Magnetotaxis in bacteria (and some protist) is passive. The organism will biomineralize a ferromagnetic mineral, such as magnetite or greigite, and in a magnetic field they will passively orientate. This is not active orientation, meaning that even when the organism is dead it will still orientate in the field.

Their movement in the magnetic field is however active. The theory behind the magnetotaxis is that it allows them to know what direction is up. In the northern hemisphere, the magnetic poles come in from above and go down. So to a bacterium, North is down.

Why care what direction North is? if you are sensitive to oxygen, which MMB are, and oxygen diffuses in from the atmosphere above, your magnetotaxis would tell you the direction to swim to get away from toxic levels of oxygen. Wild how evolution works!