  The microaerophilic magnetotactic bacterium Magnetospirillum gryphiswaldense
swims along magnetic field lines using a single flagellum at each cell pole. It
is believed that this magnetotactic behavior enables cells to seek optimal
oxygen concentration with maximal efficiency. We analyse the trajectories of
swimming M. gryphiswaldense cells in external magnetic fields larger than the
earth's field, and show that each cell can switch very rapidly (in < 0.2 s)
between a fast and a slow swimming mode. Close to a glass surface, a variety of
trajectories was observed, from straight swimming that systematically deviates
from field lines to various helices. A model in which fast (slow) swimming is
solely due to the rotation of the trailing (leading) flagellum can account for
these observations. We determined the magnetic moment of this bacterium using a
new method, and obtained a value of (2.0 $\pm$ 0.6) $\times$ $10^{-16}$ Am$^2$.
This value is found to be consistent with parameters emerging from quantitative
fitting of trajectories to our model.
