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Swarming Escherichia coliIntroductionWhen grown on 0.45% Eiken agar in rich medium, cells of E. coli elongate, produce more flagella, and move over the surface of the agar in a coordinated manner. The chemotaxis system in not thought to be required. As you will see in the movies, taken at 30 degrees C, the cells swirl about in rafts or packs. At the edge of the swarm they form a monolayer. At the very edge, cells are nearly stationary. Farther back, closer to the point of inoculation, cells pile up in multilayers and are very active. The videos were made in bright-phase contrast. Smoke particles on the top surface near the swarm edge were visualized in dark field by adding a fiber-optic illuminator oriented about 10 degrees from horizontal. They diffuse locally but are not perturbed by the swarming cells, which shows that the surface of the swarm is stationary. We think it is covered by a surfactant monolayer pinned at its edges. The smoke particles were about 0.2 microns in diameter. The cells were about 1 micron in diameter by 5 microns long.Cells in phase contrastAn E. coli swarmSmoke particles in darkfield and cells in phase contrastSwarm overtaking smoke particleSwarm overtaking a second smoke particle TrackingEach avi file is 1 s of video recorded at 30 frames/s. To play back at slow speed, choose 1/2x in QuickTime A/V Controls. The computer display is an overlay of the phase image where each cell is identified by a line running from its head to tail, colored either by cell identity (e.g., Edge by identity.avi) or by cell heading (Edge by heading.avi). The latter is shown in Figure 1. The files are organized by region of the swarm recorded, listed at the top of Figure 2. Figure 1. Swarm cell tracking:
The upper image is a phase-contrast video image of an E. coli swarm. The lower image shows the computer display colored (in this case) according to the cell's heading. For example, cells moving toward the right are magenta, toward the top blue, toward the left green, and toward the bottom yellow. The coloring changes as a cell's trajectory changes but each cell's identity remains fixed. Tracks were plotted for every cell and their features compared. Figure 2. Across the swarm:
The first row shows phase-contrast images of a swarm, where each image is taken farther from the swarm edge, from right (swarm edge, 0 mm) to left (plateau 2, ~1 mm). The second row is the computer display for the same region, colored as in Figure 1, summed over five successive frames. Cells in fluorescenceA few fluorescently-labeled cells were mixed in with unlabeled cells, and simultaneous phase-contrast and fluorescence images (with laser illumination at 496 nm) were recorded at 30 frames/s. It was not possible to distinguish all of the different regions of the swarm, so movies are designated as either Edge or Monolayer. Interesting behaviors observed included cell reversals and interactions between different flagellar bundles. ReferencesZhang, R., Turner, L. and Berg, H.C. The upper surface of an Escherichia coli swarm is stationary. PNAS 107: 288-290 (2010). Darnton, N.C., Turner, L, Rojevsky, S., and Berg, H.C. Dynamics of bacterial swarming. Biophys J. (2010), in press. Turner, L., Zhang, R., Darnton, N.C., and Berg, H.C. Visualization of flagella during bacterial swarming. J. Bacteriol. (2010), in press.
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Overview
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Last modified Sunday, April 18, 2010.
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