Collective motion of living creatures is found in nature at all scales, from the tiny bacterial swarms to large mammal herds. In most large intelligent animals, such as in schools of fish and flocks of birds, the individuals communicate within a hierarchical organization, thus benefit from the collective dynamics. Bacteria may also benefit from the collective motion by relying on several different mechanisms including hydrodynamic interactions and short repulsive steric effects. Collectively, the cells show increased motility, facilitating expansion into new territories, and even enhanced resistance to antibiotics. While swarming, the cells move in whirls and jets where hundreds of individuals swim in dynamic clusters.

Despite the intensive work on swarming, only theoretical works suggested that cell aspect ratio affects swarm dynamics, but how and whether it dictates the motion in real colonies is still unclear.

In our experimental study we used 9 variants of Bacillus subtilis of different cell aspect ratio. From the optical flow of the moving cells we have studied the velocity distribution, the temporal and spatial correlations, and the vorticity. The results showed that wild type cells, and similar in size strains, exhibit the fastest mean speed, a Gaussian velocity and vorticity distribution, and an exponential decay in the spatial and temporal correlations. However, the long and short cells exhibited a non-Gaussian velocity and vorticity distribution with slower speeds, and a non-trivial decay of the temporal correlation, indicating a non-Markovian process and possible intricate interactions between cells.

Our results suggest that cell aspect ratio, close to the one obtained for wild type cells, will yield the best results as far as it concerns successful spreading and food foraging for both the individual and the group.