Ultrastructure and characterization of anisotropic magnetic inclusions in magnetotactic bacteria

Abstract

Ovoid magnetotactic bacteria extracted from the Exeter River, New Hampshire, U.S.A., contain chains of 20-35 anisotropic magnetic inclusions running longitudinally in each of three lateral cell positions adjacent to the inner surface of the cytoplasmic membrane. The inclusions are bullet-shaped and have characteristic flattened end faces. Some particles show kinking and curvature in their morphology. In cross section the particles have a hexagonal shape. The length of the inclusions varies over a wide range (45-135 nm) with a mean value of 97.8 nm. In contrast, the width of the particles is restricted to a range of 30-45 nm with a mean value of 36.9 nm. Many particles are surrounded by an organic electrondense envelope. The crystallographic structure of the inclusions has been determined by electron diffraction and corresponds to the mineral magnetite (Fe$_{3}$O$_{4}$). The dimensions of the crystals fall within the magnetic single-domain range for magnetite and the magnetic moment of one cell is approximately 4 $\times $ 10$^{-12}$ emu (4 fJT$^{-1}$).