  Motility is a fundamental survival strategy of bacteria to navigate porous
environments. Swimming cells thrive in quiescent wetlands and sediments at the
bottom of the marine water column, where they mediate many essential
biogeochemical processes. While swimming motility in bulk fluid is now well
established, a comprehensive understanding of the mechanisms regulating
self-transport in the confined interstices of porous media is lacking, and
determining the interactions between cells and surfaces of the solid matrix
becomes paramount. Here, we precisely track the movement of bacteria
(\emph{Magnetococcus marinus}) through a series of microfluidic porous media
with broadly varying geometries and show that cell motility results in a
succession of scattering events from the porous microstructure. Order or
disorder can impact the cells' motility over short ranges, but we directly
demonstrate that their large-scale transport properties are regulated by the
cutoff of their persistent swimming, which is dictated primarily by the
porosity and scale of the porous geometry. The effective mean free path is
established as the key geometrical parameter controlling transport, and along
with with minimal knowledge of cell swimming motility and surface scattering
properties, we implement a theoretical model that universally predicts the
effective diffusion of cells for the geometries studied here. These results are
an important step toward predicting the physical ecology of swimming cells in
quiescent porous media and understanding their role in environmental and health
hazards in stagnant water.
