To cross or not to cross: Collective swimming of Escherichia coli under two-dimensional confinement

Abstract

Bacteria suspended in fluids swim collectively and display fascinating emergent dynamics. Although bacterial collective swimming in bulk suspensions has been well studied, its counterpart in confined two-dimensional (2D) geometries relevant to many natural habitats of bacteria is still poorly understood. Here, through carefully designed experiments on Escherichia coli in a Hele-Shaw geometry, we show that a small change in the degree of confinement leads to a drastic change in bacterial collective swimming. While a long-range nematic order emerges for bacteria that can cross over each other during encounters, a slight decrease of the confining height that prevents the crossing leads to the formation of bacterial clusters with a short-range polar order. By tracking the swimming kinetics of individual bacteria, we reveal the microscopic origins of the two emergent collective phases and illustrate the effect of the collective dynamics on the swimming behaviors of single bacteria. Our study provides insights into bacterial collective swimming under confinement and demonstrates a simple way to control the emergent symmetry of collective phases.