Trait-specific dispersal of bacteria in heterogeneous porous environments: from pore to porous medium scale

The dispersal of organisms controls the structure and dynamics of populations and communities, and can regulate ecosystem functioning. Predicting dispersal patterns across scales is important to understand microbial life in heterogeneous porous environments such as soils and sediments. We developed a multi-scale approach, combining experiments with microfluidic devices and time-lapse microscopy to track individual bacterial trajectories and measure the overall breakthrough curves and bacterial deposition profiles: we, then, linked the two scales with a novel stochastic model. We show that motile cells of Pseudomonas putida disperse more efficiently than non-motile mutants through a designed heterogeneous porous system. Motile cells can evade flow-imposed trajectories, enabling them to explore larger pore areas than non-motile cells. While transported cells exhibited a rotation in response to hydrodynamic shear, motile cells were less susceptible to the torque, maintaining their body oriented towards the flow direction and thus changing the population velocity distribution with a significant impact on the overall transport properties. We also found, in a separate set of experiments, that if the suspension flows through a porous system already colonized by a biofilm, P. putida cells are channelled into preferential flow paths and the cell attachment rate is increased. These two effects were more pronounced for non-motile than for motile cells. Our findings suggest that motility coupled with heterogeneous flows can be beneficial to motile bacteria in confined environments as it enables them to actively explore the space for resources or evade regions with unfavourable conditions. Our study also underlines the benefit of a multi-scale approach to the study of bacterial dispersal in porous systems.