Abstract
Diverse processes—e.g. bioremediation, biofertilization, and microbial drug delivery—rely on bacterial migration in disordered, three-dimensional (3D) porous media. However, how pore-scale confinement alters bacterial motility is unknown due to the opacity of typical 3D media. As a result, models of migration are limited and often employ ad hoc assumptions. Here we reveal that the paradigm of run-and-tumble motility is dramatically altered in a porous medium. By directly visualizing individual Escherichia coli, we find that the cells are intermittently and transiently trapped as they navigate the pore space, exhibiting diffusive behavior at long time scales. The trapping durations and the lengths of “hops” between traps are broadly distributed, reminiscent of transport in diverse other disordered systems; nevertheless, we show that these quantities can together predict the long-time bacterial translational diffusivity. Our work thus provides a revised picture of bacterial motility in complex media and yields principles for predicting cellular migration.
Highlights
Diverse processes—e.g. bioremediation, biofertilization, and microbial drug delivery—rely on bacterial migration in disordered, three-dimensional (3D) porous media
Our results overturn the assumption that the bacteria exhibit run-andtumble motility with shorter runs; instead, we find a different form of motility in which individual cells are intermittently and transiently trapped as they move through the pore space
We find that bacteria do not exhibit run-and-tumble motility with runs shortened by confinement, as is commonly assumed
Summary
Diverse processes—e.g. bioremediation, biofertilization, and microbial drug delivery—rely on bacterial migration in disordered, three-dimensional (3D) porous media. Peritrichous bacteria are propelled by a rotating bundle of flagella along ballistic runs of mean speed 〈vr〉 and length 〈Lr〉; these are punctuated by rapid tumbles, arising when flagella spontaneously unbundle, that randomly reorient the cells This run-and-tumble motion is diffusive over time scales larger than the run duration, with a translational diffusivity given by 〈vr〉〈Lr〉/316. A common assumption is that the bacteria continue to perform runs with a mean run speed 〈vr〉, but with a shorter length hLr′i < hLri due to collisions with the solid matrix of the medium These are thought to reorient the cells in a manner similar to tumbles, leading to a decreased diffusivity hvrihLr′i=317–21. Our work provides a revised picture of bacterial motility in 3D porous media and yields principles for predicting cellular migration over large time and length scales
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