Advection-dominated transport dynamics of pili and flagella-mediated motile bacteria in porous media

The transport of motile bacteria in porous media is highly relevant to many fields, ranging from ecology to human health. Still, critical gaps remain in our understanding of the impacts of hydrodynamics and pore structure on bacterial transport. Here, we present direct visualizations of three species of motile bacteria under variable flow rates and porosities. We find that at higher flow rates, motility is less critical to the transport of bacteria, as motion is controlled by hydrodynamic advection, making it difficult for bacteria to move across streamlines. We show that this lack of motion across streamlines results in increased velocity autocorrelation and bacterial spreading in the direction of flow. Furthermore, we find that transport of bacteria with different motility types are impacted by flow rates to different extents. At low flow rates, the transport of bacteria with pili-mediated twitching motility is strongly controlled by advection, whereas bacteria with flagella still display active motility. At higher flow rates, we show that bacteria with peritrichous flagella maintain their motility characteristics to a greater degree than bacteria with pili or monotrichous flagella. We also examine experimental net speeds of bacteria in relation to the simulated flow fields and find that the interactions between hydrodynamics, motility, and porous media geometry lead to oversampling of medium-velocity regions of a pore network by all three species. The study presents new perspectives on how different types of motile bacteria are transported and dispersed in porous media aided by strength of differentially advecting fluid.