Water contaminated with toxic or radioactive metals such as lead, cadmium or uranium can leak into the environment from old mines. Bacteria Magnetospirillum magnetotacticum can bind and accumulate these heavy metals. They possess so-called magnetosomes, small magnetic crystals they use for orientation on the magnetic field of the earth. Scientists proposed to let the bacteria clean contaminated waters and remove the loaded cells with magnets. Interesting fact: M. magnetotacticum also binds gold and silver!
Other interesting stuff bacteria do: