Bacteria Motility in Porous Media: Not a Random Walk

While bacterial motility is well-studied for motion on flat surfaces or in unconfined liquid media, most bacteria are found in heterogeneous porous media, such as biological gels and tissues, soils, sediments, and subsurface formations. Understanding how confinement alters bacterial motility is therefore critical to model the progression of infections, apply beneficial bacteria for drug delivery, and bioremediation. Unconfined bacteria move via runs and tumbles, leading to random walk-like motion; in a porous medium, previous research has assumed random walk-like motion for bacteria, with a reduced diffusivity due to collisions with obstacles. However, this assumption has never been directly tested due to the inability to visualize processes in opaque 3D media. Here, we directly visualize the motion of single E. coli cells inside a model 3D porous medium, having controlled pore structure. By analyzing the individual cell trajectories, we find that the bacteria do not move via a random walk process. We will present how bacterial motility depends sensitively on pore-scale confinement. Our findings overturn standard assumptions made in the field and provide guidance for the development of more accurate macroscopic models of bacterial motion.