Magnetic properties and effective temperature of magnetotactic bacteria

Magnetotactic bacteria are motile prokaryotes that synthesize magnetosomes, which are magnetic single domain crystals surrounded by a lipid membrane. These organelles confer to the cells a permanent magnetic moment that makes them align passively in external magnetic fields and therefore behave as active micro-compasses that swim along magnetic field lines.
We use single-cell tracking experiments based on optical microscopy to study the magnetic properties of these bacteria. For each cell, we first measure the magnetic moment by applying an alternating magnetic field and studying the angular motion of the bacterium during field reversals. We then analyze the statistics of cell alignment with a constant field. Our data show that considering the susceptibility of the magnetosomes is needed to obtain reliable results, therefore we first measure it on dead cells. The orientation histogram of each live bacterium is then fitted to a Boltzmann distribution with temperature as the only fit parameter. A value close to room temperature is obtained for dead cells while a larger effective temperature is needed for live cells, showing that these bacteria are a great tool for measuring non-thermal noise originating from biological processes.