Microbubbles reveal chiral fluid flows in bacterial swarms
Edited by Tom C. Lubensky, University of Pennsylvania, Philadelphia, PA, and approved December 30, 2010 (received for review November 5, 2010)
Abstract
Flagellated bacteria can swim within a thin film of fluid that coats a solid surface, such as agar; this is a means for colony expansion known as swarming. We found that micrometer-sized bubbles make excellent tracers for the motion of this fluid. The microbubbles form explosively when small aliquots of an aqueous suspension of droplets of a water-insoluble surfactant (Span 83) are placed on the agar ahead of a swarm, as the water is absorbed by the agar and the droplets are exposed to air. Using these bubbles, we discovered an extensive stream (or river) of swarm fluid flowing clockwise along the leading edge of an Escherichia coli swarm, at speeds of order 10 μm/s, about three times faster than the swarm expansion. The flow is generated by the action of counterclockwise rotating flagella of cells stuck to the substratum, which drives fluid clockwise around isolated cells (when viewed from above), counterclockwise between cells in dilute arrays, and clockwise in front of cells at the swarm edge. The river provides an avenue for long-range communication in the swarming colony, ideally suited for secretory vesicles that diffuse poorly. These findings broaden our understanding of swarming dynamics and have implications for the engineering of bacterial-driven microfluidic devices.
Acknowledgments
We thank Peko Hosoi, Daniel Kearns, and Richard Losick for comments on the manuscript, and Linda Turner, Rongjing Zhang, and Junhua Yuan for discussions and help with the experiments. This work was supported by Grants AI065540 and AI016478 from the National Institutes of Health.
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Published online: February 7, 2011
Published in issue: March 8, 2011
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Acknowledgments
We thank Peko Hosoi, Daniel Kearns, and Richard Losick for comments on the manuscript, and Linda Turner, Rongjing Zhang, and Junhua Yuan for discussions and help with the experiments. This work was supported by Grants AI065540 and AI016478 from the National Institutes of Health.
Notes
This article is a PNAS Direct Submission.
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Competing Interests
The authors declare no conflict of interest.
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Microbubbles reveal chiral fluid flows in bacterial swarms, Proc. Natl. Acad. Sci. U.S.A.
108 (10) 4147-4151,
https://doi.org/10.1073/pnas.1016693108
(2011).
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