RESEARCH HIGHLIGHTS Nature Reviews Physics | https://doi.org/10.1038/s42254-020-0172- x | Published online 7 April 2020 ACTIVE MATTER A catalogue of bacterial swarm behaviour Even under adverse conditions length of its cells can be controlled such as starvation, some bacteria by known genetic manipulations. can efficiently expand and move This controllability enabled their colonies by rapidly migrating Be’er et al. to select between four en masse, a process known as cell aspect ratios for their colonies. Credit: Adapted from Be’er, A. et al. Commun. Phys. swarming. Although bacterial They recorded the motion of the 3, 66 (2020), CC BY 4.0 colonies have been studied by colonies on a surface for a range of physicists for years, there is not colony densities. Very sparse colonies of similar self-propelled rods with yet a complete picture of how the of B. subtilis do not move, whereas known interactions indicates that physical properties of the cells, very dense colonies are jammed. the bacterial behaviour is domina- such as their shape, govern the However, for intermediate densities, ted by short-range interactions. behaviour of the swarm. Now, cell aspect ratio plays a role in In contrast to long cells, shorter writing in Communications Physics, the colony’s behaviour. Longer cells swarm in clusters that have Avraham Be’er and colleagues fill cells form high-density clusters uniform density across the surface in another piece of the puzzle, of moving cells that are separated (lower panels of figure). Be’er et al. cataloguing the swarming behaviour by low- density regions containing posit that this uniform density of the rod-shaped Bacillus subtilis, only immobile cells (upper left arises because long-range hydro- as a function of the density of panel of figure). If the mean density dynamic interactions suppress the colony and the aspect ratio of the colony is large enough, inhomogeneities. of the cells. then the moving clusters span the Zoe Budrikis B. subtilis is a bacterium found whole field of view of the experi- ORIGINAL articLE Be’er, A. et al. A phase in soil and the gut of humans and ment (upper right panel of figure). diagram for bacterial swarming. Commun. Phys. 3, 66 (2020) animals. It is well-studied and the Comparison with earlier studies 226 | MAY 2020 | VOLUME 2 www.nature.com/natrevphys.