The bacteria orient and can move along earth's magnetic field lines. One end of the bacterium acts as the north, and the other end as the south magnetic pole - when the bacteria are not attached to something, the bacteria will move rapidly by flagellar rotation along a magnetic field line until they run into a place for attachment. Interestingly, north-seeking bacteria seem to be the most prevalent. If the bacteria are placed onto a microscope slide, and a magnet applied, they will move in unison like crazy - either away from or towards the pole of the magnet. This process of movement is called magnetotaxis .
Honey bees have such magnet-like things, as do some birds and some dolphins.
Bacteria Aren't the Only Cells Which Can Move We, too, have cells which respond to chemotactic signals. Some very inmportant cells such as neutrophils, macrophages, etc., which help us to fight infection respond to chemical substances released as a result of trauma or inflammation, and will move toward the source of the inflammation. These cells must physically move from inside a vein, cross the vein wall and enter the tissue. This type of movement across a vein is called diapedesis . So, we are back to where we started, a tail of Thucidydes.