes where a population accumulates does not guarantee success for every swimmer.
In a uniform environment, movement provides no improving side to discover. And this run-and-tumble example is not a rule for every microbe: organisms use different sensing and movement strategies.
What did the researchers measure?
The 1972 tracking study followed swimming cells in three dimensions. Later experiments examined responses to changing chemical stimuli. Together they help connect the movement with the comparison that influences it.
The paths on this page are original explanatory drawings, not recorded cells. No measured swimming speed, response time or universal memory duration is being inferred from them.
Follow the evidence
- Berg and Brown: Chemotaxis in Escherichia coli analysed by Three-dimensional TrackingNature (1972). Tracking runs and directional changes in an attractant environment.
- Macnab and Koshland: The gradient-sensing mechanism in bacterial chemotaxisPNAS (1972). Experimental responses to changing chemical conditions.
- Segall, Block and Berg: Temporal comparisons in bacterial chemotaxisPNAS (1986). Experimental evidence for comparing present conditions with the recent past.
- Read the temporal-comparison paperAccessible research article and abstract.
All paths are explanatory drawings. No microscope recording or research figure is reproduced.