Collective movement in bacteria EU-funded microbiologists investigated the movement of a single bacterial cell within a swarming colony in three dimensions. Fundamental Research © Kristijan Puljek, Shutterstock Bacterial ‘swarming’ is the most rapid method by which motile bacteria are able to colonise surfaces. The 3D SWARMING (Complex dynamics of swarming bacteria in three dimensions) project provided new and innovative insights into several aspects of swarming bacteria. Researchers first studied the collective movement of the Gram-negative bacteria Serratia marcescens, which formed a dense culture of spherical cells. The bacteria were found to migrate to the upper surface of drops of culture placed on a glass slide, moving in correlated whirls and jets for several seconds. This indicated that steric forces are not essential for the generation of collective motion of self-propelled particles. The team investigated the dynamics of the rod-shaped swarming bacteria Bacillus subtilis on agar plates. When antibiotics were applied to some of the plates to determine how the swarm reacted to stress, the bacteria moved differently than for other forms of stress such as starvation, dryness and oxygen depletion. A mathematical model revealed that the swarm comprised two populations – non-affected cells and cells with defective motility caused by the antibiotic kanamycin. Interactions between the two populations explained the difference in swarming. In addition, scientists analysed the trajectories of fluorescently labelled individual bacteria as they were pushed by their neighbours in the dense swarm. It was found that the cells performed super-diffusion, consistent with Levy walk, a mix of long straight trajectories and short, random movements. This suggested that this strategy may have evolved considerably earlier than previously thought. Finally, study of the two different rod-shaped species (B, subtilis and S. marcescens) helped determine the independence of the motion of a single cell embedded in the swarm and whether it followed the flow, or was able to ‘decide’ and move on its own. Results showed that the bacteria move in almost every direction and are not aligned to the flow. However, immotile cells that were embedded in the active swarm served as tracers, did follow the flow. 3D SWARMING therefore demonstrated how motile cells can manoeuvre to enhance spreading and dissemination. Keywords Bacteria, swarming, 3D SWARMING, super-diffusion, Levy walk
