Crossing and non-crossing dictates the collective swimming of bacteria under two-dimensional confinement

The collective behavior of swimming bacteria in confined spaces, such as pores of the soil and interstices of tissues, is poorly understood. To study the effect of geometric confinement on bacterial collective behavior, we image a suspension of swimming Escherichia coli confined in a Hele-Shaw cell using bright-field microscopy. We find that the emergent collective phase formed by bacteria has two different symmetries depending on the gap thickness of the Hele-Shaw cell. For a gap thickness that is large enough to allow a pair of bacteria to cross over during a collision, the emergent collective phase exhibits long-range nematic order. In contrast, when bacteria are not able to cross over under stronger confinement, they assemble into transient clusters with short-range polar order. We explain the origin of these two different emergent symmetries by tracking the swimming behavior of individual bacteria in the two geometries with different gap thicknesses. Our experiments reveal the unusual dynamics of bacterial suspensions under geometric confinement and demonstrate a simple and effective way to control the emergent collective behavior of active matter.