Pairwise encounters boost bacterial motion by transient velocity spikes.

For swimming bacteria near surfaces, pairwise encounters inevitably occur and impact their social behavior. However, we know little about how the encounter events influence bacterial dynamics due to the limitations in tracking interplaying bacteria in 3D. Herein, we elucidated the motions of encountering E. coli using a combination of 3D holographic tracking experiments and hydrodynamic simulations. We find encounters with other cells induce transient yet remarkable fluctuations in the swimming speed and angle of E. coli, concurrently diminishing their temporal correlations, in contrast to solitary cells. Notably, bacteria approaching each other in a face-to-face fashion both accelerate, whereas they both decelerate during pursuits. Generally, the motion of a pair of smooth-swimming E. coli is dictated by the relative angle, velocity, and intercellular distance, as validated by hydrodynamic simulations. The presence of the surface mitigates the velocity spikes during the encounter process. Additionally, the encounter process influences the timing of tumbles; i.e., tumble tends to occur before the two bacteria get in close proximity. Despite the impact of one encounter being transient, we reveal that smooth-swimming E. coli gains a propulsion advantage from the encounter, thus providing insights into bacterial physiology and guidance for designing active microdevices.