Anyone who has looked through the microscope at tiny objects suspended in water cannot fail to notice how they are all in constant vibrating motion, even in living cells. This 'Brownian' motion results from random jostling of molecules, and the warmer they are, the faster they move. The magnetite particles, however, must not be jostled by Brownian motion, otherwise there would be no magnetic orientation. Thus each particle must be able to orient itself firmly in the direction of the earth's magnetic field. Biologists believe that this ability may provide a mechanism to enable bacteria to orient themselves in space.
Large organisms know where they are relative to Earth's surface because gravity provides the cue.
This is much harder for bacteria, however, because of their small size gravity is very weak in comparison to other forces like surface tension for example. The further north one is located, the steeper is the incline of the magnet's needle. At the north pole the needle points straight down. In the southern hemisphere the needle points more steeply up, the further one moves to the south. Scientists suspect that magnets in the northern hemisphere may indicate to these tiny cells which direction is down.