Chiral Symmetry Breaking and Vortical States in Bacterial Biofilm

Bacterial biofilms are one of the most primordial forms of collective interaction leading to complex coordinated functional communities. Biofilms are observed across the whole bacterial kingdom and play a central role in many biological processes with many repercussions on human life.

But how do biofilms regulate their growth and self-assemble into large-scale dynamical structures? And how can we control them?

In this work we make use of experimental, theoretical and computational techniques to investigate the transition between the swarming and biofilm regime in a system of genetically modified Bacillus subtilis. These mutations lead to longer, highly flagellate bacteria with low tumbling rates. First, we observe that these features result into the formation of large motile clusters of bacteria which can either develop translational or rotational motion according to the internal bacterial organization. Even more importantly, we observe these dynamical states to be chirally biased and systematically rotate in the clockwise direction.

We rationalize these findings through the active gel theory, by observing that the localized coupling between bacterial dynamics and the no-slip substrate results into an effective perturbative torque, which breaks chiral symmetry and ultimately acts as a selection mechanism between two -otherwise equivalent- rotational states.