Swarming is a specialized form of bacterial motility that develops when cells that can swim are grown in a rich medium on a moist surface. The cells become multinucleate, elongate, synthesize large numbers of flagella, excrete wetting agents, and advance across the surface in coordinated packs. Most studies of swarming have sought to define the developmental processes leading from the vegetative to the swarming state. Such studies ask why cells swarm.
Our aim i s to understand how cells swarm. The questions to be answered are pertinent not only to basic flagellar mechanics near a surface, but also to larger ramifications of this process, such as the group behavior of cells during surface colonization, including pattern generation and biofilm formation. Cell spreading is an important mechanism that helps bacteria establish infections. If enough can be learned about what cells do, then it should be possible to accurately model the process and perhaps even interfere with it by novel means. In prior work we videotaped cells of the model organism Escherichia coli in small microscopic fields (42 x 57 μm), identified every cell by hand, and measured 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. We extended this work to larger spatial scales by automated tracking of individual cells and groups of cells. We distinguished four kinds of tracks, defining stalls, reversals, lateral movement, and forward movement. We fluorescently-labeled flagella of cells exhibiting such movement and identified a remarkable maneuver wherein cells back up by swimming through the middle of the flagellar bundle, a process promoted by changes in shape of flagellar filaments. This trick enables cells to escape from confined environments. We found that the swarm-air interface is covered by a stationary surfactant monolayer, i.e., that cells do not swarm at a free surface but rather in a thin film of liquid between two fixed surfaces, a talent clearly useful for invading tissue. We discovered an extended stream of fluid flowing CW (when viewed from above) ahead of the swarm at its leading edge, a potential channel for long-range communication. We propose 1) to compare the behavior of smooth-swimming mutants to wild-type cells to determine whether cell reversals are essential for swarming, 2) to complete measurements of the thickness and motion of the fluid in front of the swarm and extend these measurements to the body of the swarm, and 3) to learn whether fluid is drawn from the underlying agar by osmotic flow. We expect to understand the importance of flagellar reversals, the nature of the extra fluid seen in front of a swarm, and the role that this fluid plays in swarm expansion.
We are trying to understand how flagellated bacteria move over moist surfaces, i.e., the mechanics of bacterial swarming. This motion enables pathogenic bacteria to invade and colonize human tissue or medical devices, and thus is an important factor in the development of disease.
| Wu, Yilin; Berg, Howard C (2012) Water reservoir maintained by cell growth fuels the spreading of a bacterial swarm. Proc Natl Acad Sci U S A 109:4128-33 |
| Wu, Yilin; Hosu, Basarab G; Berg, Howard C (2011) Microbubbles reveal chiral fluid flows in bacterial swarms. Proc Natl Acad Sci U S A 108:4147-51 |
