Spatial arrangement of chains of magnetosomes in magnetotactic bacteria

Scanning and transmission electron microscopy (SEM/TEM) were used to investigate the spatial arrangement of chains of magnetosomes in cells of two morphologically different types of magnetotactic bacteria which possess at least two chains: Magnetobacterium bavaricum, and wildtype magnetic cocci. The TEM pictures show apparently very different arrangements of the chains within the cells. Stereo-micrographs obtained by tilting individual bacteria in the TEM, however, revealed that magnetosome chains in magnetic cocci always lie on opposite sides of the cell body and in close proximity to the cell envelope. The rod-shaped cells of M. bavaricum contain up to 1000 bullet-shaped magnetosomes forming 3-5 rope-shaped bundles of magnetosomes arranged such that they are separated by the maximum possible distance from each other and positioned adjacent to the cell envelope. These observations can be understood in terms of repulsion forces between parallel magnetic dipoles driving the chains apart from each other and forcing them to be in direct mechanical contact to the cell envelope. Thus, the magnetic torque acting on the chains under the influence of the geomagnetic field can be transferred very effectively to the whole cell body. Moreover, the formation of two or more chains inside the cell body is an effective means of achieving a magnetic moment large enough for a given cell size to overcome the viscous resistance with respect to the surrounding medium, which is required to ensure an alignment with the geomagnetic field as fast as possible. Based on these two arguments and on the electron optical observations, we hypothesize that all coccoid magnetotactic bacteria contain at least two chains of magnetosomes.