<i>E</i> . <i>coli</i> bacterium tumbling in bulk and close to surfaces: a simulation study

Motility is fundamental to the survival and proliferation of microorganisms. The <i>E. coli</i> bacterium propels itself using a bundle of rotating helical flagella. If one flagellum reverses its rotational direction, it leaves the bundle, performs a polymorphic transformation, and the bacterium tumbles. The <i>E. coli</i> bacterium is hydrodynamically attracted to surfaces. This prolongs its residence time, while tumbling facilitates surface detachment. We develop a model of <i>E. coli</i> that uses an extended Kirchhoff-rod theory to implement flagellar flexibility as well as different polymorphic conformations and perform hydrodynamic simulations with the method of multi-particle collision dynamics (MPCD). To establish a reference case, we determine the distribution of tumble angles in the bulk fluid. It shows good agreement with experiments, when we always choose the same tumble time. Increasing the hook stiffness, narrows the tumble angle distribution and reduces the flagellar dispersion during tumbling. Close to a bounding surface, the tumble angle distribution is shifted to smaller angles, while flagellar dispersion is reduced. Reorientation within the plane favors the forward direction, which might be an explanation for prolonged run times observed in experiments.