Individual to Collective Dynamics of Swimming Bacteria

Spatial order and fast collective coherent dynamics of populations of the swimming bacteria Bacillus subtilis emerges from local interactions and from flows generated by the organisms’ locomotion. The transition from dilute, to intermediate, to high concentrations of cells is analyzed and presented as probability density functions for swimming velocity. The low concentration phase, which exhibits swimming speeds characteristic of individual bacteria, arrives at the anomalously high speed phase,the ZoomingBioNematic (ZBN), via an intermediate phase that exhibits surprisingly low mean speeds. We show that these low speeds at intermediate concentrations are due to transitional speeds that occur after collisions of the organisms, while the flagella that propel the bacteria re-form into a “bundle”. Measurement of individual and collective velocities, as well as correlation of speeds with alignment of velocity directions, within and adjacent to coherent patches, were found by Particle Imaging Velocimetry (PIV). The significance for mixing of the ZBN dynamic is demonstrated.