Fig. 2

Bacteria move through 3D porous media via intermittent trapping and hopping. a Ensemble average mean-squared displacement (MSD) as a function of lag time for unconfined bacteria (red) and for bacteria in porous media with increasing amounts of confinement (blue, green, magenta, black). Stars indicate deviation from ballistic motion for unconfined bacteria, or deviation from superdiffusive motion for bacteria in porous media. Legend indicates characteristic pore sizes of the different media. b Rescaling by the crossover length and time scales (stars in a) indicates two regimes of motion for bacteria in porous media: superdiffusive motion at short times with the MSD scaling as τ^1.5, and subdiffusive motion at long times with the MSD scaling as τ^v with the exponent 0 < v ≤ 1 decreasing with pore-scale confinement. This behavior is in stark contrast to simple run-and-tumble motion and instead reflects intermittent trapping of cells as they move. Insets show crossover lengths and times for different media; crossover lengths do not scale linearly with the measured characteristic pore sizes due to pore-size heterogeneity in the media. c Representative single-cell trajectories reveal switching between two modes of motion: hopping, in which bacteria move through extended, directed paths through the pore space, and trapping, in which bacteria are confined for extended periods of time. Insets show time projections of the cell body in the hopping and trapping state; trapped cells continue to reorient their bodies until they can escape and continue to hop through the pore space. Decreasing the pore size decreases the hop lengths, indicated by the green and black trajectories (characteristic pore sizes are 3.6, 2.5, and 1.5 μm from left to right). Scale bar represents 10 μm