Dynamics of Magnetotactic Bacteria in a Rotating Magnetic Field

Abstract

Magnetotactic bacteria (MTB) contain chains of ferrimagnetic nanoparticles which allow them to align with external magnetic fields and self-propel along the field lines. They are a simple yet elegant example of biological interaction with the Earth’s magnetic field. In this thesis, I study the dynamics of MTB in a uniform, rotating magnetic field at a range of rotational frequencies. I show that the cells’ rotational dynamics are in qualitative agreement with the theoretical dynamics of a self-propelled magnetic dipole in a uniform, rotating field. I measure the cells’ swim speed, the curvature along their trajectory, and their orientation relative to the magnetic field. I compare each of these quantities to theoretical models, confirming the predicted relationships between rotational period, swim speed, and trajectory curvature. Finally, I apply a novel method to estimate the cells’ magnetic moments and drag coefficients using their orientational fluctuations, and discuss the advantages and limitations of this method.