Bacteria were observed under an optical Nikon Eclipse
E600 phase-contrast microscope. The average number of
bacteria was estimated by direct cell counting, using a
Thoma chamber. MTB were counted after magnetic collection
by hanging-drop assay [4]. Average numbers were
calculated from three separate cell counts. Samples for
electron microscopy observations were adsorbed on formvar-
carbon copper grids (200 mesh) and examined with a
Zeiss EM912 electron microscope operating at 100 kV.
Photographs were taken with a Gatan Bioscan 792 camera.
The size of the MTB and of their magnetosomes was
measured using the GIMP 2.8 software (http://www.gimp.
org/). Length and width averages were made on all of the
MTB morphotypes observed by transmission electron
microscopy. Thirty crystals were measured to compute a
mean magnetosome length