Dr Marco Mazza - Boundary-interior principle for microbial navigation in geometric confinement

The motion of microbial cells has enormous impact: from swimming sperm cells to bacteria or microalgae colonizing new environments. Upon close observation, this motion often appears erratic, and yet the combination of nonequilibrium forces and confining surfaces can produce striking examples of organization in microbial systems. It remains elusive how and at which length scale self-organization emerges in complex geometries.  In this talk, we will describe experiments and analytical and numerical calculations on the motion of motile cells under controlled microfluidic conditions.  We find that probability flux loops organize active motion, even at the level of a single cell exploring an isolated compartment of nontrivial geometry. By accounting for the interplay of activity and interfacial forces, we find that the boundary’s curvature determines the nonequilibrium probability fluxes of the motion. We theoretically predict a universal relation between fluxes and global geometric properties that is directly confirmed by experiments. Our findings open the possibility to decipher the most probable trajectories of motile cells and may enable the design of geometries guiding their time-averaged motion.