Introduction to magnetotactic bacteria. The first magnetotactic bacterium, Magnetospirillum magnetotacticum , was isolated by Richard Blakemore in 1975. It is a Gram-negative, microaerophilic, motile, spirillum, 0.5 x 5 µ, belonging to the α-proteobacteria. Its most interesting property is that it forms single-domain crystals of magnetite (Fe3O4). Each crystal is enclosed in a membrane and is called a magnetosome. The magnetosomes are arranged in long chains, keeping the north-south orientation, so that the whole structure functions as a bar magnet and causes the organism to exhibit magnetotaxis. Specifically, it is thought to follow the magnetic field lines of the earth downward to regions favorable to growth. Since Blakemore’s discovery many other magnetotactic bacteria, with different morphologies and producing different magnetic minerals, have been observed in both fresh and salt water environments, although relative few have been cultured. There is considerable interest in understanding both the mechanism by which these organisms synthesize magnetite and the crystalline structure of the magnetite product. Biologically they are studied as possible models for the process of biomineralization and for their roles in the evolution of the magnetotactic response in higher organisms. They are of geological interest for their contributions to the magnetization of sediments and for their potential as geobiological tracers, since they leave detectable fossil remains. Finally, because of the exceptionally fine quality of the single-domain magnetic crystals, the product, magnetite, has many practical uses. Commercial uses include magnetic tapes, magnetic targeting of pharmaceuticals, cell separation, and applications in magnetic resonance imaging. .