Bacteria have developed a large array of motility mechanisms to exploit available resources and environments. These mechanisms can be broadly classified into swimming in aqueous media and movement ...over solid surfaces. Swimming motility involves either the rotation of rigid helical filaments through the external medium or gyration of the cell body in response to the rotation of internal filaments. On surfaces, bacteria swarm collectively in a thin layer of fluid powered by the rotation of rigid helical filaments, they twitch by assembling and disassembling type IV pili, they glide by driving adhesins along tracks fixed to the cell surface and, finally, non-motile cells slide over surfaces in response to outward forces due to colony growth. Recent technological advances, especially in cryo-electron microscopy, have greatly improved our knowledge of the molecular machinery that powers the various forms of bacterial motility. In this Review, we describe the current understanding of the physical and molecular mechanisms that allow bacteria to move around.
A complete description of the swimming behavior of a bacterium requires measurement of the displacement and orientation of the cell body together with a description of the movement of the flagella. ...We rebuilt a tracking microscope so that we could visualize flagellar filaments of tracked cells by fluorescence. We studied Escherichia coli (cells of various lengths, including swarm cells), Bacillus subtilis (wild-type and a mutant with fewer flagella), and a motile Streptococcus (now Enterococcus). The run-and-tumble statistics were nearly the same regardless of cell shape, length, and flagellation; however, swarm cells rarely tumbled, and cells of Enterococcus tended to swim in loops when moving slowly. There were events in which filaments underwent polymorphic transformations but remained in bundles, leading to small deflections in direction of travel. Tumble speeds were ∼2/3 as large as run speeds, and the rates of change of swimming direction while running or tumbling were smaller when cells swam more rapidly. If a smaller fraction of filaments were involved in tumbles, the tumble intervals were shorter and the angles between runs were smaller.
Multisubunit protein complexes are ubiquitous in biology and perform a plethora of essential functions. Most of the scientific literature treats such assemblies as static: their function is assumed ...to be independent of their manner of assembly, and their structure is assumed to remain intact until they are degraded. Recent observations of the bacterial flagellar motor, among others, bring these notions into question. The torque-generating stator units of the motor assemble and disassemble in response to changes in load. Here, we used electrorotation to drive tethered cells forward, which decreases motor load, and measured the resulting stator dynamics. No disassembly occurred while the torque remained high, but all of the stator units were released when the motor was spun near the zero-torque speed. When the electrorotation was turned off, so that the load was again high, stator units were recruited, increasing motor speed in a stepwise fashion. A model in which speed affects the binding rate and torque affects the free energy of bound stator units captures the observed torque-dependent stator assembly dynamics, providing a quantitative framework for the environmentally regulated self-assembly of a major macromolecular machine.
Flagellated bacteria, such as
Escherichia coli
, swim by rotating thin
helical filaments, each driven at its base by a reversible rotary motor,
powered by an ion flux. A motor is about 45 nm in ...diameter and is assembled
from about 20 different kinds of parts. It develops maximum torque at stall but
can spin several hundred Hz. Its direction of rotation is controlled by a
sensory system that enables cells to accumulate in regions deemed more
favorable. We know a great deal about motor structure, genetics, assembly, and
function, but we do not really understand how it works. We need more crystal
structures. All of this is reviewed, but the emphasis is on function.
Mechanosensing by flagella is thought to trigger bacterial swarmer-cell differentiation, an important step in pathogenesis. How flagellar motors sense mechanical stimuli is not known. To study this ...problem, we suddenly increased the viscous drag on motors by a large factor, from very low loads experienced by motors driving hooks or hooks with short filament stubs, to high loads, experienced by motors driving tethered cells or 1-μm latex beads. From the initial speed (after the load change), we inferred that motors running at very low loads are driven by one or at most two force-generating units. Following the load change, motors gradually adapted by increasing their speeds in a stepwise manner (over a period of a few minutes). Motors initially spun exclusively counterclockwise, but then increased the fraction of time that they spun clockwise over a time span similar to that observed for adaptation in speed. Single-motor total internal reflection fluorescence imaging of YFP–MotB (part of a stator force-generating unit) confirmed that the response to sudden increments in load occurred by the addition of new force-generating units. We estimate that 6–11 force-generating units drive motors at high loads. Wild-type motors and motors locked in the clockwise or counterclockwise state behaved in a similar manner, as did motors in cells deleted for the motor protein gene fliL or for genes in the chemotaxis signaling pathway. Thus, it appears that stators themselves act as dynamic mechanosensors. They change their structure in response to changes in external load. How such changes might impact cellular functions other than motility remains an interesting question.
Many bacteria use the flagellum for locomotion and chemotaxis. Its bidirectional rotation is driven by a membrane-embedded motor, which uses energy from the transmembrane ion gradient to generate ...torque at the interface between stator units and rotor. The structural organization of the stator unit (MotAB), its conformational changes upon ion transport, and how these changes power rotation of the flagellum remain unknown. Here, we present ~3 Å-resolution cryoelectron microscopy reconstructions of the stator unit in different functional states. We show that the stator unit consists of a dimer of MotB surrounded by a pentamer of MotA. Combining structural data with mutagenesis and functional studies, we identify key residues involved in torque generation and present a detailed mechanistic model for motor function and switching of rotational direction.
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•Structure of MotAB flagellar stator unit in different functional states by cryo-EM•5:2 stoichiometry is conserved across the MotAB/PomAB family•Conformational changes upon protonation•MotB2 drives rotation of surrounding MotA5, which engages the rotor to generate torque
Structures of the MotAB stator unit reveal how its conformational changes, coupled to ion transport, provide torque to power the rotation of the bacterial flagellum.
Motility is important for the survival and dispersal of many bacteria, and it often plays a role during infections. Regulation of bacterial motility by chemical stimuli is well studied, but recent ...work has added a new dimension to the problem of motility control. The bidirectional flagellar motor of the bacterium
recruits or releases torque-generating units (stator units) in response to changes in load. Here, we show that this mechanosensitive remodeling of the flagellar motor is independent of direction of rotation. Remodeling rate constants in clockwise rotating motors and in counterclockwise rotating motors, measured previously, fall on the same curve if plotted against torque. Increased torque decreases the off rate of stator units from the motor, thereby increasing the number of active stator units at steady state. A simple mathematical model based on observed dynamics provides quantitative insight into the underlying molecular interactions. The torque-dependent remodeling mechanism represents a robust strategy to quickly regulate output (torque) in response to changes in demand (load).
Cells swimming in confined environments are attracted by surfaces. We measure the steady-state distribution of smooth-swimming bacteria (Escherichia coli) between two glass plates. In agreement with ...earlier studies, we find a strong increase of the cell concentration at the boundaries. We demonstrate theoretically that hydrodynamic interactions of the swimming cells with solid surfaces lead to their reorientation in the direction parallel to the surfaces, as well as their attraction by the closest wall. A model is derived for the steady-state distribution of swimming cells, which compares favorably with our measurements. We exploit our data to estimate the flagellar propulsive force in swimming E. coli.
Flavobacterium johnsoniae, a rod-shaped bacterium, glides over surfaces at speeds of ∼2 μm/s. The propulsion of a cell-surface adhesin, SprB, is known to enable gliding. We used cephalexin to ...generate elongated cells with irregular shapes and followed their displacement in three dimensions. These cells rolled about their long axes as they moved forward, following a right-handed trajectory. We coated gold nanoparticles with an SprB antibody and tracked them in three dimensions in an evanescent field where the nanoparticles appeared brighter when they were closer to the glass. The nanoparticles followed a right-handed spiral trajectory on the surface of the cell. Thus, if SprB were to adhere to the glass rather than to a nanoparticle, the cell would move forward along a right-handed trajectory, as observed, but in a direction opposite to that of the nanoparticle.
Cells of Flavobacterium johnsoniae, a rod-shaped bacterium devoid of pili or flagella, glide over glass at speeds of 2–4 μm/s 1. Gliding is powered by a protonmotive force 2, but the machinery ...required for this motion is not known. Usually, cells move along straight paths, but sometimes they exhibit a reciprocal motion, attach near one pole and flip end over end, or rotate. This behavior is similar to that of a Cytophaga species described earlier 3. Development of genetic tools for F. johnsoniae led to discovery of proteins involved in gliding 4. These include the surface adhesin SprB that forms filaments about 160 nm long by 6 nm in diameter, which, when labeled with a fluorescent antibody 2 or a latex bead 5, are seen to move longitudinally down the length of a cell, occasionally shifting positions to the right or the left. Evidently, interaction of these filaments with a surface produces gliding. To learn more about the gliding motor, we sheared cells to reduce the number and size of SprB filaments and tethered cells to glass by adding anti-SprB antibody. Cells spun about fixed points, mostly counterclockwise, rotating at speeds of 1 Hz or more. The torques required to sustain such speeds were large, comparable to those generated by the flagellar rotary motor. However, we found that a gliding motor runs at constant speed rather than at constant torque. Now, there are three rotary motors powered by protonmotive force: the bacterial flagellar motor, the Fo ATP synthase, and the gliding motor.
•The gliding motor, a novel rotary motor, spins tethered F. johnsoniae cells•The gliding motor generates high torque•The gliding motor runs at constant speed rather than at constant torque
Shrivastava et al. describe a novel rotary motor that spins tethered gliding cells. It generates high torque and runs at constant speed. The catalog of biological rotary motors now contains three motors powered by protonmotive force: the Fo ATP synthase, the bacterial flagellar motor, and the gliding motor.
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