Bacteria solve the problem of crowding by moving slowly

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Synopsis

O. J. Meacock, A. Doostmohammadi, K. R. Foster, J. M. Yeomans and W. M. Durham
Nature Physics 17, 205-210 (2021).
https://www.nature.com/articles/s41567-020-01070-6

Authors' Abstract:
Bacteria commonly live attached to surfaces in dense collec- tives containing billions of cells. While it is known that motility allows these groups to expand en masse into new territory, how bacteria collectively move across surfaces under such tightly packed conditions remains poorly understood. Here we combine experiments, cell tracking and individual-based mod- elling to study the pathogen Pseudomonas aeruginosa as it col- lectively migrates across surfaces using grappling-hook-like pili. We show that the fast-moving cells of a hyperpilated mutant are overtaken and outcompeted by the slower-moving wild type at high cell densities. Using theory developed to study liquid crystals, we demonstrate that this effect is mediated by the physics of topological defects, points where cells with different orientations meet one another. Our analy- ses reveal that when defects with topological charge +1/2 col- lide with one another, the fast-moving mutant cells rotate to point vertically and become trapped. By moving more slowly, wild-type cells avoid this trapping mechanism and generate collective behaviour that results in faster migration. In this way, the physics of liquid crystals explains how slow bacteria can outcompete faster cells in the race for new territory.