Flagellar elasticity and the multiple swimming modes of interfacial bacteria

In peritrichous bacteria, such as E. coli, flagella join into a compact bundle that is usually assumed to be rigidly connected to the cell body allowing only counter-rotations around a common axis. This simple microswimmer model has been very successful in providing quantitative predictions on swimming behavior in bulk fluids and in the proximity of different kinds of interfaces and confinement. Here, we show that, when bacteria colonize a water-air interface, capillary forces can strongly deform the body-bundle complex, giving rise to unusual and heterogeneous swimming modes. We find that all trajectories can be classified into four main modes, with cells tracing either clockwise or counterclockwise circles while the cell body can be locked to the swimming direction or spin freely. All the observed phenomenology can be reproduced by simply allowing elastic bending of the bundle axis, where stiffness is the main factor in selecting the swimming mode. Our results allow us to experimentally test flexible models of microswimmers in highly perturbed contexts and provide physical insights into the early stages of bacterial pellicles.