Collective behaviors of motile bacteria

Swimming bacteria are among the best characterized examples of biological active matter. Most early studies of bacterial motility focused on movement of individual bacterial cells at low densities, where interactions among bacteria could be neglected. However, motile bacteria can also exhibit a range of collective behaviors. As shown by the example of the model Escherichia coli, chemical or physical interactions between swimming bacteria can lead to emergence of different collective phenomena. Here we first focus on swirling collective motion at high densities of bacteria in microfluidic channels of different height and show that this motion could be primarily explained by hydrodynamic interactions between swimmers. We further demonstrate that collective swimming impedes bacterial ability to follow chemical gradients. We also investigate heterogeneous mixtures of motile and non-motile bacteria, demonstrating that non-motile bacteria can exhibit non-equilibrium dynamic clustering in presence of even small fraction of motile bacteria in the population.