Phases of active matter composed of multicellular magnetotactic bacteria near a hard surface

Abstract

Multicellular magnetotactic bacteria (MMB) -- the only known obligately multicellular species of bacteria -- use the geomagnetic field to navigate the pore space of water-saturated sediment. To investigate how MMB navigate around obstructions, we orient a magnetic field to direct thousands of MMB toward the hard surface of a microfluidic chamber. MMB in a sub-millitesla field distribute exponentially about the surface as an active gas with a penetration length $\ensuremath{\lambda}$ proportional to the distance a colony swims before aligning with the applied field. At a critical magnetic field $\ensuremath{\lambda}$ shrinks to the size of a single MMB colony and the active gas condenses into an active two-dimensional fluid.