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We propose a simple, cost-effective method for synchronized phase contrast and fluorescence video acquisition in live samples. Counter-phased pulses of phase contrast illumination and fluorescence excitation light are synchronized with the exposure of the two fields of an interlaced camera sensor. This results in a video sequence in which each frame contains both exposure modes, each in half of its pixels. The method allows real-time acquisition and display of synchronized and spatially aligned phase contrast and fluorescence image sequences that can be separated by de-interlacing in two independent videos. The method can be implemented on any fluorescence microscope with a camera port without needing to modify the optical path.

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The molecular cascade that controls switching of the direction of rotation of Escherichia coli flagellar motors is well known, but the conformational changes that allow the rotor to switch are still unclear. The signaling molecule CheY, when phosphorylated, binds to the C-ring at the base of the rotor, raising the probability that the motor spins clockwise. When the concentration of CheY-P is so low that the motor rotates exclusively counterclockwise (CCW), the C-ring recruits more monomers of FliM and tetramers of FliN, the proteins to which CheY-P binds, thus increasing the motor's sensitivity to CheY-P and allowing it to switch once again. Motors that rotate exclusively CCW have more FliM and FliN subunits in their C-rings than motors that rotate exclusively clockwise. How are the new subunits accommodated? Does the diameter of the C-ring increase, or do FliM and FliN get packed in a different pattern, keeping the overall diameter of the C-ring constant? Here, by measuring fluorescence anisotropy of yellow fluorescent protein-labeled motors, we show that the CCW C-rings accommodate more FliM monomers without changing the spacing between them, and more FliN monomers at the same time as increasing their effective spacing and/or changing their orientation within the tetrameric structure.

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Most bacteria that swim, including Escherichia coli, are propelled by helical filaments, each driven at its base by a rotary motor powered by a proton or a sodium ion electrochemical gradient. Each motor contains a number of stator complexes, comprising 4MotA 2MotB or 4PomA 2PomB, proteins anchored to the rigid peptidoglycan layer of the cell wall. These proteins exert torque on a rotor that spans the inner membrane. A shaft connected to the rotor passes through the peptidoglycan and the outer membrane through bushings, the P and L rings, connecting to the filament by a flexible coupling known as the hook. Although the external components, the hook and the filament, are known to rotate, having been tethered to glass or marked by latex beads, the rotation of the internal components has remained only a reasonable assumption. Here, by using polarized light to bleach and probe an internal YFP-FliN fusion, we show that the innermost components of the cytoplasmic ring rotate at a rate similar to that of the hook.

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Using Escherichia coli as a model organism, we studied how water is recruited by a bacterial swarm. A previous analysis of trajectories of small air bubbles revealed a stream of fluid flowing in a clockwise direction ahead of the swarm. A companion study suggested that water moves out of the agar into the swarm in a narrow region centered ∼ 30 µm from the leading edge of the swarm and then back into the agar (at a smaller rate) in a region centered ∼ 120 µm back from the leading edge. Presumably, these flows are driven by changes in osmolarity. Here, we utilized green/red fluorescent liposomes as reporters of osmolarity to verify this hypothesis. The stream of fluid that flows in front of the swarm contains osmolytes. Two distinct regions are observed inside the swarm near its leading edge: an outer high-osmolarity band (∼ 30 mOsm higher than the agar baseline) and an inner low-osmolarity band (isotonic or slightly hypotonic to the agar baseline). This profile supports the fluid-flow model derived from the drift of air bubbles and provides new (to our knowledge) insights into water maintenance in bacterial swarms. High osmotic pressure at the leading edge of the swarm extracts water from the underlying agar and promotes motility. The osmolyte is of high molecular weight and probably is lipopolysaccharide.

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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.

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In the bacterial chemotaxis network, receptor clusters process input, and flagellar motors generate output. Receptor and motor complexes are coupled by the diffusible protein CheY-P. Receptor output (the steady-state concentration of CheY-P) varies from cell to cell. However, the motor is ultrasensitive, with a narrow operating range of CheY-P concentrations. How the match between receptor output and motor input might be optimized is unclear. Here we show that the motor can shift its operating range by changing its composition. The number of FliM subunits in the C-ring increases in response to a decrement in the concentration of CheY-P, increasing motor sensitivity. This shift in sensitivity explains the slow partial adaptation observed in mutants that lack the receptor methyltransferase and methylesterase and why motors show signal-dependent FliM turnover. Adaptive remodelling is likely to be a common feature in the operation of many molecular machines.

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Flagellated bacteria can swim within a thin film of fluid that coats a solid surface, such as agar; this is a means for colony expansion known as swarming. We found that micrometer-sized bubbles make excellent tracers for the motion of this fluid. The microbubbles form explosively when small aliquots of an aqueous suspension of droplets of a water-insoluble surfactant (Span 83) are placed on the agar ahead of a swarm, as the water is absorbed by the agar and the droplets are exposed to air. Using these bubbles, we discovered an extensive stream (or river) of swarm fluid flowing clockwise along the leading edge of an Escherichia coli swarm, at speeds of order 10 µm/s, about three times faster than the swarm expansion. The flow is generated by the action of counterclockwise rotating flagella of cells stuck to the substratum, which drives fluid clockwise around isolated cells (when viewed from above), counterclockwise between cells in dilute arrays, and clockwise in front of cells at the swarm edge. The river provides an avenue for long-range communication in the swarming colony, ideally suited for secretory vesicles that diffuse poorly. These findings broaden our understanding of swarming dynamics and have implications for the engineering of bacterial-driven microfluidic devices.

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Effective cryopreservation of oocytes is critically needed in many areas of human reproductive medicine and basic science, such as stem cell research. Currently, oocyte cryopreservation has a low success rate. The goal of this study was to understand the mechanisms associated with oocyte cryopreservation through biophysical means using a mouse model. Specifically, we experimentally investigated the biomechanical properties of the ooplasm prior and after cryopreservation as well as the consequences of reversible dismantling of the F-actin network in mouse oocytes prior to freezing. The study was complemented with the evaluation of post-thaw developmental competence of oocytes after in vitro fertilization. Our results show that the freezing-thawing process markedly alters the physiological viscoelastic properties of the actin cytoskeleton. The reversible depolymerization of the F-actin network prior to freezing preserves normal ooplasm viscoelastic properties, results in high post-thaw survival and significantly improves developmental competence. These findings provide new information on the biophysical characteristics of mammalian oocytes, identify a pathophysiological mechanism underlying cryodamage and suggest a novel cryopreservation method.

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Membrane nanotubes, under physiological conditions, typically form en masse. We employed magnetic tweezers (MTW) to extract tethers from human brain tumor cells and compared their biophysical properties with tethers extracted after disruption of the cytoskeleton and from a strongly differing cell type, Chinese hamster ovary cells. In this method, the constant force produced with the MTW is transduced to cells through super-paramagnetic beads attached to the cell membrane. Multiple sudden jumps in bead velocity were manifest in the recorded bead displacement-time profiles. These discrete events were interpreted as successive ruptures of individual tethers. Observation with scanning electron microscopy supported the simultaneous existence of multiple tethers. The physical characteristics, in particular, the number and viscoelastic properties of the extracted tethers were determined from the analytic fit to bead trajectories, provided by a standard model of viscoelasticity. Comparison of tethers formed with MTW and atomic force microscopy (AFM), a technique where the cantilever-force transducer is moved at constant velocity, revealed significant differences in the two methods of tether formation. Our findings imply that extreme care must be used to interpret the outcome of tether pulling experiments performed with single molecular techniques (MTW, AFM, optical tweezers, etc). First, the different methods may be testing distinct membrane structures with distinct properties. Second, as soon as a true cell membrane (as opposed to that of a vesicle) can attach to a substrate, upon pulling on it, multiple nonspecific membrane tethers may be generated. Therefore, under physiological conditions, distinguishing between tethers formed through specific and nonspecific interactions is highly nontrivial if at all possible.

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