Resumo

Magnetotactic bacteria are able to align to magnetic field lines while swimming propelled by flagella, due to the presence of magnetosomes, intracytoplasmic membrane-bound magnetic crystals. They are phylogenetically diverse Bacteria and include cocci, rods, spirilla, vibrios, and multicellular ensembles of cells, which are known as magnetotactic multicellular prokaryotes or MMPs. ‘Candidatus Magnetoglobus multicellularis’ (CMm) is the best known MMP, despite the lack of pure cultures. It swims in helical, clockwise trajectories, while its body rotates clockwise. At the Earth’s magnetic field, they swim in complex, looping trajectories (walking), but at higher fields their trajectories are parallel to the applied magnetic field (free motion). When they find an obstacle, they remain rotating (rotation) and eventually undergo backward excursions followed by a forward movement (escape motility). In this work, we investigate the transitions between these four types of movement, and the effects of the magnetic field intensity on CMm motility. To obtain CMm, samples of water and sediment were collected in Araruama Lagoon (RJ, Brazil) and maintained in the lab for some months. When needed, a sub-sample was transferred to a specially designed flask containing a lateral capillary aperture, which was placed inside a homemade coil to generate a magnetic field aligned to the capillary aperture. CMm swam towards the capillary and, after 20 minutes, enriched samples were collected and placed between slide and coverslip separated by an O-ring. Samples were observed with a light microscope equipped with a CCD camera. A pair of Helmholtz coils was used to generate vertical magnetic fields of 2.5, 5.0, 10, 20, 40 and 80 Gauss (G). We observed that all CMm performed “walking” at 2.5 G but, at 5 G, rotation motility emerged. The proportion of CMm performing rotation increased at 10 and 20 G, and at 40 and 80 G, all CMm performed “rotation”. Thus, “walking” is probably the main type of movement at the Earth’s magnetic field (0.25-0.75G), whereas “rotation” seems to occur only under stronger magnetic fields (>10G). During “rotation”, CMm always rotated clockwise, which is also the direction of rotation during free motion. For “escape motility”, they rotated counterclockwise, suggesting that it is due to inversion of the flagella rotation direction. Financial support: CNPq.