(443h) Bacterial Swarm Simulations Highlight the Roles of Hydrodynamic Interactions, Cell Morphology and Steric Interactions on Emergent Patterns

Bacterial swarming is a rapid, multicellular mode of flagella-based motility observed in multiple model bacteria species including S. marcescens and P. mirabilis. Swarming involves a high degree of cell-cell coordination that manifests in the emergence of collective structures such as rafting, clustering, and spatiotemporally organized velocity fields, enabling rapid colonization and procurement of nutrients. Recent experimental studies by us and others on dense bacteria swarms suggest that collective features are controlled by cell aspect ratio and morphology, cell self-propulsion, and cell-cell hydrodynamic interactions. Acting in isolation or synergistically, these biophysical aspects control the emergence, the intensity and lifetime of swarm features. It is experimentally difficult to dissect and quantify the effects of each in dividual factor on swarming. This motivates the use of detailed agent-based modeling -- where interaction modalities may be switched off or turned on – in combination with experiments to understand swarm initiation and evolution. Here, we combine experimental observations on swarming Proteus mirabilis and Serratia marcescens with agent-based dynamical simulations that incorporate short-range steric interactions, cell geometry effects as well as long-range Stokesian hydrodynamic interactions to interpret experimental results and explore parametric regimes that are difficult to access experimentally. The simulations treat bacterial cells as self-propelling dipolar structures moving in a quasi-two-dimensional space and interacting with each other. These idealized cells may all have the same intrinsic properties or may have different properties as seen for instance in two-species bidisperse swarms. Our computational work capture features and structures consistent with our experimental observations on bacterial swarms. As swarming bacteria increase their aspect ratio, clustering and rafting of cells are promoted in both the dilute and dense phases. In homogenous systems, increasing the cell aspect ratio – an effect seen when cells switch to their swarming phenotypes - increases the size and persistence time of rafts and clusters. Conversely simulations at aspect ratios comparable to the shorter, wild-type cells do not form efficient clusters and rafts. The lifetime of the clusters as well as typical cluster sizes and polarity within each cluster are dominated by short-range steric interactions and aspect ratio. Hydrodynamic interactions meanwhile result in active and persistent reorientation of these aligned clusters resulting in mixing and the formation of spatiotemporally aligned fluctuating vortical flows and jets. Rafting is enhanced by weak fluid mediated interactions but hindered for strong interactions as mixing breaks up aligned clusters and reduces cluster size. We hypothesize that bacteria may swarm most efficiently when there is a balance between cell-cell and cell-fluid interactions. Combining experimental observations and simulation analysis, we conclude that steric and hydrodynamic interactions enable and impact different aspects of collective dynamics, cell, and swarm statistics. Acting synergistically, the overall effect is to significantly enhance swarming rates and mixing thereby providing an efficient means to colony expansion.