Living Lodestones: Magnetotactic Bacteria
By Catherine Faber
2 July 2001
How bacteria swim -- objects in motion tend to come to rest.
As far as bacteria are concerned, friction is everything and inertia doesn't exist. They live out their lives wriggling among water molecules (and the stuff dissolved in the water). It's sort of as if we lived all our lives in a building full of shifting golf balls. Falling just doesn't happen, because the golf balls don't get out of the way -- if you want to go downstairs you have to swim. Bacteria have very little mass, and the water, for them, is very viscous. They can't glide; they must keep swimming to get anywhere.
The most efficient way to swim under these circumstances is to use a propeller, and that's just what bacteria do. They have one or more flagella (from the Latin for whip) -- long helical springy filaments that poke out from the cell membrane. A little protein disk in the cell membrane rotates the flagella, making them twist in the water.
Some bacteria, like the laboratory favorite Escherichia coli, have a number of flagella. When the flagella rotate counterclockwise they tend to tangle together in a bundle on one side of the cell and push it rapidly in one direction. This behavior is called "running." When the flagella rotate clockwise they fly apart and each flagellum pushes or pulls on one side or another of the cell, so that it turns randomly, a behavior called "tumbling."
As it moves through the water, an E. coli cell compares the conditions it finds to the conditions it just left. If things are getting better (for instance there's more sugar in the water here), the cell is more likely to keep running, going in the same direction. If things are getting worse (for instance there's a toxin in the water) the cell is more likely to tumble and change direction at random. This is called run-and-tumble motility, and while it isn't a very efficient way to find food and flee trouble, it's a very simple system that doesn't call for a cell with no brain to make a lot of decisions.
Even when bacteria are running, they don't run quite in a straight line. The disk in the outer membrane that makes the flagellum rotate also puts a torque on the bacterium, making it rotate in the opposite direction. The combination of these rotations means that the bacterium travels in a tight helix, spiraling around its direction of motion.
Return to Article ("Finding your way down").
Return to Article ("Axial vs. polar magnetoaerotaxis").