This applet simulates a model of how swimming bacteria follow
a chemical gradient to find food.
You can set the total number of germs. They do not reproduce or die.
|Run||Animate the germs|
|Step||Single-step the germs|
|Restart||Put the germs at random locations|
|Faster,Slower||There are four speeds|
Click the mouse to add more food.
A bacterium has several problems to overcome when trying to find food:
- It is so small that there is no detectable chemical gradient
along its body.
- It is constantly buffeted by brownian motion, so it cannot keep
going a given direction for any length of time.
- The gradients it is detecting change over time.
- The mechanism must be pretty simple to fit inside a bacterium.
Because a bacterium is so small, it can detect a gradient only by moving,
and comparing the chemical concentration where it is now to the concentration
where it was recently.
If the 'now' concentration is greater, it is moving in a good direction; otherwise it is not.
The bacterium swims by spinning its screw-shaped flagellum.
It can also spin the flagellum in the direction opposite the screw.
This causes the bacterium to spin and tumble, but not move much.
These two actions provide the bacterium's response:
This works surprising well, as the simulation demonstrates.
- Concentration increasing--swim
- Concentration decreasing--tumble (try a new direction), then swim
In the simulation, at each time-step, each germ compares the concentration where
it is with the concentration at the last time-step.
The concentration from each food source (yellow dot) is the source's value divided by the distance to the germ, squared.
The value decreases when a germ is near it ('eats'). When the value is zero, the source disappears
and, if there are fewer then two sources left, a new source is created.
Adding the concentrations from all the food sources gives the concentration each germ sees.
- If it is now greater, it moves in the same direction it was moving.
- If it is less, it picks a new direction at random and moves that way.
The mechanisms used by real bacteria are considerably more complex than the one shown here.
A search for 'bacterial navigation' will turn up lots of information.
Here are two URLs that worked when this page was created:
Sensing: Bacterial Navigation & Applications To Marine Sensing
Neural Networks in Bacteria
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