  Synchronization plays a crucial role in the dynamics of living organisms,
from fireflies flashing in unison to pacemaker cells that jointly generate
heartbeats. Uncovering the mechanism behind these phenomena requires an
understanding of individual biological oscillators and the coupling forces
between them. Here, we develop a single-cell assay that studies rhythmic
behavior in the motility of individual E.coli cells that can be mutually
synchronized. Circular microcavities are used to isolate E.coli cells that swim
along the cavity wall, resulting in self-sustained oscillations. Upon
connecting these cavities by microchannels the bacterial motions can be
coupled, yielding nonlinear dynamic synchronization patterns with phase slips.
We demonstrate that the coordinated movement observed in coupled E. coli
oscillators follows mathematical rules of synchronization which we use to
quantify the coupling strength. These findings advance our understanding of
motility in confinement, and lay the foundation for engineering desired
dynamics in microbial active matter.
