An E. coli swarm.
Escherichia coli. Swarming is a common yet specialized form of surface translocation exhibited by flagellated bacteria, distinct from swimming. When grown on a moist nutrient-rich surface, cells differentiate from a vegetative to a swarm state: they elongate, make more flagella, secrete wetting agents, and move across the surface in coordinated packs. In our lab, we focus on the mechanics of bacterial swarming, as exhibited by the model organism Escherichia coli. The chemotaxis system is thought not to be required. At the edge of the swarm E. coli forms a monolayer where cells move in packs. More centrally, closer to the point of inoculation, cells pile up in multilayers and move in active swirls.
How do cells in E. coli swarms move across an agar surface? Nick Darnton and Linda Turner sought to answer this question by performing a global analysis of videotaped data (of phase-contrast images) collected from 5 regions of 2 swarms, plotting body lengths, speeds, propulsion angles, local track curvatures, and temporal and spatial correlations, finding that cells reorient on the time scale of a few tenths of a second, primarily by colliding with one another.
What are their flagella doing? Using flourescent labeling of flagella Linda found that most of the time, swarm cells are driven forwards by a flagellar bundle in the usual way. Flagellar filaments from different cells can intertwine and form common bundles, but this is rare. However, cells in swarms do something not ordinarily seen with swimming cells: they back up. They do this without changing the orientation of the cell body by moving back through the middle of the flagellar bundle. This involves changes in filament shape (in polymorphic form), from normal, to curly, and back to normal. The changes observed with swarm cells are driven by application of torque, i.e. when motors switch from CCW to CW. When swimming cells tumble, polymorphic transformations also occur, in the order normal, semi-coiled, curly, and back to normal. But we rarely see the semi-coiled form with cells in swarms, and when it appears it is quite transient. We wonder whether polymorphic transformations evolved to enable cells to escape when trapped in confined environments, when the only way out is to back up, keeping the filaments close to the sides of the cell body.
Rongjing Zhang found that the upper surface of a swarm is stationary. If small particles, i.e. smoke particles, are put on the top of the swarm then visualized in dark-field, the particles diffuse locally but are not perturbed by the cells swarming. We think the surface of the swarm is covered by a surfactant monolayer pinned at its edges.
Yilin Wu studied fluid motions ahead of swarms and within swarms. Uptake of fluid from the underlying agar appears to be driven by cell growth, presumably through local perturbations of osmolarity, a hypothesis currently under test.
Zhang, R., Turner, L., and Berg, H.C. The upper surface of an Escherichia coli swarm is stationary. Proc. Nat'l Acad. Sci. USA 5, 288-290 (2010).
Darnton, N.C., Turner, L., Rojevsky, S., and Berg, H.C. Dynamics of bacterial swarming. Biophys. J., 98, 2082-2090 (2010).
Turner, L., Zhang, R., Darnton, N., and Berg, H.C. Visualization of flagella during bacterial swarming. J. Bacteriol. 192, 3259-3267 (2010).
Wu, Y., Hosu, B.G., and Berg, H.C. Microbubbles reveal chiral fluid flows in bacterial swarms. Proc. Natl. Acad. Sci. USA 108, 4147-4151 (2011).
Wu, Y., and Berg, H.C. A water reservoir maintained by cell growth fuels the spreading of a bacterial swarm. Proc. Natl. Acad. Sci. USA 109, 4128-4133 (2012).