Movements of Mycoplasma mobile Gliding Machinery Detected by High-Speed Atomic Force Microscopy

Rheotaxis in Mycoplasma gliding

This review describes the upstream-directed movement in the small parasitic bacterium Mycoplasma. Many Mycoplasma species exhibit gliding motility, a form of biological motion over surfaces without the aid of general surface appendages such as flagella. The gliding motility is characterized by a constant unidirectional movement without changes in direction or backward motion. Unlike flagellated bacteria, Mycoplasma lacks the general chemotactic signaling system to control their moving direction. Therefore, the physiological role of directionless travel in Mycoplasma gliding remains unclear. Recently, high-precision measurements under an optical microscope have revealed that three species of Mycoplasma exhibited rheotaxis, that is, the direction of gliding motility is lead upstream by the water flow. This intriguing response appears to be optimized for the flow patterns encountered at host surfaces. This review provides a comprehensive overview of the morphology, behavior, and habitat of Mycoplasma gliding, and discusses the possibility that the rheotaxis is ubiquitous among them.

Recent advances in sensing the inter-biomolecular interactions at the nanoscale - A comprehensive review of AFM-based force spectroscopy

Biomolecular interactions underpin most processes inside the cell. Hence, a precise and quantitative understanding of molecular association and dissociation events is crucial, not only from a fundamental perspective, but also for the rational design of biomolecular platforms for state-of-the-art biomedical and industrial applications. In this context, atomic force microscopy (AFM) appears as an invaluable experimental technique, allowing the measurement of the mechanical strength of biomolecular complexes to provide a quantitative characterization of their interaction properties from a single molecule perspective. In the present review, the most recent methodological advances in this field are presented with special focus on bioconjugation, immobilization and AFM tip functionalization, dynamic force spectroscopy measurements, molecular recognition imaging and theoretical modeling. We expect this work to significantly aid in grasping the principles of AFM-based force spectroscopy (AFM-FS) technique and provide the necessary tools to acquaint the type of data that can be achieved from this type of experiments. Furthermore, a critical assessment is done with other nanotechnology techniques to better visualize the future prospects of AFM-FS.

Purification and Structural Analysis of the Gliding Motility Machinery in Mycoplasma mobile

Isolating functional units from large insoluble protein complexes are a complex but valuable approach for quantitative and structural analysis. Mycoplasma mobile, a gliding bacterium, contains a large insoluble protein complex called gliding machinery. The machinery contains several chain structures formed by motors that are evolutionarily related to the F₁-ATPase. Recently, we developed a method to purify functional motors and their chain structures using Triton X-100 and a high salt concentration buffer and resolved their structures using electron microscopy. In this chapter, we describe the processes of purification and structural analysis of functional motors for the gliding of M. mobile using negative-staining electron microscopy.

Detection of Steps and Rotation in the Gliding Motility of Mycoplasma mobile

Mycoplasma mobile is one of the fastest gliding bacteria, gliding with a speed of 4.5 μm s^-1. This gliding motility is driven by a concerted movement of 450 supramolecular motor units composed of three proteins, Gli123, Gli349, and Gli521, in the gliding motility machinery. With general experimental setups, it is difficult to obtain the information on how each motor unit works. This chapter describes strategies to decrease the number of active motor units to extract stepwise cell movements driven by a minimum number of motor units. We also describe an unforeseen motility mode in which the leg motions convert the gliding motion into rotary motion, which enables us to characterize the motor torque and energy-conversion efficiency by adding some more assumptions.

Mechanical limitation of bacterial motility mediated by growing cell chains

Contrasting most known bacterial motility mechanisms, a bacterial sliding motility discovered in at least two Gram-positive bacterial families does not depend on designated motors. Instead, the cells maintain end-to-end connections following cell divisions to form long chains and exploit cell growth and division to push the cells forward. To investigate the dynamics of this motility mechanism, we constructed a mechanical model that depicts the interplay of the forces acting on and between the cells comprising the chain. Due to the exponential growth of individual cells, the tips of the chains can, in principle, accelerate to speeds faster than any known single-cell motility mechanism can achieve. However, analysis of the mechanical model shows that the exponential acceleration comes at the cost of an exponential buildup in mechanical stress in the chain, making overly long chains prone to breakage. Additionally, the mechanical model reveals that the dynamics of the chain expansion hinges on a single non-dimensional parameter. Perturbation analysis of the mechanical model further predicts the critical stress leading to chain breakage and its dependence on the non-dimensional parameter. Finally, we developed a simplistic population expansion model that uses the predicted breaking behavior to estimate the physical limit of chain-mediated population expansion. Predictions from the models provide critical insights into how this motility depends on key physical properties of the cell and the substrate. Overall, our models present a generically applicable theoretical framework for cell chain-mediated bacterial sliding motility and provide guidance for future experimental studies on such motility.

High-speed Atomic Force Microscopy Observation of Internal Structure Movements in Living Mycoplasma

Dozens of Mycoplasma species belonging to the class Mollicutes bind to solid surfaces through the organelle formed at a cell pole and glide in its direction by a unique mechanism. In Mycoplasma mobile, the fastest gliding species in Mycoplasma, the force for gliding is generated by ATP hydrolysis on an internal structure. However, the spatial and temporal behaviors of the internal structures in living cells were unclear. High-speed atomic force microscopy (HS-AFM) is a powerful method to monitor the dynamic behaviors of biomolecules and cells that can be captured while maintaining their active state in aqueous solution. In this protocol, we describe a method to detect their movements using HS-AFM. This protocol should be useful for the studies of many kinds of microorganisms. Graphic abstract: Scanning Mycoplasma cell.

A Single Shot of Vesicles

Bacteria communicate through signaling molecules that coordinate group behavior. Hydrophobic signals that do not diffuse in aqueous environments are used as signaling molecules by several bacteria. However, limited information is currently available on the mechanisms by which these molecules are transported between cells. Membrane vesicles (MVs) with diverse functions play important roles in the release and delivery of hydrophobic signaling molecules, leading to differences in the dynamics of signal transportation from those of free diffusion. Studies on Paracoccus denitrificans, which produces a hydrophobic long-chain N-acyl homoserine lactone (AHL), showed that signals were loaded into MVs at a concentration with the potential to trigger the quorum sensing (QS) response with a "single shot" to the cell. Furthermore, stimulating the formation of MVs increased the release of signals from the cell; therefore, a basic understanding of MV formation is important. Novel findings revealed the formation of MVs through different routes, resulting in the production of different types of MVs. Methods such as high-speed atomic force microscopy (AFM) phase imaging allow the physical properties of MVs to be analyzed at a nanometer resolution, revealing their heterogeneity. In this special minireview, we introduce the role of MVs in bacterial communication and highlight recent findings on MV formation and their physical heterogeneity by referring to our research. We hope that this minireview will provide basic information for understanding the functionality of MVs in ecological systems.

Investigating the morphological dynamics of the plasma membrane by high-speed atomic force microscopy

Despite numerous recent developments in bioimaging techniques, nanoscale and live-cell imaging of the plasma membrane has been challenging because of the insufficient z-resolution of optical microscopes, as well as the lack of fluorescent probes to specifically label small membrane structures. High-speed atomic force microscopy (HS-AFM) is a powerful tool for visualising the dynamics of a specimen surface and is therefore suitable for observing plasma membrane dynamics. Recent developments in HS-AFM for live-cell imaging have enabled the visualisation of the plasma membrane and the network of cortical actin underneath the membrane in a living cell. Furthermore, correlative imaging with fluorescence microscopy allows for the direct visualisation of morphological changes of the plasma membrane together with the dynamic assembly or disassembly of proteins during the entire course of endocytosis in a living cell. Here, we review these recent advances in HS-AFM in order to analyse various cellular events occurring at the cell surface.

Chained Structure of Dimeric F1-like ATPase in Mycoplasma mobile Gliding Machinery

Mycoplasma mobile, a fish pathogen, exhibits gliding motility using ATP hydrolysis on solid surfaces, including animal cells. The gliding machinery can be divided into surface and internal structures. The internal structure of the motor is composed of 28 so-called “chains” that are each composed of 17 repeating protein units called “particles.” These proteins include homologs of the catalytic α and β subunits of F₁-ATPase. In this study, we isolated the particles and determined their structures using negative-staining electron microscopy and high-speed atomic force microscopy. The isolated particles were composed of five proteins, MMOB1660 (α-subunit homolog), -1670 (β-subunit homolog), -1630, -1620, and -4530, and showed ATP hydrolyzing activity. The two-dimensional (2D) structure, with dimensions of 35 and 26 nm, showed a dimer of hexameric ring approximately 12 nm in diameter, resembling F₁-ATPase catalytic (αβ)₃. We isolated the F₁-like ATPase unit, which is composed of MMOB1660, -1670, and -1630. Furthermore, we isolated the chain and analyzed the three-dimensional (3D) structure, showing that dimers of mushroom-like structures resembling F₁-ATPase were connected and aligned along the dimer axis at 31-nm intervals. An atomic model of F₁-ATPase catalytic (αβ)₃ from Bacillus PS3 was successfully fitted to each hexameric ring of the mushroom-like structure. These results suggest that the motor for M. mobile gliding shares an evolutionary origin with F₁-ATPase. Based on the obtained structure, we propose possible force transmission processes in the gliding mechanism.