Magnetic colloids from magnetotactic bacteria: Chain formation and colloidal stability

Single-domain magnetite (Fe3O4) crystals, harvested from magnetotactic bacteria, display on transmission electron micrographs the cluster morphologies (folded chains, flux-closure rings) predicted for magnetic Colloids with dominant dipolar attractions. These strong attractions are responsible for the linear magnetite chains inside bacteria but do not affect the colloidal stability of the bacteria, as confirmed by analytic sedimentation experiments. Calculations of the interaction energy between dipole chains show that the magnetic component of the interbacteria interaction is negligible due to screening of dipolar forces: the bacteria sense only the geomagnetic field but not each other's compass.