Active matter
Can swimming bacteria spin a perfectly symmetric disc?
Open access · cc by · source: Europe PMC
A bacterium trapped under a symmetric disc makes it rotate because its body and its tail spin in opposite directions, even without pushing on any wall.
Study at a glance
- Design
- Other — Fluorescence microscopy of 3D-printed microdiscs in baths of swimming E. coli, compared against a boundary-element hydrodynamic model with one fitted parameter
- N
- No single N; many pucks and individual bacterial crossings tracked across disc designs (plain discs, four-chamber discs, open-channel discs).
- Population
- Symmetric polymer microdiscs (radius 5, 10 or 20 micrometres) in suspensions of motile E. coli MG1655
- Outcome
- Rotation angle and rotation rate of the discs versus bacterial position, body length and disc size
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Key findings
Plain discs rotated slowly clockwise because bacteria swimming in curved clockwise paths hit their edge asymmetrically, with rotation rate falling as 1/R. A bacterium inside a chamber made the disc spin about ten times faster than a simple wall-pushing estimate predicts, and each extra trapped bacterium sped it up further. In the open channel the disc turned clockwise, then reversed as the cell body left, independent of the swimming direction; longer bacteria gave bigger turns, and the model reproduced this with one fitted factor of about 1.5.
Methodology
The researchers 3D-printed tiny flat discs ('pucks') and let them sink to the bottom of a capillary filled with swimming E. coli. Some discs were plain, some had four dead-end chambers and some had one open channel through the middle that a single bacterium could swim through. They tracked the disc angle while fluorescent bacteria entered, and built a low-Reynolds-number fluid model in which the bacterium is two opposite point torques.
Limitations
The model treats the flagellar bundle as a point torque and the channel as infinitely long, so it misplaces where the rotation reverses; the authors say near-field effects need more work. Only one bacterial species and strain in narrow, gravity-confined geometry was studied, so how important this effect is in dense or natural suspensions is untested. The dipole length is fitted rather than measured directly.
How this study connects
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