Bacterial Swarming: Role of Flagella in Emergent Behavior

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Flagellated bacteria exhibit distinct environmental dependent modes of locomotion. In a liquid environment, individual (planktonic) cells exhibit the so-called swimming motility. A different mode of motion is denoted as bacterial swarming, where bacteria migrate collectively over surfaces (in biofilms) and are able to form stable aggregates, which can become highly motile. Bacteria swarms are densely packed and exhibit large-scale swirling and streaming motions. Various bacteria strains show distinctly different morphologies in the swarming mode compared to swimmer cells as they are more elongated by suppression of cell division and their number of flagella is significantly increased. Hence, it is expected that flagella play a major role in the swarming behavior, aside from simply providing the propulsion of a bacterium. However, so far little is known on how bacteria use flagella to migrate across surfaces and their impact on swarming.In this project, we want to elucidate the properties of flagella in bacteria assemblies and the role in their emergent collective behavior. Moreover, we want to unravel the influence of a viscoelastic fluid on the swimming properties of individual cells as well as on swarming. Thereby, we will apply mesoscale hydrodynamic simulations, combining the multiparticle collision dynamics approach for the (viscoelastic) fluid with molecular dynamics simulations for the embedded bacteria. To address viscoelastic effects, we will systematically increase the complexity of a microswimmer from a spherical squirmer through a swimming E. coli cell up to swarmer cells and resolve their specificities. Two types of flagellated cells will be studied in order to shed light on the influence of the flagellum number on swarming: E. coli-like swarmer cells with on the order of 20 flagella and P. mirabilis-like cells with about 200 flagella. Specific attention will be paid to bundle formation of flagella. Here, we will perform systematic studies on the influence of the flagella-driving torque on flagella bundles and cell migration for small cells. The collective migration behavior of E. coli-type cells is distinctly different from that of very elongated cells. For E. coli-type cells, we will study the migration behavior of large assemblies of cells (rafts) with particular emphasise on the role of body shape, flagella, and hydrodynamic interactions. Intercellular flagella interactions are most important for P. mirabilis-like cells, as shown in the previous funding period. We will systematically analyse the appearing interwoven flagella bundles between adjacent cells. We want to understand the dynamics of the bundles and the way cells are collectively propelled. In addition, we are interested in the collective behavior of rafts and larger assemblies and want to resolve the role of flagella on the raft structure and motility. This comprises merging (splitting) of individual cells with rafts and the formation of intercellular bundles.