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 behaviour

Even under adverse conditions length of its cells can be controlled such as starvation, some 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. . 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 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 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

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