Dynamics of bacterial swarming.

When vegetative bacteria that can swim are grown in a rich medium on an agar surface, they become multinucleate, elongate, synthesize large numbers of flagella, produce wetting agents, and move across the surface in coordinated packs: they swarm. We examined the motion of swarming Escherichia coli, comparing the motion of individual cells to their motion during swimming. Swarming cells' speeds are comparable to bulk swimming speeds, but very broadly distributed. Their speeds and orientations are correlated over a short distance (several cell lengths), but this correlation is not isotropic. We observe the swirling that is conspicuous in many swarming systems, probably due to increasingly long-lived correlations among cells that associate into groups. The normal run-tumble behavior seen in swimming chemotaxis is largely suppressed, instead, cells are continually reoriented by random jostling by their neighbors, randomizing their directions in a few tenths of a second. At the edge of the swarm, cells often pause, then swim back toward the center of the swarm or along its edge. Local alignment among cells, a necessary condition of many flocking theories, is accomplished by cell body collisions and/or short-range hydrodynamic interactions.

Visualization of Flagella during bacterial Swarming.

When cells of Escherichia coli are grown in broth and suspended at low density in a motility medium, they swim independently, exploring a homogeneous, isotropic environment. Cell trajectories and the way in which these trajectories are determined by flagellar dynamics are well understood. When cells are grown in a rich medium on agar instead, they elongate, produce more flagella, and swarm. They move in coordinated packs within a thin film of fluid, in intimate contact with one another and with two fixed surfaces, a surfactant monolayer above and an agar matrix below: they move in an inhomogeneous, anisotropic environment. Here we examine swarm-cell trajectories and ways in which these trajectories are determined by flagellar motion, visualizing the cell bodies by phase-contrast microscopy and the flagellar filaments by fluorescence microscopy. We distinguish four kinds of tracks, defining stalls, reversals, lateral movement, and forward movement. When cells are stalled at the edge of a colony, they extend their flagellar filaments outwards, moving fluid over the virgin agar; when cells reverse, changes in filament chirality play a crucial role; when cells move laterally, they are pushed sideways by adjacent cells; and when cells move forward, they are pushed by flagellar bundles in the same way as when they are swimming in bulk aqueous media. These maneuvers are described in this report.

Bacterial flagella are firmly anchored.

There are mutants of Salmonella enterica (with mutations in fliF and fliL) that shed flagella when they are swimming in a viscous medium or on the surface of soft agar. Filaments with hooks and the distal rod segment FlgG are recovered. We tried to extract flagellar filaments from such cells by pulling on them with an optical trap but failed, even when we used forces large enough to straighten the filaments. Thus, flagella are firmly anchored.

Force-extension measurements on bacterial flagella: triggering polymorphic transformations.

Bacterial flagella can adopt several different helical shapes in response to varying environmental conditions. A geometric model by Calladine ascribes these discrete shape changes to cooperative transitions between two stable tertiary structures of the constituent protein, flagellin, and predicts an ordered set of 12 helical states called polymorphic forms. Using long polymers of purified flagellin, we demonstrate controlled, reversible transformations between different polymorphic forms. While pulling on a single filament using an optical tweezer, we record the progressive transformation of the filament and also measure the force-extension curve. Both normal and coiled polymorphic forms stretch elastically with a bending stiffness of 3.5 pN x microm(2). At a force threshold of 4-7 pN or 3-5 pN (for normal and coiled forms, respectively), a fraction of the filament suddenly transforms to the next, longer, polymorphic form. This transformation is not deterministic because the force and amount of transformation vary from pull to pull. In addition, the force is highly dependent on stretching rate, suggesting that polymorphic transformation is associated with an activation energy.

On torque and tumbling in swimming Escherichia coli.

Bacteria swim by rotating long thin helical filaments, each driven at its base by a reversible rotary motor. When the motors of peritrichous cells turn counterclockwise (CCW), their filaments form bundles that drive the cells forward. We imaged fluorescently labeled cells of Escherichia coli with a high-speed charge-coupled-device camera (500 frames/s) and measured swimming speeds, rotation rates of cell bodies, and rotation rates of flagellar bundles. Using cells stuck to glass, we studied individual filaments, stopping their rotation by exposing the cells to high-intensity light. From these measurements we calculated approximate values for bundle torque and thrust and body torque and drag, and we estimated the filament stiffness. For both immobilized and swimming cells, the motor torque, as estimated using resistive force theory, was significantly lower than the motor torque reported previously. Also, a bundle of several flagella produced little more torque than a single flagellum produced. Motors driving individual filaments frequently changed directions of rotation. Usually, but not always, this led to a change in the handedness of the filament, which went through a sequence of polymorphic transformations, from normal to semicoiled to curly 1 and then, when the motor again spun CCW, back to normal. Motor reversals were necessary, although not always sufficient, to cause changes in filament chirality. Polymorphic transformations among helices having the same handedness occurred without changes in the sign of the applied torque.

Influence of topology on bacterial social interaction.

The environmental topology of complex structures is used by Escherichia coli to create traveling waves of high cell density, a prelude to quorum sensing. When cells are grown to a moderate density within a confining microenvironment, these traveling waves of cell density allow the cells to find and collapse into confining topologies, which are unstable to population fluctuations above a critical threshold. This was first observed in mazes designed to mimic complex environments, then more clearly in a simpler geometry consisting of a large open area surrounding a square (250 x 250 microm) with a narrow opening of 10-30 microm. Our results thus show that under nutrient-deprived conditions bacteria search out each other in a collective manner and that the bacteria can dynamically confine themselves to highly enclosed spaces.