Bacterial motility dynamics in open and confined porous media

Diverse processes such as bioremediation, biofertilization, and microbial drug delivery rely on bacterial migration in porous media. However, how pore-scale confinement alters bacterial motility is unknown due to the inherent physical heterogeneities on porous media. As a result, models of migration are limited and often employ ad hoc assumptions.

We aim to determine the impact of pore confinement in the spreading dynamics of two populations of motile metal-reducing bacteria by directly visualizing individual Acidovorax and Pelosinus in an unconfined liquid medium and a microfluidic chip containing arrays of pillars placed at regular intervals. We observe that the length of runs of the two species differs between the unconfined and confined medium. Results show that bacteria in the confined medium display systematic shorter jumps due to grain obstacles when compared to the open porous medium. Close inspection of the trajectories reveals that cells are intermittently and transiently trapped, producing superdiffusive motion at early and subdiffusion behavior at late times as they navigate through the confined pore spaces. While in the open medium, we observe a linearly increasing variance with respect to time for Acidovorax, and for Pelosinus the variance increases at a much faster rate showing superdiffusive behavior at early times. At late times, the rate of growth in spreading increases for Acidovorax while it reduces for Pelosinus. We finally discuss that the paradigm of run-and-tumble motility is dramatically altered in confined porous medium, which can have strong implications for large-scale transport.