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Vélez-González F, Marcos-Vilchis A, Vega-Baray B, Dreyfus G, Poggio S, Camarena L. Rotation of the Fla2 flagella of Cereibacter sphaeroides requires the periplasmic proteins MotK and MotE that interact with the flagellar stator protein MotB2. PLoS One 2024; 19:e0298028. [PMID: 38507361 PMCID: PMC10954123 DOI: 10.1371/journal.pone.0298028] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/05/2023] [Accepted: 01/16/2024] [Indexed: 03/22/2024] Open
Abstract
The bacterial flagellum is a complex structure formed by more than 25 different proteins, this appendage comprises three conserved structures: the basal body, the hook and filament. The basal body, embedded in the cell envelope, is the most complex structure and houses the export apparatus and the motor. In situ images of the flagellar motor in different species have revealed a huge diversity of structures that surround the well-conserved periplasmic components of the basal body. The identity of the proteins that form these novel structures in many cases has been elucidated genetically and biochemically, but in others they remain to be identified or characterized. In this work, we report that in the alpha proteobacteria Cereibacter sphaeroides the novel protein MotK along with MotE are essential for flagellar rotation. We show evidence that these periplasmic proteins interact with each other and with MotB2. Moreover, these proteins localize to the flagellated pole and MotK localization is dependent on MotB2 and MotA2. These results together suggest that the role of MotK and MotE is to activate or recruit the flagellar stators to the flagellar structure.
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Affiliation(s)
- Fernanda Vélez-González
- Instituto de Investigaciones Biomédicas, Universidad Nacional Autónoma de México, Mexico City, Mexico
| | - Arely Marcos-Vilchis
- Instituto de Fisiología Celular, Universidad Nacional Autónoma de México, Mexico City, Mexico
| | - Benjamín Vega-Baray
- Instituto de Investigaciones Biomédicas, Universidad Nacional Autónoma de México, Mexico City, Mexico
| | - Georges Dreyfus
- Instituto de Fisiología Celular, Universidad Nacional Autónoma de México, Mexico City, Mexico
| | - Sebastian Poggio
- Instituto de Investigaciones Biomédicas, Universidad Nacional Autónoma de México, Mexico City, Mexico
| | - Laura Camarena
- Instituto de Investigaciones Biomédicas, Universidad Nacional Autónoma de México, Mexico City, Mexico
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2
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Su-Arcaro C, Liao W, Bieniek K, Constantino MA, Decker SM, Turner BS, Bansil R. Unraveling the Intertwined Effect of pH on Helicobacter pylori Motility and the Microrheology of the Mucin-Based Medium It Swims in. Microorganisms 2023; 11:2745. [PMID: 38004756 PMCID: PMC10673263 DOI: 10.3390/microorganisms11112745] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/16/2023] [Revised: 10/16/2023] [Accepted: 11/03/2023] [Indexed: 11/26/2023] Open
Abstract
The gastric pathogen, Helicobacter pylori bacteria have to swim across a pH gradient from 2 to 7 in the mucus layer to colonize the gastric epithelium. Previous studies from our group have shown that porcine gastric mucin (PGM) gels at an acidic pH < 4, and H. pylori bacteria are unable to swim in the gel, although their flagella rotate. Changing pH impacts both the rheological properties of gastric mucin and also influences the proton (H+)-pumped flagellar motors of H. pylori as well as their anti-pH sensing receptors. To unravel these intertwined effects of acidic pH on both the viscoelastic properties of the mucin-based mucus as well as the flagellar motors and chemo-receptors of the bacterium, we compared the motility of H. pylori in PGM with that in Brucella broth (BB10) at different pH values using phase contrast microscopy to track the motion of the bacteria. The results show that the distribution of swimming speeds and other characteristics of the bacteria trajectories exhibit pH-dependent differences in both media. The swimming speed exhibits a peak at pH 4 in BB10, and a less pronounced peak at a higher pH of 5 in PGM. At all pH values, the bacteria swam faster and had a longer net displacement in BB10 compared to PGM. While the bacteria were stuck in PGM gels at pH < 4, they swam at these acidic pH values in BB10, although with reduced speed. Decreasing pH leads to a decreased fraction of motile bacteria, with a decreased contribution of the faster swimmers to the distributions of speeds and net displacement of trajectories. The body rotation rate is weakly dependent on pH in BB10, whereas in PGM bacteria that are immobilized in the low pH gel are capable of mechano-sensing and rotate faster. Bacteria can be stuck in the gel in various ways, including the flagella getting entangled in the fibers of the gel or the cell body being stuck to the gel. Our results show that in BB10, swimming is optimized at pH4, reflecting the combined effects of pH sensing by anti-pH tactic receptors and impact on H+ pumping of flagellar motors, while the increase in viscosity of PGM with decreasing pH and gelation below pH 4 lead to further reduction in swimming speed, with optimal swimming at pH 5 and immobilization of bacteria below pH 4.
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Affiliation(s)
- Clover Su-Arcaro
- Department of Physics, Boston University, Boston, MA 02215, USA; (C.S.-A.); (W.L.); (K.B.); (M.A.C.); (S.M.D.)
| | - Wentian Liao
- Department of Physics, Boston University, Boston, MA 02215, USA; (C.S.-A.); (W.L.); (K.B.); (M.A.C.); (S.M.D.)
| | - Katarzyna Bieniek
- Department of Physics, Boston University, Boston, MA 02215, USA; (C.S.-A.); (W.L.); (K.B.); (M.A.C.); (S.M.D.)
| | - Maira A. Constantino
- Department of Physics, Boston University, Boston, MA 02215, USA; (C.S.-A.); (W.L.); (K.B.); (M.A.C.); (S.M.D.)
| | - Savannah M. Decker
- Department of Physics, Boston University, Boston, MA 02215, USA; (C.S.-A.); (W.L.); (K.B.); (M.A.C.); (S.M.D.)
| | - Bradley S. Turner
- Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA;
| | - Rama Bansil
- Department of Physics, Boston University, Boston, MA 02215, USA; (C.S.-A.); (W.L.); (K.B.); (M.A.C.); (S.M.D.)
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3
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Morimoto YV, Minamino T. Measurements of the Ion Channel Activity of the Transmembrane Stator Complex in the Bacterial Flagellar Motor. Methods Mol Biol 2023; 2646:83-94. [PMID: 36842108 DOI: 10.1007/978-1-0716-3060-0_8] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/27/2023]
Abstract
The bacterial flagellum is driven by a rotational motor located at the base of the flagellum. The stator unit complex conducts cations such as protons (H+) and sodium ions (Na+) along the electrochemical potential across the cytoplasmic membrane and interacts with the rotor to generate the rotational force. Escherichia coli and Salmonella have the H+-type stator complex, which serves as a transmembrane H+ channel that couples H+ flow through an ion channel to torque generation whereas Vibrio and some Bacillus species have the Na+-type stator complex. In this chapter, we describe how to measure the ion conductivity of the transmembrane stator complex over-expressed in E. coli cells using fluorescent indicators. Intensity measurements of fluorescent indicators using either a fluorescence spectrophotometer or microscope allow quantitative detection of changes in the intracellular ion concentrations due to the ion channel activity of the transmembrane protein complex.
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Affiliation(s)
- Yusuke V Morimoto
- Department of Physics and Information Technology, Faculty of Computer Science and Systems Engineering, Kyushu Institute of Technology, Iizuka, Fukuoka, Japan.
- Japan Science and Technology Agency, PRESTO, Kawaguchi, Saitama, Japan.
| | - Tohru Minamino
- Graduate School of Frontier Biosciences, Osaka University, Suita, Osaka, Japan
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4
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Lin TS, Kojima S, Fukuoka H, Ishijima A, Homma M, Lo CJ. Stator Dynamics Depending on Sodium Concentration in Sodium-Driven Bacterial Flagellar Motors. Front Microbiol 2021; 12:765739. [PMID: 34899649 PMCID: PMC8661058 DOI: 10.3389/fmicb.2021.765739] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/27/2021] [Accepted: 10/19/2021] [Indexed: 11/17/2022] Open
Abstract
Bacterial flagellar motor (BFM) is a large membrane-spanning molecular rotary machine for swimming motility. Torque is generated by the interaction between the rotor and multiple stator units powered by ion-motive force (IMF). The number of bound stator units is dynamically changed in response to the external load and the IMF. However, the detailed dynamics of stator unit exchange process remains unclear. Here, we directly measured the speed changes of sodium-driven chimeric BFMs under fast perfusion of different sodium concentration conditions using computer-controlled, high-throughput microfluidic devices. We found the sodium-driven chimeric BFMs maintained constant speed over a wide range of sodium concentrations by adjusting stator units in compensation to the sodium-motive force (SMF) changes. The BFM has the maximum number of stator units and is most stable at 5 mM sodium concentration rather than higher sodium concentration. Upon rapid exchange from high to low sodium concentration, the number of functional stator units shows a rapidly excessive reduction and then resurrection that is different from predictions of simple absorption model. This may imply the existence of a metastable hidden state of the stator unit during the sudden loss of sodium ions.
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Affiliation(s)
- Tsai-Shun Lin
- Department of Physics and Center for Complex Systems, National Central University, Taoyuan City, Taiwan
| | - Seiji Kojima
- Division of Biological Science, Graduate School of Science, Nagoya University, Nagoya, Japan
| | - Hajime Fukuoka
- Graduate School of Frontier Biosciences, Osaka University, Suita, Japan
| | - Akihiko Ishijima
- Graduate School of Frontier Biosciences, Osaka University, Suita, Japan
| | - Michio Homma
- Division of Biological Science, Graduate School of Science, Nagoya University, Nagoya, Japan
| | - Chien-Jung Lo
- Department of Physics and Center for Complex Systems, National Central University, Taoyuan City, Taiwan
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5
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Biquet-Bisquert A, Labesse G, Pedaci F, Nord AL. The Dynamic Ion Motive Force Powering the Bacterial Flagellar Motor. Front Microbiol 2021; 12:659464. [PMID: 33927708 PMCID: PMC8076557 DOI: 10.3389/fmicb.2021.659464] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/27/2021] [Accepted: 03/02/2021] [Indexed: 11/13/2022] Open
Abstract
The 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.
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Affiliation(s)
- Anaïs Biquet-Bisquert
- Centre de Biologie Structurale (CBS), INSERM, CNRS, Université Montpellier, Montpellier, France
| | - Gilles Labesse
- Centre de Biologie Structurale (CBS), INSERM, CNRS, Université Montpellier, Montpellier, France
| | - Francesco Pedaci
- Centre de Biologie Structurale (CBS), INSERM, CNRS, Université Montpellier, Montpellier, France
| | - Ashley L Nord
- Centre de Biologie Structurale (CBS), INSERM, CNRS, Université Montpellier, Montpellier, France
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6
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Li QM, Zhou YL, Wei ZF, Wang Y. Phylogenomic Insights into Distribution and Adaptation of Bdellovibrionota in Marine Waters. Microorganisms 2021; 9:757. [PMID: 33916768 PMCID: PMC8067016 DOI: 10.3390/microorganisms9040757] [Citation(s) in RCA: 12] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/22/2021] [Revised: 03/26/2021] [Accepted: 04/01/2021] [Indexed: 11/23/2022] Open
Abstract
Bdellovibrionota is composed of obligate predators that can consume some Gram-negative bacteria inhabiting various environments. However, whether genomic traits influence their distribution and marine adaptation remains to be answered. In this study, we performed phylogenomics and comparative genomics studies using 132 Bdellovibrionota genomes along with five metagenome-assembled genomes (MAGs) from deep sea zones. Four phylogenetic groups, Oligoflexia, Bdello-group1, Bdello-group2 and Bacteriovoracia, were revealed by constructing a phylogenetic tree, of which 53.84% of Bdello-group2 and 48.94% of Bacteriovoracia were derived from the ocean. Bacteriovoracia was more prevalent in deep sea zones, whereas Bdello-group2 was largely distributed in the epipelagic zone. Metabolic reconstruction indicated that genes involved in chemotaxis, flagellar (mobility), type II secretion system, ATP-binding cassette (ABC) transporters and penicillin-binding protein were necessary for the predatory lifestyle of Bdellovibrionota. Genes involved in glycerol metabolism, hydrogen peroxide (H2O2) degradation, cell wall recycling and peptide utilization were ubiquitously present in Bdellovibrionota genomes. Comparative genomics between marine and non-marine Bdellovibrionota demonstrated that betaine as an osmoprotectant is probably widely used by marine Bdellovibrionota, and all the marine genomes have a number of genes for adaptation to marine environments. The genes encoding chitinase and chitin-binding protein were identified for the first time in Oligoflexia, which implied that Oligoflexia may prey on a wider spectrum of microbes. This study expands our knowledge on adaption strategies of Bdellovibrionota inhabiting deep seas and the potential usage of Oligoflexia for biological control.
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Affiliation(s)
- Qing-Mei Li
- Institute of Deep Sea Science and Engineering, Chinese Academy of Sciences, Sanya 572000, China; (Q.-M.L.); (Y.-L.Z.); (Z.-F.W.)
- University of Chinese Academy of Sciences, Beijing 100049, China
| | - Ying-Li Zhou
- Institute of Deep Sea Science and Engineering, Chinese Academy of Sciences, Sanya 572000, China; (Q.-M.L.); (Y.-L.Z.); (Z.-F.W.)
- University of Chinese Academy of Sciences, Beijing 100049, China
| | - Zhan-Fei Wei
- Institute of Deep Sea Science and Engineering, Chinese Academy of Sciences, Sanya 572000, China; (Q.-M.L.); (Y.-L.Z.); (Z.-F.W.)
- University of Chinese Academy of Sciences, Beijing 100049, China
| | - Yong Wang
- Institute of Deep Sea Science and Engineering, Chinese Academy of Sciences, Sanya 572000, China; (Q.-M.L.); (Y.-L.Z.); (Z.-F.W.)
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7
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Morimoto YV, Minamino T. Architecture and Assembly of the Bacterial Flagellar Motor Complex. Subcell Biochem 2021; 96:297-321. [PMID: 33252734 DOI: 10.1007/978-3-030-58971-4_8] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
One of the central systems responsible for bacterial motility is the flagellum. The bacterial flagellum is a macromolecular protein complex that is more than five times the cell length. Flagella-driven motility is coordinated via a chemosensory signal transduction pathway, and so bacterial cells sense changes in the environment and migrate towards more desirable locations. The flagellum of Salmonella enterica serovar Typhimurium is composed of a bi-directional rotary motor, a universal joint and a helical propeller. The flagellar motor, which structurally resembles an artificial motor, is embedded within the cell envelop and spins at several hundred revolutions per second. In contrast to an artificial motor, the energy utilized for high-speed flagellar motor rotation is the inward-directed proton flow through a transmembrane proton channel of the stator unit of the flagellar motor. The flagellar motor realizes efficient chemotaxis while performing high-speed movement by an ingenious directional switching mechanism of the motor rotation. To build the universal joint and helical propeller structures outside the cell body, the flagellar motor contains its own protein transporter called a type III protein export apparatus. In this chapter we summarize the structure and assembly of the Salmonella flagellar motor complex.
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Affiliation(s)
- Yusuke V Morimoto
- Department of Physics and Information Technology, Faculty of Computer Science and Systems Engineering, Kyushu Institute of Technology, 680-4 Kawazu, Iizuka, Fukuoka, 820-8502, Japan
| | - Tohru Minamino
- Graduate School of Frontier Biosciences, Osaka University, 1-3 Yamadaoka, Suita, Osaka, 565-0871, Japan.
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8
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Santiveri M, Roa-Eguiara A, Kühne C, Wadhwa N, Hu H, Berg HC, Erhardt M, Taylor NM. Structure and Function of Stator Units of the Bacterial Flagellar Motor. Cell 2020; 183:244-257.e16. [DOI: 10.1016/j.cell.2020.08.016] [Citation(s) in RCA: 95] [Impact Index Per Article: 23.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/25/2020] [Revised: 07/09/2020] [Accepted: 08/11/2020] [Indexed: 12/17/2022]
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9
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Chawla R, Gupta R, Lele TP, Lele PP. A Skeptic's Guide to Bacterial Mechanosensing. J Mol Biol 2020; 432:523-533. [PMID: 31629771 PMCID: PMC7002054 DOI: 10.1016/j.jmb.2019.09.004] [Citation(s) in RCA: 33] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/11/2019] [Revised: 09/03/2019] [Accepted: 09/11/2019] [Indexed: 12/28/2022]
Abstract
Surface sensing in bacteria is a precursor to the colonization of biotic and abiotic surfaces, and an important cause of drug resistance and virulence. As a motile bacterium approaches and adheres to a surface from the bulk fluid, the mechanical forces that act on it change. Bacteria are able to sense these changes in the mechanical load through a process termed mechanosensing. Bacterial mechanosensing has featured prominently in recent literature as playing a key role in surface sensing. However, the changes in mechanical loads on different parts of the cell at a surface vary in magnitudes as well as in signs. This confounds the determination of a causal relationship between the activation of specific mechanosensors and surface sensing. Here, we explain how contrasting mechanical stimuli arise on a surface-adherent cell and how known mechanosensors respond to these stimuli. The evidence for mechanosensing in select bacterial species is reinterpreted, with a focus on mechanosensitive molecular motors. We conclude with proposed criteria that bacterial mechanosensors must satisfy to successfully mediate surface sensing.
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Affiliation(s)
- Ravi Chawla
- Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX, USA
| | - Rachit Gupta
- Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX, USA
| | - Tanmay P Lele
- Department of Chemical Engineering, University of Florida, Gainesville, Fl, USA
| | - Pushkar P Lele
- Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX, USA.
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Nord AL, Pedaci F. Mechanisms and Dynamics of the Bacterial Flagellar Motor. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2020; 1267:81-100. [PMID: 32894478 DOI: 10.1007/978-3-030-46886-6_5] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
Many bacteria are able to actively propel themselves through their complex environment, in search of resources and suitable niches. The source of this propulsion is the Bacterial Flagellar Motor (BFM), a molecular complex embedded in the bacterial membrane which rotates a flagellum. In this chapter we review the known physical mechanisms at work in the motor. The BFM shows a highly dynamic behavior in its power output, its structure, and in the stoichiometry of its components. Changes in speed, rotation direction, constituent protein conformations, and the number of constituent subunits are dynamically controlled in accordance to external chemical and mechanical cues. The mechano-sensitivity of the motor is likely related to the surface-sensing ability of bacteria, relevant in the initial stage of biofilm formation.
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Affiliation(s)
- A L Nord
- Centre de Biochimie Structurale (CBS), INSERM, CNRS, University of Montpellier, Montpellier, France
| | - F Pedaci
- Centre de Biochimie Structurale (CBS), INSERM, CNRS, University of Montpellier, Montpellier, France.
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11
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Nakamura S, Hanaizumi Y, Morimoto YV, Inoue Y, Erhardt M, Minamino T, Namba K. Direct observation of speed fluctuations of flagellar motor rotation at extremely low load close to zero. Mol Microbiol 2019; 113:755-765. [DOI: 10.1111/mmi.14440] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/15/2019] [Revised: 12/10/2019] [Accepted: 12/10/2019] [Indexed: 12/22/2022]
Affiliation(s)
- Shuichi Nakamura
- Department of Applied Physics, Graduate School of Engineering Tohoku University Sendai Japan
| | - Yuta Hanaizumi
- Department of Applied Physics, Graduate School of Engineering Tohoku University Sendai Japan
| | - Yusuke V. Morimoto
- Faculty of Computer Science and Systems Engineering, Department of Physics and Information Technology Kyushu Institute of Technology Fukuoka Japan
| | - Yumi Inoue
- Graduate School of Frontier Biosciences Osaka University Osaka Japan
| | - Marc Erhardt
- Institut für Biologie/Bakterienphysiologie Humboldt‐Universität zu Berlin Berlin Germany
| | - Tohru Minamino
- Graduate School of Frontier Biosciences Osaka University Osaka Japan
| | - Keiichi Namba
- Graduate School of Frontier Biosciences Osaka University Osaka Japan
- RIKEN Spring‐8 Center and Center for Biosystems Dynamics Research Osaka Japan
- JEOL YOKOGUSHI Research Alliance Laboratories Osaka University Osaka Japan
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12
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Flagella-Driven Motility of Bacteria. Biomolecules 2019; 9:biom9070279. [PMID: 31337100 PMCID: PMC6680979 DOI: 10.3390/biom9070279] [Citation(s) in RCA: 181] [Impact Index Per Article: 36.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/27/2019] [Revised: 07/11/2019] [Accepted: 07/12/2019] [Indexed: 01/17/2023] Open
Abstract
The bacterial flagellum is a helical filamentous organelle responsible for motility. In bacterial species possessing flagella at the cell exterior, the long helical flagellar filament acts as a molecular screw to generate thrust. Meanwhile, the flagella of spirochetes reside within the periplasmic space and not only act as a cytoskeleton to determine the helicity of the cell body, but also rotate or undulate the helical cell body for propulsion. Despite structural diversity of the flagella among bacterial species, flagellated bacteria share a common rotary nanomachine, namely the flagellar motor, which is located at the base of the filament. The flagellar motor is composed of a rotor ring complex and multiple transmembrane stator units and converts the ion flux through an ion channel of each stator unit into the mechanical work required for motor rotation. Intracellular chemotactic signaling pathways regulate the direction of flagella-driven motility in response to changes in the environments, allowing bacteria to migrate towards more desirable environments for their survival. Recent experimental and theoretical studies have been deepening our understanding of the molecular mechanisms of the flagellar motor. In this review article, we describe the current understanding of the structure and dynamics of the bacterial flagellum.
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