Robust edge flows in bacterial colonies: a topological insulator in living matter?

Microorganisms frequently display chiral motility at individual level. For example, bacteria with flagella, appendages used for movement, exhibit helical swimming patterns. Curiously, at collective level, this inherent handedness often appears to be absent.

Now, new work shows that in growing colonies of curved-body bacteria, cells self-organize into a wide flow all along the typically tortuous centimeter-scale external boundary that is always clockwise, while standard turbulent collective motion without manifest chirality is seen in their interior. The robustness and definite chirality of the boundary flows is reminiscent of the edge modes actively researched in condensed matter physics, and marks another irruption of modern physics concepts in living matter.

Edge flows in slowly growing colonies of swarming Paenibacillus vortex bacteria. (a) global view, note the contorted external boundary and the interior islands. (b) close up showing local alignment of cells. (c) transport of passive tracer particles, note the long-distance displacement along the edge of particles 1 and 2. (d) velocity field of bacteria (streamlines colored by local speed), note the strong flow near the boundary.

Experimental results show how the outer boundaries of the growing colony self-assemble into a wide, consistently clockwise flow, while counterclockwise flows arise along internal boundaries such as ‘islands’ not yet invaded by the colony. This is similar to the quantum Hall effect and related phenomena, where individual particles all perform chiral motion with the same chirality. But here, detailed experiments in sparse conditions show that individual cells typically perform both clockwise and counterclockwise motion, with only a small very small, hardly measurable bias in favor of counterclockwise trajectories. This small cell-level bias itself remains mysterious –although it is probably due to the chiral architecture of flagella—but at collective level it is strikingly amplified along boundaries and erased in the bulk.

Crosstalk between the experimentalists and the theorists involved have guided these experiments and led to the construction of a mathematical model that reproduces all the phenomena uncovered. This hybrid model describes the collective swimming of elongated particles in a fluid is simple enough to simulate millions of swimmers and to be the starting point of a full-blown theory of such chiral living matter.

These robust edge flows could play a role in the long-distance transport of information and nutrients through bacterial colonies.