I always learned to wash my hands with my arms pointed down.
This was to use the force of gravity to my advantage in killing bacteria. Point your hands down, bacteria have to climb to escape the soap. The other way around, it's smooth sailing: just drop. Bacteria run from soap. That's creepy.
Bacteria are clumsy creatures.
Watch one of them swim under a microscope. It isn't in the straight line or graceful zigzag in which a fish or insect swims. Rather, it's short straight lines breached with random tumbles and weaves that reorient it.
Flagellum is synonymous with tail. Bacteria have flagella, some more than others. Bacterial flagella operation is as follows:
- Counter-clockwise rotation aligns the flagella into a single bundle and facilitates forward movement. Picture an ice skater spinning. As she draws her limbs inward she spins faster.
- Clockwise rotation breaks the bundle, creating friction that causes the bacteria to tumble. The ice skater releases her limbs and falls.
Vertebrates have taken advantage of flagella.
Evolution has programmed our immune cells to recognize these little curves of protein: much of the time our pathogens recognize bacteria because of their tails.
Bacteria don't have much control over the direction they swim. The movement of a bacterium serves two purposes: to find food and avoid poison. Tumbling points the bacterium in a random direction; a bacterium heading in a good direction tumbles less, while a bacterium heading in a bad direction tumbles more. The random flips in good conditions are called rotational diffusion, a fancy way of saying that bacteria forget which direction they're going.
Bacteria cannot go for more than a few seconds without forgetting where they are. But they are the most successful and prolific creatures on this planet.
Memory is a swindle.
Bacteria sense their environment with three types of transmembrane receptors: one for attractants, one for repellents, and one for periplasmatic proteins. Signals received are transmitted to the cytosol, which activate che proteins, which alter tumbling frequency according to environment. Movement is made through chance, and because directional alterations occur more in negative environments averages dictate that overall movement is beneficial.
You could program a bacterium in a graphing calculator.
Of course, bacteria aren't the only organisms that chemotax.
Eukaryotes, like immune cells, are also capable of self-contained movement. However, the devices they use to sense their environment are somewhat more complex than those of bacteria — these sense codes make up most of the cell's genome. A eukaryote's senses are distant cousins of sight and smell, members of the same superfamily of genes. Additionally, the way in which they physically get from one point to another is unclear. Since eukaryotes are without flagella, it's believed that movement is facilitated with the use of actin filaments called into action by signaling gradients and other biological things gleaned from the stuff floating in the environment.
I could write you an entire book that you wouldn't want to read.
Sources
Cells Alive
http://www.cellsalive.com/chemotx.htm
Wikipedia
http://en.wikipedia.org/wiki/Chemotaxis
University of Edinburgh
http://www.bms.ed.ac.uk/research/others/smaciver/Chemotaxis.htm