Applying torque to the Escherichia coli flagellar motor using magnetic tweezers

The bacterial flagellar motor of Escherichia coli is a nanoscale rotary engine essential for bacterial propulsion. Studies on the power output of single motors rely on the measurement of motor torque and rotation under external load. Here, we investigate the use of magnetic tweezers, which in principle allow the application and active control of a calibrated load torque, to study single flagellar motors in Escherichia coli. We manipulate the external load on the motor by adjusting the magnetic field experienced by a magnetic bead linked to the motor, and we probe the motor's response. A simple model describes the average motor speed over the entire range of applied fields. We extract the motor torque at stall and find it to be similar to the motor torque at drag-limited speed. In addition, use of the magnetic tweezers allows us to force motor rotation in both forward and backward directions. We monitor the motor's performance before and after periods of forced rotation and observe no destructive effects on the motor. Our experiments show how magnetic tweezers can provide active and fast control of the external load while also exposing remaining challenges in calibration. Through their non-invasive character and straightforward parallelization, magnetic tweezers provide an attractive platform to study nanoscale rotary motors at the single-motor level

Roadmap for Optical Tweezers 2023

Optical tweezers are tools made of light that enable contactless pushing, trapping, and manipulation of objects ranging from atoms to space light sails. Since the pioneering work by Arthur Ashkin in the 1970s, optical tweezers have evolved into sophisticated instruments and have been employed in a broad range of applications in life sciences, physics, and engineering. These include accurate force and torque measurement at the femtonewton level, microrheology of complex fluids, single micro- and nanoparticle spectroscopy, single-cell analysis, and statistical-physics experiments. This roadmap provides insights into current investigations involving optical forces and optical tweezers from their theoretical foundations to designs and setups. It also offers perspectives for applications to a wide range of research fields, from biophysics to space exploration

Contrôle des solitons de cavité et dynamique modale dans les lasers à semiconducteur (étude expérimentale)

Le travail présenté dans cette thèse consiste de l'étude expérimentale de deux systèmes de lasers à semiconducteurs. Dans la première partie, nous étudions la dynamique modale des lasers à émission latérale, dits bulk. L'émission de ces lasers présente en général un seul mode de cavité interne. Dans certaines régions des paramètres, toutefois, on observe bistabilité et mode-hopping'' induit par le bruit entre deux modes de cavité principaux. Nous analysons expérimentalement cette dynamique modale qui peut être décrite en une dimension en termes d'un (quasi-)potentiel bistable et bruit, par une équation de Langevin. On observe que une modulation symétrique du courant de pompage du laser modifie la symétrie d'émission entre les deux modes. Une hypothèse dynamique est donc formulée, faisant intervenir, dans l'équation décrivant le comportement temporel modal, les fluctuations du courant comme un terme de bruit multiplicatif. Dans un tel système il est possible observer le phénomène de résonance stochastique. Enfin, a partir des équations d'évolution des variables du laser, en considérant les échelles temporelles relatives, il a été possible dériver une équation de Langevin mono dimensionnelle, avec bruit multiplicative, qui bien reproduit la caractérisation expérimentale. La deuxième partie est consacrée au contrôle expérimental des solitons de cavité''. Dans ce travail, ces structures localisées non linéaires sont crées dans le plan transverse des lasers à cavité verticale (VCSELs) avec injection externe. Nous étudions leur déplacement sous l'influence de forces externes, montrons les effets des gradients de phase et intensité dans le champs d'injection et démontrons la possibilité de construire un registre à décalage optique en utilisant leurs propriétés. Enfin, en utilisant une masque de phase reconfigurable pour l'injection, on montre que, grâce à leur plasticité, les solitons peuvent être fixés dans des différentes configurations dans le plan transverse, comme prévu par les résultats théoriques. Ces résultats sont encourageants en vue de possibles applications où le solitons de cavité peuvent constituer les bits (1-0) d'un dispositif de stockage et/ou routage optique reconfigurable.NICE-BU Sciences (060882101) / SudocSudocFranceF

Efficient illumination for microsecond tracking microscopy.

The possibility to observe microsecond dynamics at the sub-micron scale, opened by recent technological advances in fast camera sensors, will affect many biophysical studies based on particle tracking in optical microscopy. A main limiting factor for further development of fast video microscopy remains the illumination of the sample, which must deliver sufficient light to the camera to allow microsecond exposure times. Here we systematically compare the main illumination systems employed in holographic tracking microscopy, and we show that a superluminescent diode and a modulated laser diode perform the best in terms of image quality and acquisition speed, respectively. In particular, we show that the simple and inexpensive laser illumination enables less than 1 μs camera exposure time at high magnification on a large field of view without coherence image artifacts, together with a good hologram quality that allows nm-tracking of microscopic beads to be performed. This comparison of sources can guide in choosing the most efficient illumination system with respect to the specific application

The Dynamic Ion Motive Force Powering the Bacterial Flagellar Motor

International audienceThe bacterial flagellar motor (BFM) is a rotary molecular motor embedded in the cell membrane of numerous bacteria. It turns a flagellum which acts as a propeller, enabling bacterial motility and chemotaxis. The BFM is rotated by stator units, inner membrane protein complexes that stochastically associate to and dissociate from individual motors at a rate which depends on the mechanical and electrochemical environment. Stator units consume the ion motive force (IMF), the electrochemical gradient across the inner membrane that results from cellular respiration, converting the electrochemical energy of translocated ions into mechanical energy, imparted to the rotor. Here, we review some of the main results that form the base of our current understanding of the relationship between the IMF and the functioning of the flagellar motor. We examine a series of studies that establish a linear proportionality between IMF and motor speed, and we discuss more recent evidence that the stator units sense the IMF, altering their rates of dynamic assembly. This, in turn, raises the question of to what degree the classical dependence of motor speed on IMF is due to stator dynamics vs. the rate of ion flow through the stators. Finally, while long assumed to be static and homogeneous, there is mounting evidence that the IMF is dynamic, and that its fluctuations control important phenomena such as cell-to-cell signaling and mechanotransduction. Within the growing toolbox of single cell bacterial electrophysiology, one of the best tools to probe IMF fluctuations may, ironically, be the motor that consumes it. Perfecting our incomplete understanding of how the BFM employs the energy of ion flow will help decipher the dynamical behavior of the bacterial IMF

Kinetic analysis methods applied to single motor protein trajectories

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