Why do swimming bacteria drift sideways in a flowing channel?
The corkscrew shape of E. coli's flagella makes the bacteria drift across the flow at a speed that rises linearly with shear at first and then levels off well below their swimming speed.
Source
Chirality-induced bacterial rheotaxis in bulk shear flows
Study at a glance
- Design
- Other — Microfluidic Poiseuille-flow experiments tracking fluorescent E. coli and passive tracers, compared with Brownian dynamics simulations and an analytical orientation model.
- N
- No single N; many bacterial tracks recorded at several flow rates and heights in one channel geometry, plus simulated ensembles (e.g. 1000 averaged trajectories for orientation maps).
- Population
- Wild-type run-and-tumble Escherichia coli (strain RP437) swimming in a PDMS microchannel
- Outcome
- Mean rheotactic (vorticity-direction) drift velocity and distributions of swimming velocity and orientation versus local shear rate
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What they did
The authors pumped a dilute suspension of swimming E. coli through a microchannel 600 μm wide and 100 μm high and tracked thousands of bacteria and passive tracer beads at different heights and flow rates. They measured how fast the bacteria drifted perpendicular to the flow and how their swimming directions were distributed. They then built a model that adds a chirality-induced rotation to the classic Jeffery rotation of elongated bodies plus rotational noise and tumbling, simulated it, and analysed its fixed points.
What they found
Bacteria drifted to one side in the lower half of the channel and the other side in the upper half, with drift growing with local shear rate and saturating at around half the free swimming speed. Plotting drift against local shear rate collapsed data from different heights and flow rates, and the simulations matched both the means and the full velocity and orientation distributions once the chiral strength was set to 0.06. A dimensionless chirality number (shear rate times chiral strength divided by effective rotational diffusion) collapsed simulations with different parameters at low shear, giving a drift length of about 0.75 μm per unit shear rate. The saturation arises because the sideways orientation is only a marginally stable fixed point, so bacteria circle around it instead of locking in.
The limits
What it doesn't show
The chiral strength and aspect ratio were fitted to the same experimental data the model is compared against, so the agreement is partly by construction. Experiments only see 2D projections of 3D trajectories, and at the highest flow rates the camera could not resolve tracks, so the high-shear regime relies on simulations. Only one wild-type strain in one channel height was used, trajectories near the walls were excluded, and the scaling collapse for tumbling versus non-tumbling bacteria was not exact.
Key terms
- Rheotaxis
- Reorientation or drift of a swimmer in response to a flow; here the sideways drift of bacteria along the vorticity direction.
- Poiseuille flow
- Pressure-driven flow in a channel with a parabolic velocity profile: fastest in the centre, zero at the walls, so shear rate is highest near the walls.
- Jeffery orbit
- The periodic tumbling rotation of an elongated particle in a shear flow.
- Chiral strength
- A single number encoding how strongly the helical flagella and the cell body combine to rotate the bacterium in response to shear.
- Marginally stable fixed point
- An equilibrium where perturbations neither grow nor decay, so trajectories orbit around it instead of settling onto it.
- Rotational Péclet number
- Shear rate divided by rotational diffusion rate; compares how strongly flow rotates a particle versus how fast noise randomises it.
Flashcards
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Quiz yourself
What physical feature of E. coli does the model credit for the sideways drift?
Common questions
Why don't the bacteria eventually swim straight sideways at full speed?
The sideways-facing orientation is only marginally stable, so without noise bacteria orbit around it, and with noise the orientation distribution stays broad; the average drift therefore approaches the swimming speed only very slowly.
Do bacteria need a wall nearby to drift sideways?
No. The drift was measured in the bulk of the channel away from the top and bottom walls, and the model attributes it to the helical flagella shape, not to surface interactions.
Why does the drift direction flip between the upper and lower halves of the channel?
The local shear rate changes sign across the channel centre, and the chirality-induced rotation depends on the sign of the shear, so the drift reverses.
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