Single Cell Dynamics Drive Turbulent Flow In The Collective Motion Of Bacteria

In most areas of biology, the principal confounding factor is the complexity. Biology from the cellular level to the ecosystem involves the action of a multitude of individuals that come together and achieve specific tasks. For example, at the single cell level, the binding and unbinding of cytoskeletal proteins conspire to allow a cell to move across surfaces or through the extracellular matrix. At the tissue level, these motile cells act together to heal wounds or form cancer metastases. Tissues come together to form organisms, which form societies, and so on. Here we use dense communities of swimming bacteria to understand how collective behavior arises out of the actions of an individual. At high density, rod-shaped bacteria produce complex fluid flows that include vortices and jets. These flows arise partially due to the dipole forces that each bacterium exerts on the surrounding fluid. By confining Bacillus subtilis or Escherichia coli within Hele-Shaw cells of controllable depth, we probe how individual biophysical parameters, such as shape, speed, external drag, and chemotaxis, affect the resulting collective behavior in this system. We then compare our results to predictions from a two-phase fluid model that is based on the single-cell physics of a swimming bacterium. Comparison of our experimental and simulation results show that the collective behavior in this system is largely determined by the biophysics of the single organism. The physics of these dense communities has many similarities to actomyosin systems, as well as to collective systems of epithelial cells. Therefore, our results are likely broadly applicable to a wide range of problems in cell migration, from the single cell to the collective.