Microbubbles reveal chiral fluid flows in bacterial swarms

Rowland Institute at Harvard and Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA 02138, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 02/2011; 108(10):4147-51. DOI: 10.1073/pnas.1016693108
Source: PubMed


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.

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    • "In fact, water is commonly observed in swarms formed by gram-negative bacteria, such as Escherichia coli and Salmonella enterica (Chen et al., 2007; Zhang et al., 2010; Wu et al., 2011; Ping et al., 2014). It is known that an E. coli swarm has water extending about 30 µm ahead of the edge of the swarm (Wu and Berg, 2012), and flagellar motion has been shown to induce fluid flows in E. coli swarms, pointing to the presence of substantial amounts of water (Wu et al., 2011). Fluid osmolarity within a swarm remains high, allowing water in the agar to be extracted, and a recent study found that an E. coli swarm has a high-osmolarity region at the edge and a low-osmolarity region within (Ping et al., 2014). "
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    • "Data from combined phase contrast and epifluorescent microscopy suggest that the observed flow patterns are generated by the action of CCW rotating flagella of cells stuck to the substratum. The results provide a mechanistic insight into the hydrodynamics that contribute to swarm expansion and suggest an avenue for longrange communication in the swarming colony by fluid flows (Wu et al., 2011 "
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