  Many bacteria share the fascinating ability to sense Earth's magnetic field
-- a process known as magnetotaxis. These bacteria synthesize magnetic
nanoparticles, called magnetosomes, within their own cell body and arrange them
to form a linear magnetic chain. The chain, which behaves like a compass
needle, aligns the microorganisms with the geomagnetic field. Here, we measure
the magnetic hysteresis of an individual bacterium of the species
Magnetospirillum gryphiswaldense via ultrasensitive torque magnetometry. These
measurements, in combination with transmission electron microscopy and
micromagnetic simulations, reveal the magnetic configurations of the
magnetosomes, their progression as a function of applied field, as well as the
total remanent magnetic moment and effective magnetic anisotropy of a chain
within a single bacterium. Knowledge of magnetic properties is crucial both for
understanding the mechanisms behind magnetotaxis and for the design of systems
exploiting magnetotactic bacteria in biomedical applications.
