Visualization of bacterial flagella by video-enhanced light microscopy

Flagellar rotational features of an optically confined bacterium at high frequency and temporal resolution reveal the microorganism's response to changes in the fluid environment

Rotations of the flagella control the movement of a peritrichous (multiflagellar) bacterium in fluids, the run and tumble events being caused through modulations in the flagella's collective rotation speed and pattern. Observing such modulations is a challenge in free swimming bacteria. In this work, we present a setup to measure the collective flagellar rotational features of an optically confined Bacillus subtilis bacterium. We adopt a Continuous Wavelet Technique (CWT) while monitoring the rotational patterns in frequency and time, thus achieving optimal resolution in both the domains. This enables in marking the events wherein subtle changes in the flagellar rotational pattern occur. These studies unravel a fact, hitherto unknown, that variations in swimming speed that are seen in pure run sequences are also caused by modulations in the rotating flagella. Further, we have monitored the flagellar rotation for durations over a minute and observe a gradual slowing down of the rotation before ceasing completely due to the trapping laser induced photodamage. We have observed a significant alteration in the rate of rotational fall off in real time with changes in pH or the nutrient concentration in the fluid. This work serves to demonstrate the advantage of optical confinement of a bacterium in its pristine form for carrying out such studies and can serve as a marker for work that assesses membrane photodamage in active matter. Details on the role of flagella in propulsion and on other factors influencing the rotations, can be of significance in the design of artificial microswimmers.

The bank of swimming organisms at the micron scale (BOSO-Micro)

Unicellular microscopic organisms living in aqueous environments outnumber all other creatures on Earth. A large proportion of them are able to self-propel in fluids with a vast diversity of swimming gaits and motility patterns. In this paper we present a biophysical survey of the available experimental data produced to date on the characteristics of motile behaviour in unicellular microswimmers. We assemble from the available literature empirical data on the motility of four broad categories of organisms: bacteria (and archaea), flagellated eukaryotes, spermatozoa and ciliates. Whenever possible, we gather the following biological, morphological, kinematic and dynamical parameters: species, geometry and size of the organisms, swimming speeds, actuation frequencies, actuation amplitudes, number of flagella and properties of the surrounding fluid. We then organise the data using the established fluid mechanics principles for propulsion at low Reynolds number. Specifically, we use theoretical biophysical models for the locomotion of cells within the same taxonomic groups of organisms as a means of rationalising the raw material we have assembled, while demonstrating the variability for organisms of different species within the same group. The material gathered in our work is an attempt to summarise the available experimental data in the field, providing a convenient and practical reference point for future studies.

Direct Mapping from Intracellular Chemotaxis Signaling to Single-Cell Swimming Behavior

Bacterial chemotaxis allows bacteria to sense the chemical environment and modulate their swimming behavior accordingly. Although the intracellular chemotaxis signaling pathway has been studied extensively, experimental studies are still lacking that could provide direct link from the pathway output (the intracellular concentration of the phosphorylated form of the response regulator phosphorylated CheY (CheY-P)) to single-cell swimming behavior. Here, we measured the swimming behavior of individual Escherichia coli cells while simultaneously detecting the intracellular CheY-P concentration, thereby providing a direct relationship between the intracellular CheY-P concentration and the single-cell run-and-tumble behavior. The measured relationship is consistent with the ultrasensitivity of the motor switch and a "veto model" that describes the interaction among individual flagella, although contribution from the voting mechanism could not be ruled out.

Bacteria-inspired nanorobots with flagellar polymorphic transformations and bundling

Wirelessly controlled nanoscale robots have the potential to be used for both in vitro and in vivo biomedical applications. So far, the vast majority of reported micro- and nanoscale swimmers have taken the approach of mimicking the rotary motion of helical bacterial flagella for propulsion, and are often composed of monolithic inorganic materials or photoactive polymers. However, currently no man-made soft nanohelix has the ability to rapidly reconfigure its geometry in response to multiple forms of environmental stimuli, which has the potential to enhance motility in tortuous heterogeneous biological environments. Here, we report magnetic actuation of self-assembled bacterial flagellar nanorobotic swimmers. Bacterial flagella change their helical form in response to environmental stimuli, leading to a difference in propulsion before and after the change in flagellar form. We experimentally and numerically characterize this response by studying the swimming of three flagellar forms. Also, we demonstrate the ability to steer these devices and induce flagellar bundling in multi-flagellated nanoswimmers.

Modelling and analysis of bacterial tracks suggest an active reorientation mechanism in Rhodobacter sphaeroides

Most free-swimming bacteria move in approximately straight lines, interspersed with random reorientation phases. A key open question concerns varying mechanisms by which reorientation occurs. We combine mathematical modelling with analysis of a large tracking dataset to study the poorly understood reorientation mechanism in the monoflagellate species Rhodobacter sphaeroides. The flagellum on this species rotates counterclockwise to propel the bacterium, periodically ceasing rotation to enable reorientation. When rotation restarts the cell body usually points in a new direction. It has been assumed that the new direction is simply the result of Brownian rotation. We consider three variants of a self-propelled particle model of bacterial motility. The first considers rotational diffusion only, corresponding to a non-chemotactic mutant strain. Two further models incorporate stochastic reorientations, describing 'run-and-tumble' motility. We derive expressions for key summary statistics and simulate each model using a stochastic computational algorithm. We also discuss the effect of cell geometry on rotational diffusion. Working with a previously published tracking dataset, we compare predictions of the models with data on individual stopping events in R. sphaeroides. This provides strong evidence that this species undergoes some form of active reorientation rather than simple reorientation by Brownian rotation.

Chemotaxis in Campylobacter jejuni

Chemotaxis is the common way of flagellated bacteria to direct their locomotion to sites of most favourable living conditions, that are sites with the highest concentrations of energy sources and the lowest amounts of bacteriotoxic substances. The general prerequisites for chemotaxis are chemoreceptors, a chemosensory signal-transduction system and the flagellar apparatus. Epsilonproteobacteria like Campylobacter sp. show specific variations of the common chemotaxis components. CheV, a CheW-like linking-protein with an additional response regulator (RR) domain, was identified as commonly used coupling scaffold protein of Campylobacter jejuni. It attaches the histidine autokinase (CheAY), which also has an additional RR-domain, to the chemoreceptors signalling domains. These additional RR-domains seem to play an important role in the regulation of the CheAY-phosphorylation state and thereby in sensory adaptation. The Campylobacter-chemoreceptors are arranged into the three groups A, B, and C. Group A contains membrane-anchored receptors sensing periplasmic signals, group B consists only of one receptor with two cytoplasmic ligand-proteins representing a bipartite energy taxis system that senses pyruvate and fumarate, and group C receptors are cytoplasmic signalling domains with mostly unknown cytoplasmic ligand-binding proteins as sensory constituents. Recent findings demonstrating different alleles of the TLP7 chemoreceptor, specific for formic acid, led to an amendment of this grouping.

Label-free optical imaging of membrane patches for atomic force microscopy

In atomic force microscopy (AFM), finding sparsely distributed regions of interest can be difficult and time-consuming. Typically, the tip is scanned until the desired object is located. This process can mechanically or chemically degrade the tip, as well as damage fragile biological samples. Protein assemblies can be detected using the back-scattered light from a focused laser beam. We previously used back-scattered light from a pair of laser foci to stabilize an AFM. In the present work, we integrate these techniques to optically image patches of purple membranes prior to AFM investigation. These rapidly acquired optical images were aligned to the subsequent AFM images to ~40 nm, since the tip position was aligned to the optical axis of the imaging laser. Thus, this label-free imaging efficiently locates sparsely distributed protein assemblies for subsequent AFM study while simultaneously minimizing degradation of the tip and the sample.

Optical microscopy in photosynthesis

Emerging as well as the most frequently used optical microscopy techniques are reviewed and image contrast generation methods in a microscope are presented, focusing on the nonlinear contrasts such as harmonic generation and multiphoton excitation fluorescence. Nonlinear microscopy presents numerous advantages over linear microscopy techniques including improved deep tissue imaging, optical sectioning, and imaging of live unstained samples. Nonetheless, with the exception of multiphoton excitation fluorescence, nonlinear microscopy is in its infancy, lacking protocols, users and applications; hence, this review focuses on the potential of nonlinear microscopy for studying photosynthetic organisms. Examples of nonlinear microscopic imaging are presented including isolated light-harvesting antenna complexes from higher plants, starch granules, chloroplasts, unicellular alga Chlamydomonas reinhardtii, and cyanobacteria Leptolyngbya sp. and Anabaena sp. While focusing on nonlinear microscopy techniques, second and third harmonic generation and multiphoton excitation fluorescence microscopy, other emerging nonlinear imaging modalities are described and several linear optical microscopy techniques are reviewed in order to clearly describe their capabilities and to highlight the advantages of nonlinear microscopy.

Rehinging biflagellar locomotion in a viscous fluid

A means of swimming in a viscous fluid is presented, in which a swimmer with only two links rotates around a joint and then rehinges in a periodic fashion in what is here termed rehinging locomotion. This two-link rigid swimmer is shown to locomote with an efficiency similar to that of Purcell's well-studied three-link swimmer, but with a simpler morphology. The hydrodynamically optimal stroke of an analogous flexible biflagellated swimmer is also considered. The introduction of flexibility is found to increase the swimming efficiency by up to 520% as the body begins to exhibit wavelike dynamics, with an upper bound on the efficiency determined by a degeneracy in the limit of infinite flexibility.

Bacterial flagellar motor

The bacterial flagellar motor is a reversible rotary nano-machine, about 45 nm in diameter, embedded in the bacterial cell envelope. It is powered by the flux of H+or Na+ions across the cytoplasmic membrane driven by an electrochemical gradient, the proton-motive force or the sodium-motive force. Each motor rotates a helical filament at several hundreds of revolutions per second (hertz). In many species, the motor switches direction stochastically, with the switching rates controlled by a network of sensory and signalling proteins. The bacterial flagellar motor was confirmed as a rotary motor in the early 1970s, the first direct observation of the function of a single molecular motor. However, because of the large size and complexity of the motor, much remains to be discovered, in particular, the structural details of the torque-generating mechanism. This review outlines what has been learned about the structure and function of the motor using a combination of genetics, single-molecule and biophysical techniques, with a focus on recent results and single-molecule techniques.

Bacterial motility: handedness and symmetry

Many bacteria swim by rotating thin helical filaments that extend into the external medium, as with common bacteria, or run beneath the outer membrane, as with spirochetes. Each filament is driven at its base by a motor that turns alternately clockwise and counterclockwise. The motor-filament complex is called a flagellum. Other kinds of bacteria glide, but their organelles of locomotion are not known. Since bacteria are microscopic and live in an aqueous environment, they swim at low Reynolds' number; cyclic motion works (e.g. rotation of a helix) but reciprocal motion does not (e.g. stroking of a singly hinged oar). By measuring concentrations of certain chemicals as they move through their environment, making temporal comparisons and modulating the direction of flagellar rotation, bacteria accumulate in regions that they find more favourable. Studies of bacterial chemotaxis are highly advanced, particularly for the peritrichously flagellated species Escherichia coli. A great deal is known about chemoreception, receptor-flagellar coupling and adaptation. Recently it has been found that E. coli can aggregate in response to signals generated by the cells themselves. Complex patterns form with remarkable symmetries.

The rotary motor of bacterial flagella

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.

Real-time imaging of fluorescent flagellar filaments

Bacteria swim by rotating flagellar filaments that are several micrometers long, but only about 20 nm in diameter. The filaments can exist in different polymorphic forms, having distinct values of curvature and twist. Rotation rates are on the order of 100 Hz. In the past, the motion of individual filaments has been visualized by dark-field or differential-interference-contrast microscopy, methods hampered by intense scattering from the cell body or shallow depth of field, respectively. We have found a simple procedure for fluorescently labeling cells and filaments that allows recording their motion in real time with an inexpensive video camera and an ordinary fluorescence microscope with mercury-arc or strobed laser illumination. We report our initial findings with cells of Escherichia coli. Tumbles (events that enable swimming cells to alter course) are remarkably varied. Not every filament on a cell needs to change its direction of rotation: different filaments can change directions at different times, and a tumble can result from the change in direction of only one. Polymorphic transformations tend to occur in the sequence normal, semicoiled, curly 1, with changes in the direction of movement of the cell body correlated with transformations to the semicoiled form.

The bacterial flagella motor

The bacterial flagellum is probably the most complex organelle found in bacteria. Although the ribosome may be made of slightly more subunits, the bacterial flagellum is a more organized and complex structure. The limited number of flagella must be targeted to the correct place on the cell membrane and a structure with cytoplasmic, cytoplasmic membrane, outer membrane and extracellular components must be assembled. The process of controlled transcription and assembly is still not fully understood. Once assembled, the motor complex in the cytoplasmic membrane rotates, driven by the transmembrane ion gradient, at speeds that can reach many 100 Hz, driving the bacterial cell at several body lengths a second. This coupling of an electrochemical gradient to mechanical rotational work is another fascinating feature of the bacterial motor. A significant percentage of a bacterium's energy may be used in synthesizing the complex structure of the flagellum and driving its rotation. Although patterns of swimming may be random in uniform environments, in the natural environment, where cells are confronted with gradients of metabolites and toxins, motility is used to move bacteria towards their optimum environment for growth and survival. A sensory system therefore controls the switching frequency of the rotating flagellum. This review deals primarily with the structure and operation of the bacterial flagellum. There has been a great deal of research in this area over the past 20 years and only some of this has been included. We apologize in advance if certain areas are covered rather thinly, but hope that interested readers will look at the excellent detailed reviews on those areas cited at those points.

Transformations in flagellar structure of Rhodobacter sphaeroides and possible relationship to changes in swimming speed

Rhodobacter sphaeroides is a photosynthetic bacterium which swims by rotating a single flagellum in one direction, periodically stopping, and reorienting during these stops. Free-swimming R. sphaeroides was examined by both differential interference contrast (DIC) microscopy, which allows the flagella of swimming cells to be seen in vivo, and tracking microscopy, which tracks swimming patterns in three dimensions. DIC microscopy showed that when rotation stopped, the helical flagellum relaxed into a high-amplitude, short-wavelength coiled form, confirming previous observations. However, DIC microscopy also revealed that the coiled filament could rotate slowly, reorienting the cell before a transition back to the functional helix. The time taken to reform a functional helix depended on the rate of rotation of the helix and the length of the filament. In addition to these coiled and helical forms, a third conformation was observed: a rapidly rotating, apparently straight form. This form took shape from the cell body out and was seen to form directly from flagella that were initially in either the coiled or the helical conformation. This form was always significantly longer than the coiled or helical form from which it was derived. The resolution of DIC microscopy made it impossible to identify whether this form was genuinely in a straight conformation or was a low-amplitude, long-wavelength helix. Examination of the three-dimensional swimming pattern showed that R. sphaeroides changed speed while swimming, sometimes doubling the swimming speed between stops. The rate of acceleration out of stops was also variable. The transformations in waveform are assumed to be torsionally driven and may be related to the changes in speed measured in free-swimming cells. The roles of and mechanisms that may be involved in the transformations of filament conformations and changes in swimming speed are discussed.

The measurement of swimming velocity of Vibrio cholerae and Pseudomonas aeruginosa using the video tracking methods

The swimming velocities of two monotrichous flagellated bacteria were measured by a computer-assisted video tracking method. Tracing the moving path of the individual bacterium revealed that the bacterial cell did not swim continuously in a straight direction, but frequently changed swimming direction and velocity. The average swimming velocities calculated from the 3-sec path were 75.4 +/- 9.4 microns/sec in four strains of Vibrio cholerae and 51.3 +/- 8.4 microns/sec in five strains of Pseudomonas aeruginosa. These results suggest that V. cholerae swim faster than P. aeruginosa at 30 C in nutrient broth. This method is useful for a detailed analysis of bacterial movement and moving patterns in different environmental conditions.

The unidirectional flagellar motor of Rhodobacter sphaeroides WS8 can rotate either clockwise or counterclockwise: characterization of the flagellum under both conditions by antibody decoration

A variant of Rhodobacter sphaeroides WS8 has been isolated which when tethered has a cell body that rotates counterclockwise instead of clockwise. Antibody decoration in vivo has shown that the filament on the variant has handedness opposite to that of the normal form. In both cases the cell body is pushed by the rotating flagellum.

Biological microscopy: the emergence of digital microscopy

The past few years have seen a tremendous renaissance in biological optical microscopy, mainly as a result of the application of digital image processing and video imaging techniques. We review recent developments in microscopy that are permitting unprecedented views of biological structure and function.

Conformation and elasticity of the isolated red blood cell membrane skeleton

We studied the structure and elasticity of membrane skeletons from human red blood cells (RBCs) during and after extraction of RBC ghosts with nonionic detergent. Optical tweezers were used to suspend individual cells inside a flow chamber, away from all surfaces; this procedure allowed complete exchange of medium while the low-contrast protein network of the skeleton was observed by high resolution, video-enhanced differential interference-contrast (DIC) microscopy. Immediately following extraction in a 5 mM salt buffer, skeletons assumed expanded, nearly spherical shapes that were uncorrelated with the shapes of their parent RBCs. Judging by the extent of thermal undulations and by their deformability in small flow fields, the bending rigidity of skeletons was markedly lower than that of either RBCs or ghosts. No further changes were apparent in skeletons maintained in this buffer for up to 40 min at low temperatures (T less than 10 degrees C), but skeletons shrank when the ionic strength of the buffer was increased. When the salt concentration was raised to 1.5 M, shrinkage remained reversible for approximately 1 min but thereafter became irreversible. When maintained in 1.5 M salt buffer for longer periods, skeletons continued to shrink, lost flexibility, and assumed irregular shapes: this rigidification was irreversible. At this stage, skeletons closely resembled those isolated in standard bulk preparations. We propose that the transformation to the rigid, irreversibly shrunken state is a consequence of spectrin dimer-dimer reconnections and that these structural rearrangements are thermally activated. We also measured the salt-dependent size of fresh and bulk extracted skeletons. Our measurements suggest that, in situ, the spectrin tethers are flexible, with a persistence length of approximately 10 nm at 150 mM salt.

Morphology and dynamics of protruding spirochete periplasmic flagella

We recently characterized the three-dimensional shape of Treponema phagedenis periplasmic flagella (PFs). In the course of these studies, we observed protrusions on swimming cells that resembled PFs. Here we present a detailed characterization of the shape, structure, and motion of these protrusions. Although protrusion formation occurred primarily in wild-type cells during the stationary phase, a large fraction of exponential-phase cells of cell cylinder helicity mutants (greater than 90% of mutant T-52) had protrusions. These results suggest that cells bearing protrusions can still participate in cell division. T. phagedenis protrusions had the identical helix handedness, pitch, and diameter to those of purified PFs. Protrusions were not present on mutants unable to synthesize PFs, but were present in all motile revertants which regained PFs. These results, taken together with electron microscope observations, suggest that protrusions consist of PFs surrounded by an outer membrane sheath. To analyze protrusion movements, we held cells against a coverglass surface with optical tweezers and observed the motion of protrusions by video-enhanced differential interference contrast light microscopy. Protrusions were found to gyrate in both clockwise and counterclockwise directions, and direct evidence was obtained that protrusions rotate. Protrusions were also observed on Treponema denticola and Borrelia burgdorferi. These were also left-handed and had the same helix handedness, pitch, and diameter as purified PFs from their respective species. The PFs from T. denticola had a helix diameter of 0.26 microns and a helix pitch of 0.78 micron; PFs from B. burgdorferi had a helix diameter of 0.28 micron and a helix pitch of 1.48 microns. Protrusions from these spirochete species had similar structures and motion to those of T. phagedenis. Our results present direct evidence that PFs rotate and support previously proposed models of spirochete motility.

Analysis of edge birefringence

We present an experimental and theoretical study of the phenomenon of edge birefringence that appears near boundaries of transparent objects which are observed with high extinction and high resolution polarized light microscopy. As test objects, thin flakes of isotropic KCl crystals were immersed in media of various refractive indices. The measured retardation near crystal edges increased linearly with both the crystal thickness (tested between 0.3 and 1 micron), and the difference in refractive indices n between crystal (n = 1.49) and immersion liquids (n between 1.36 and 1.62). The specific edge birefringence, i.e., the retardation per thickness and per refractive index difference, is 0.029 on the high refractive index side of the boundary and -0.015 on the low refractive index side. The transition through zero birefringence specifies the position of a boundary at a much higher precision than predicted by the diffraction limit of the optical setup. The theoretical study employs a ray tracing procedure modeling the change in phase and polarization of rays passing through the specimen. We find good agreement between the model calculations and the experimental results indicating that edge birefringence can be attributed to the change in polarization of light that is refracted and reflected by dielectric interfaces.

Mutations in the two flagellin genes of Rhizobium meliloti

The previously cloned DNA fragment which complements the behavioral defects of the che-1 and che-3 mutations of Rhizobium meliloti codes for two nearly identical (93%) flagellin genes. A wild-type copy of one of the two genes (flaA) but not the other (flaB) can complement the mutations. The behavior and flagellar morphology of newly isolated strains carrying insertion and deletion mutations or various combinations of these mutations demonstrated that either gene product alone can form functional flagellar filaments but when both gene products are present they interact in the formation of filaments. Both the nucleic acid sequences of the genes and the deduced amino acid sequences of the proteins from strain Rm1021 showed significant differences from the sequences determined previously for strain RU10406. (E. Pleier and R. Schmitt, J. Bacteriol. 171:1467-1475, 1989). The tandem arrangement of the two genes is stable, although in vitro recombination between them gave rise to a strain with wild-type behavior.