Multicellular Magnetotactic Bacteria under an Applied Magnetic Field Form Active Crystals

Multicellular Magnetotactic Bacteria (MMB) of the species Magnetoglobus multicellularis live in spherical colonies composed of 10-50 individual bacteria. These bacteria are the known known obligately multicellular bacteria. The colony swims as a single unit parallel to the Earth’s magnetic field. When a magnetic field is oriented normal to a glass surface, aggregates accumulate into a monolayer on the glass surface. As the magnitude of magnetic field increases, the density of the colonies increases. At a critical field strength, the mean free path of the colonies shrinks to the radius of a single colony. The colonies display a crystalline packing. Unlike previous examples of active crystals (e.g., with colloids and fast swimming bacteria), these bacteria spontaneously detach and reincorporate into the structure at rates dependent on the strength of the applied field. As a result, active crystals composed of MMB display numerous vacancies. We describe the dynamics is this new state of active matter and compare them to active crystals and active super-critical fluids.