Hydrodynamic interactions in bundles of bacterial flagella*

Flagellated swimming is an important and well-studied aspect of prokaryotic life. It is also the more "socially-distanced" form of bacterial motility, usually taking place in the bulk of the medium, compared to gliding which takes place on crowded surfaces. Bacterial flagella are composed of a slender and semi-rigid helical filament, a flexible hook, and a molecular motor which rotates continuously in order to escape the reversibility of Stokes flow and push the cell forward. The molecular motor may change the sense of rotation to induce a change in swimming direction, either through a run-reverse or run-reverse-flick mechanism for bacteria with a single flagellum, or run-and-tumble in the case of multi-flagellated bacteria like E. coli. Apart from this simple, stochastic control over the molecular motors, bacteria cannot control the individual position of flagella, so much of their swimming behaviour results naturally from steric and hydrodynamic interactions with nearby surfaces, between the flagella and cell body, or amongst the flagella themselves. We will present novel theoretical findings regarding the effect of inter-filament hydrodynamic interactions on thrust generation and synchronization in bundles of bacterial flagella.