Navigational Compass in Magnetic Bacteria R. B. Frankel

Several species of bacteria from both fresh water and marine enviornments were observed to orient and swim in a preferred direction in the geomag­ netic field (magnetotaxis) [1, 2J. Kalmijn and Blakemore [2] observed that most of the magneto­ tactic bacteria in sediments oriented and swam along magnetic field lines toward the north. Rever­ sal of the ambient magnetic field with Helmholtz coils caused the cells to make large V-turns within one second and swim in the opposite direction. Killed cells also oriented in uniform fields as low as 0.1 G. In these and other respects [3] the cells behaved like single magnetic dipoles. A magnetotactic spirillum designated strain MS-I was isolated from a fresh water swamp and grown in pure culture (4). Magnetic cells of this new bacterial isolate each had an intracytoplasmic chain of approximately 22 electron opaque par­ ticles (fig. I). Each particle was roughly octahedral, 500 A along each major axis. Energy dispersive X-ray measurements [1] (and unpublished results) showed that the particles in strain MS-l had high iron content. Mossbauer spectroscopy of 57Fe in freeze dried magnetic and nonmagnetic cells {5] showed conclusively that iron in magnetic cells was present primarily in the form of magnetite, Fe30 4, with a small admixture of y·Fe20 3• Another iron­ containing material with a room temperature Mossbauer spectrum similar to that of the iron storage protein ferritin was also observed. A population of nonmagnetic cells of MS-I was derived by successively cloning their nonmagneto­ tactic progeny in media low in iron. The magnetic variant of MS-I had over 10 times the total iron content of the nonmagnetic form. Nonmagne­ totactic cells lacked the magnetite crystals. Thus, magnetotaxis in MS-I is associated with intracellu­ lar magnetite. Since the bacteria were grown in a chemically defined media containing soluble (chelated) iron, the presence of intracellular mag­ netite implies a process of bacterial synthesis. The magnetic properties of magnetite particles depend on their size and shape. For a particle of roughly cubic shape with side dimension d, there is a range of d over which the particle will be a single