Structural Conservation and Adaptation of the Bacterial Flagella Motor

Advancements in bacterial chemotaxis: Utilizing the navigational intelligence of bacteria and its practical applications

The fascinating world of microscopic life unveils a captivating spectacle as bacteria effortlessly maneuver through their surroundings with astonishing accuracy, guided by the intricate mechanism of chemotaxis. This review explores the complex mechanisms behind this behavior, analyzing the flagellum as the driving force and unraveling the intricate signaling pathways that govern its movement. We delve into the hidden costs and benefits of this intricate skill, analyzing its potential to propagate antibiotic resistance gene while shedding light on its vital role in plant colonization and beneficial symbiosis. We explore the realm of human intervention, considering strategies to manipulate bacterial chemotaxis for various applications, including nutrient cycling, algal bloom and biofilm formation. This review explores the wide range of applications for bacterial capabilities, from targeted drug delivery in medicine to bioremediation and disease control in the environment. Ultimately, through unraveling the intricacies of bacterial movement, we can enhance our comprehension of the intricate web of life on our planet. This knowledge opens up avenues for progress in fields such as medicine, agriculture, and environmental conservation.

Counterclockwise rotation of the flagellum promotes biofilm initiation in Helicobacter pylori

Motility promotes biofilm initiation during the early steps of this process: microbial surface association and attachment. Motility is controlled in part by chemotaxis signaling, so it seems reasonable that chemotaxis may also affect biofilm formation. There is a gap, however, in our understanding of the interactions between chemotaxis and biofilm formation, partly because most studies analyzed the phenotype of only a single chemotaxis signaling mutant, e.g., cheA. Here, we addressed the role of chemotaxis in biofilm formation using a full set of chemotaxis signaling mutants in Helicobacter pylori, a class I carcinogen that infects more than half the world's population and forms biofilms. Using mutants that lack each chemotaxis signaling protein, we found that chemotaxis signaling affected the biofilm initiation stage, but not mature biofilm formation. Surprisingly, some chemotaxis mutants elevated biofilm initiation, while others inhibited it in a manner that was not tied to chemotaxis ability or ligand input. Instead, the biofilm phenotype correlated with flagellar rotational bias. Specifically, mutants with a counterclockwise bias promoted biofilm initiation, e.g., ∆cheA, ∆cheW, or ∆cheV1; in contrast, those with a clockwise bias inhibited it, e.g., ∆cheZ, ∆chePep, or ∆cheV3. We tested this correlation using a counterclockwise bias-locked flagellum, which induced biofilm formation independent of the chemotaxis system. These CCW flagella, however, were not sufficient to induce biofilm formation, suggesting there are downstream players. Overall, our work highlights the new finding that flagellar rotational direction promotes biofilm initiation, with the chemotaxis signaling system operating as one mechanism to control flagellar rotation.

IMPORTANCE

Chemotaxis signaling systems have been reported to contribute to biofilm formation in many bacteria; however, how they regulate biofilm formation remains largely unknown. Chemotaxis systems are composed of many distinct kinds of proteins, but most previous work analyzed the biofilm effect of loss of only a few. Here, we explored chemotaxis' role during biofilm formation in the human-associated pathogenic bacterium Helicobacter pylori. We found that chemotaxis proteins are involved in biofilm initiation in a manner that correlated with how they affected flagellar rotation. Biofilm initiation was high in mutants with counterclockwise (CCW) flagellar bias and low in those with clockwise bias. We supported the idea that a major driver of biofilm formation is flagellar rotational direction using a CCW-locked flagellar mutant, which stays CCW independent of chemotaxis input and showed elevated biofilm initiation. Our data suggest that CCW-rotating flagella, independent of chemotaxis inputs, are a biofilm-promoting signal.

Structural basis of directional switching by the bacterial flagellum

The bacterial flagellum is a macromolecular protein complex that harvests energy from uni-directional ion flow across the inner membrane to power bacterial swimming via rotation of the flagellar filament. Rotation is bi-directional, with binding of a cytoplasmic chemotactic response regulator controlling reversal, though the structural and mechanistic bases for rotational switching are not well understood. Here we present cryoelectron microscopy structures of intact Salmonella flagellar basal bodies (3.2-5.5 Å), including the cytoplasmic C-ring complexes required for power transmission, in both counter-clockwise and clockwise rotational conformations. These reveal 180° movements of both the N- and C-terminal domains of the FliG protein, which, when combined with a high-resolution cryoelectron microscopy structure of the MotA5B2 stator, show that the stator shifts from the outside to the inside of the C-ring. This enables rotational switching and reveals how uni-directional ion flow across the inner membrane is used to accomplish bi-directional rotation of the flagellum.

Rotation of the Fla2 flagella of Cereibacter sphaeroides requires the periplasmic proteins MotK and MotE that interact with the flagellar stator protein MotB2

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.

Bacterial flagella hijack type IV pili proteins to control motility

Bacterial flagella and type IV pili (TFP) are surface appendages that enable motility and mechanosensing through distinct mechanisms. These structures were previously thought to have no components in common. Here, we report that TFP and some flagella share proteins PilO, PilN, and PilM, which we identified as part of the Helicobacter pylori flagellar motor. H. pylori mutants lacking PilO or PilN migrated better than wild type in semisolid agar because they continued swimming rather than aggregated into microcolonies, mimicking the TFP-regulated surface response. Like their TFP homologs, flagellar PilO/PilN heterodimers formed a peripheral cage that encircled the flagellar motor. These results indicate that PilO and PilN act similarly in flagella and TFP by differentially regulating motility and microcolony formation when bacteria encounter surfaces.

Tuning the stator subunit of the flagellar motor with coiled-coil engineering

Many bacteria swim driven by an extracellular filament rotated by the bacterial flagellar motor. This motor is powered by the stator complex, MotA5 MotB2 , an heptameric complex which forms an ion channel which couples energy from the ion motive force to torque generation. Recent structural work revealed that stator complex consists of a ring of five MotA subunits which rotate around a central dimer of MotB subunits. Transmembrane (TM) domains TM3 and TM4 from MotA combine with the single TM domain from MotB to form two separate ion channels within this complex. Much is known about the ion binding site and ion specificity; however, to date, no modeling has been undertaken to explore the MotB-MotB dimer stability and the role of MotB conformational dynamics during rotation. Here, we modeled the central MotB dimer using coiled-coil engineering and modeling principles and calculated free energies to identify stable states in the operating cycle of the stator. We found three stable coiled-coil states with dimer interface angles of 28°, 56°, and 64°. We tested the effect of strategic mutagenesis on the comparative energy of the states and correlated motility with a specific hierarchy of stability between the three states. In general, our results indicate agreement with existing models describing a 36° rotation step of the MotA pentameric ring during the power stroke and provide an energetic basis for the coordinated rotation of the central MotB dimer based on coiled-coil modeling.

Structural Insights into Type III Secretion Systems of the Bacterial Flagellum and Injectisome

Two of the most fascinating bacterial nanomachines-the broadly disseminated rotary flagellum at the heart of cellular motility and the eukaryotic cell-puncturing injectisome essential to specific pathogenic species-utilize at their core a conserved export machinery called the type III secretion system (T3SS). The T3SS not only secretes the components that self-assemble into their extracellular appendages but also, in the case of the injectisome, subsequently directly translocates modulating effector proteins from the bacterial cell into the infected host. The injectisome is thought to have evolved from the flagellum as a minimal secretory system lacking motility, with the subsequent acquisition of additional components tailored to its specialized role in manipulating eukaryotic hosts for pathogenic advantage. Both nanomachines have long been the focus of intense interest, but advances in structural and functional understanding have taken a significant step forward since 2015, facilitated by the revolutionary advances in cryo-electron microscopy technologies. With several seminal structures of each nanomachine now captured, we review here the molecular similarities and differences that underlie their diverse functions.

Ion selectivity and rotor coupling of the Vibrio flagellar sodium-driven stator unit

Bacteria swim using a flagellar motor that is powered by stator units. Vibrio spp. are highly motile bacteria responsible for various human diseases, the polar flagella of which are exclusively driven by sodium-dependent stator units (PomAB). However, how ion selectivity is attained, how ion transport triggers the directional rotation of the stator unit, and how the stator unit is incorporated into the flagellar rotor remained largely unclear. Here, we have determined by cryo-electron microscopy the structure of Vibrio PomAB. The electrostatic potential map uncovers sodium binding sites, which together with functional experiments and molecular dynamics simulations, reveal a mechanism for ion translocation and selectivity. Bulky hydrophobic residues from PomA prime PomA for clockwise rotation. We propose that a dynamic helical motif in PomA regulates the distance between PomA subunit cytoplasmic domains, stator unit activation, and torque transmission. Together, our study provides mechanistic insights for understanding ion selectivity and rotor incorporation of the stator unit of the bacterial flagellum.

Precise Measurement of the Stoichiometry of the Adaptive Bacterial Flagellar Switch

The cytoplasmic ring (C-ring) of the bacterial flagellar motor controls the motor rotation direction, thereby controlling bacterial run-and-tumble behavior. The C-ring has been shown to undergo adaptive remodeling in response to changes in motor directional bias. However, the stoichiometry and arrangement of the C-ring is still unclear due to contradiction between the results from fluorescence studies and cryo-electron microscopy (cryo-EM) structural analysis. Here, by using the copy number of FliG molecules (34) in the C-ring as a reference, we precisely measured the copy numbers of FliM molecules in motors rotating exclusively counterclockwise (CCW) and clockwise (CW). We surprisingly found that there are on average 45 and 58 FliM molecules in CW and CCW rotating motors, respectively, which are much higher than previous estimates. Our results suggested a new mechanism of C-ring adaptation, that is, extra FliM molecules could be bound to the primary C-ring with probability depending on the motor rotational direction. We further confirmed that all of the FliM molecules in the C-ring function in chemotaxis signaling transduction because all of them could be bound by the chemotactic response regulator CheY-P. Our measurements provided new insights into the structure and arrangement of the flagellar switch. IMPORTANCE The bacterial flagellar switch can undergo adaptive remodeling in response to changes in motor rotation direction, thereby shifting its operating point to match the output of the chemotaxis signaling pathway. However, it remains unclear how the flagellar switch accomplishes this adaptive remodeling. Here, via precise fluorescence studies, we measured the absolute copy numbers of the critical component in the switch for motors rotating counterclockwise and clockwise, obtaining much larger numbers than previous relative estimates. Our results suggested a new mechanism of adaptive remodeling of the flagellar switch and provided new insights for updating the conformation spread model of the switch.

Identification of determinants for entering into a viable but nonculturable state in Vibrio alginolyticus by Tn-seq

The viable but nonculturable (VBNC) state is a dormant state of nonsporulating bacteria that enhances survival in adverse environments. Systematic genome-wide research on the genetic basis of VBNC formation is warranted. In this study, we demonstrated that the marine bacterium Vibrio alginolyticus lost culturability but remained viable and entered into the VBNC state when exposed to low nutrient concentrations for prolonged periods of time. Using transposon-insertion sequencing (Tn-seq), we identified 635 determinants governing the formation of the VBNC state, including 322 genes with defective effects on VBNC formation and 313 genes contributing to entry into the VBNC state. Tn-seq analysis revealed that genes involved in various metabolic pathways were shown to have an inhibitory effect on VBNC formation, while genes related to chemotaxis or folate biosynthesis promoted entry into the VBNC state. Moreover, the effects of these genes on the formation of VBNC were validated with the growth of deletion mutants of eight selected genes under nutrient-limited conditions. Interestingly, fleQ and pyrI were identified as essential for entry into the VBNC state, and they affected the formation of the VBNC state independent of RpoE or ToxR regulation. Collectively, these results provide new insights into the mechanism of VBNC formation. KEY POINTS: • Vibrio alginolyticus has the ability to enter into the VBNC state under low nutrient conditions at low temperature. • The 635 determinants for entry into the VBNC state were systematically identified by transposon-insertion sequencing. • PyrI and FleQ were validated to play significant roles in the formation of the VBNC state.

Antigenic Determinant of Helicobacter pylori FlaA for Developing Serological Diagnostic Methods in Children

The early diagnosis of Helicobacter pylori infection is important for gastric cancer prevention and treatment. Although endoscopic biopsy is widely used for H. pylori diagnosis, an accurate biopsy cannot be performed until a lesion becomes clear, especially in pediatric patients. Therefore, it is necessary to develop convenient and accurate methods for early diagnosis. FlaA, an essential factor for H. pylori survival, shows high antigenicity and can be used as a diagnostic marker. We attempted to identify effective antigens containing epitopes of high diagnostic value in FlaA. Full-sized FlaA was divided into several fragments and cloned, and its antigenicity was investigated using Western blotting. The FlaA fragment of 1345-1395 bp had strong immunogenicity. ELISA was performed with serum samples from children by using the 1345-1395 bp recombinant antigen fragment. IgG reactivity showed 90.0% sensitivity and 90.5% specificity, and IgM reactivity showed 100% sensitivity and specificity. The FlaA fragment of 1345-1395 bp discovered in the present study has antigenicity and is of high value as a candidate antigen for serological diagnosis. The FlaA 1345-1395 bp epitope can be used as a diagnostic marker for H. pylori infection, thereby controlling various gastric diseases such as gastric cancer and peptic ulcers caused by H. pylori.

A multi-state dynamic process confers mechano-adaptation to a biological nanomachine

Adaptation is a defining feature of living systems. The bacterial flagellar motor adapts to changes in the external mechanical load by adding or removing torque-generating (stator) units. But the molecular mechanism behind this mechano-adaptation remains unclear. Here, we combine single motor eletrorotation experiments and theoretical modeling to show that mechano-adaptation of the flagellar motor is enabled by multiple mechanosensitive internal states. Dwell time statistics from experiments suggest the existence of at least two bound states with a high and a low unbinding rate, respectively. A first-passage-time analysis of a four-state model quantitatively explains the experimental data and determines the transition rates among all four states. The torque generated by bound stator units controls their effective unbinding rate by modulating the transition between the bound states, possibly via a catch bond mechanism. Similar force-mediated feedback enabled by multiple internal states may apply to adaptation in other macromolecular complexes.

Combining experiments with modeling, Wadhwa et al. propose a model for mechano-adaptation in the bacterial flagellar motor, finding that load-dependent transitions between multiple internal states govern the binding and unbinding of subunits.

FliL and its paralog MotF have distinct roles in the stator activity of the Sinorhizobium meliloti flagellar motor

The bacterial flagellum is a complex macromolecular machine that drives bacteria through diverse fluid environments. Although many components of the flagellar motor are conserved across species, the roles of FliL are numerous and species‐specific. Here, we have characterized an additional player required for flagellar motor function in Sinorhizobium meliloti, MotF, which we have identified as a FliL paralog. We performed a comparative analysis of MotF and FliL, identified interaction partners through bacterial two‐hybrid and pull‐down assays, and investigated their roles in motility and motor rotation. Both proteins form homooligomers, and interact with each other, and with the stator proteins MotA and MotB. The ∆motF mutant exhibits normal flagellation but its swimming behavior and flagellar motor activity are severely impaired and erratic. In contrast, the ∆fliL mutant is mostly aflagellate and nonmotile. Amino acid substitutions in cytoplasmic regions of MotA or disruption of the proton channel plug of MotB partially restored motor activity to the ∆motF but not the ∆fliL mutant. Altogether, our findings indicate that both, MotF and FliL, are essential for flagellar motor torque generation in S. meliloti. FliL may serve as a scaffold for stator integration into the motor, and MotF is required for proton channel modulation.

The Bacterial Flagellar Motor: Insights Into Torque Generation, Rotational Switching, and Mechanosensing

The flagellar motor is a bidirectional rotary nanomachine used by many bacteria to sense and move through environments of varying complexity. The bidirectional rotation of the motor is governed by interactions between the inner membrane-associated stator units and the C-ring in the cytoplasm. In this review, we take a structural biology perspective to discuss the distinct conformations of the stator complex and the C-ring that regulate bacterial motility by switching rotational direction between the clockwise (CW) and counterclockwise (CCW) senses. We further contextualize recent in situ structural insights into the modulation of the stator units by accessory proteins, such as FliL, to generate full torque. The dynamic structural remodeling of the C-ring and stator complexes as well as their association with signaling and accessory molecules provide a mechanistic basis for how bacteria adjust motility to sense, move through, and survive in specific niches both outside and within host cells and tissues.

How advances in cryo-electron tomography have contributed to our current view of bacterial cell biology

Advancements in the field of cryo-electron tomography have greatly contributed to our current understanding of prokaryotic cell organization and revealed intracellular structures with remarkable architecture. In this review, we present some of the prominent advancements in cryo-electron tomography, illustrated by a subset of structural examples to demonstrate the power of the technique. More specifically, we focus on technical advances in automation of data collection and processing, sample thinning approaches, correlative cryo-light and electron microscopy, and sub-tomogram averaging methods. In turn, each of these advances enabled new insights into bacterial cell architecture, cell cycle progression, and the structure and function of molecular machines. Taken together, these significant advances within the cryo-electron tomography workflow have led to a greater understanding of prokaryotic biology. The advances made the technique available to a wider audience and more biological questions and provide the basis for continued advances in the near future.

Characterization of the Flagellar Collar Reveals Structural Plasticity Essential for Spirochete Motility

Spirochetes are a remarkable group of bacteria with distinct morphology and periplasmic flagella that enable motility in viscous environments, such as host connective tissues. The collar, a spirochete-specific complex of the periplasmic flagellum, is required for this unique spirochete motility, yet it has not been clear how the collar assembles and enables spirochetes to transit between complex host environments. Here, we characterize the collar complex in the Lyme disease spirochete Borrelia burgdorferi. We discover as well as delineate the distinct functions of two novel collar proteins, FlcB and FlcC, by combining subtractive bioinformatic, genetic, and cryo-electron tomography approaches. Our high-resolution in situ structures reveal that the multiprotein collar has a remarkable structural plasticity essential not only for assembly of flagellar motors in the highly curved membrane of spirochetes but also for generation of the high torque necessary for spirochete motility.

BB0259 Encompasses a Peptidoglycan Lytic Enzyme Function for Proper Assembly of Periplasmic Flagella in Borrelia burgdorferi

Assembly of the bacterial flagellar rod, hook, and filament requires penetration through the peptidoglycan (PG) sacculus and outer membrane. In most β- and γ-proteobacteria, the protein FlgJ has two functional domains that enable PG hydrolyzing activity to create pores, facilitating proper assembly of the flagellar rod. However, two distinct proteins performing the same functions as the dual-domain FlgJ are proposed in δ- and ε-proteobacteria as well as spirochetes. The Lyme disease spirochete Borrelia burgdorferi genome possesses a FlgJ and a PG lytic SLT enzyme protein homolog (BB0259). FlgJ in B. burgdorferi is crucial for flagellar hook and filament assembly but not for the proper rod assembly reported in other bacteria. However, BB0259 has never been characterized. Here, we use cryo-electron tomography to visualize periplasmic flagella in different bb0259 mutant strains and provide evidence that the E580 residue of BB0259 is essential for PG-hydrolyzing activity. Without the enzyme activity, the flagellar hook fails to penetrate through the pores in the cell wall to complete assembly of an intact periplasmic flagellum. Given that FlgJ and BB0259 interact with each other, they likely coordinate the penetration through the PG sacculus and assembly of a functional flagellum in B. burgdorferi and other spirochetes. Because of its role, we renamed BB0259 as flagellar-specific lytic transglycosylase or LTaseBb.

Modulation of the enzymatic activity of the flagellar lytic transglycosylase SltF by rod components, and the scaffolding protein FlgJ in Rhodobacter sphaeroides

Macromolecular cell-envelope-spanning structures such as the bacterial flagellum must traverse the cell wall. Lytic transglycosylases enzymes are capable of enlarging gaps in the peptidoglycan meshwork to allow the efficient assembly of supramolecular complexes. In the periplasmic space, the assembly of the flagellar rod requires the scaffold protein FlgJ, which includes a muramidase domain in the canonical models Salmonella enterica and Escherichia coli. In contrast, in Rhodobacter sphaeroides, FlgJ and the dedicated flagellar lytic transglycosylase SltF are separate entities that interact in the periplasm. In this study we show that sltF is expressed along with the genes encoding the early components of the flagellar hierarchy that include the hook-basal body proteins, making SltF available during the rod assembly. Protein-protein interaction experiments demonstrated that SltF interacts with the rod proteins FliE, FlgB, FlgC, FlgF and FlgG through its C-terminal region. A deletion analysis that divides the C-terminus in two halves revealed that the interacting regions for most of the rod proteins are not redundant. Our results also show that the presence of the rod proteins FliE, FlgB, FlgC, and FlgF displace the previously reported SltF-FlgJ interaction. In addition, we observed modulation of the transglycosylase activity of SltF mediated by FlgB and FlgJ that could be relevant to coordinate rod assembly with cell wall remodeling. In summary, different mechanisms regulate the flagellar lytic transglycosylase, SltF ensuring a timely transcription, a proper localization and a controlled enzymatic activity. Importance Several mechanisms participate in the assembly of cell-envelope-spanning macromolecular structures. The sequential expression of substrates to be exported, selective export, and a specific order of incorporation are some of the mechanisms that stand out to drive an efficient assembly process. In this work we analyze how the structural rod proteins, the scaffold protein FlgJ and the flagellar lytic enzyme SltF, interact in an orderly fashion to assemble the flagellar rod into the periplasmic space. A complex arrangement of transient interactions directs a dedicated flagellar muramidase towards the flagellar rod. All these interactions bring this protein to the proximity of the peptidoglycan wall while also modulating its enzymatic activity. This study suggests how a dynamic network of interactions participates in controlling SltF, a prominent component for flagellar formation.

Structural basis of torque generation in the bi-directional bacterial flagellar motor

The flagellar stator unit is an oligomeric complex of two membrane proteins (MotA₅B₂) that powers bi-directional rotation of the bacterial flagellum. Harnessing the ion motive force across the cytoplasmic membrane, the stator unit operates as a miniature rotary motor itself to provide torque for rotation of the flagellum. Recent cryo-electron microscopic (cryo-EM) structures of the stator unit provided novel insights into its assembly, function, and subunit stoichiometry, revealing the ion flux pathway and the torque generation mechanism. Furthermore, in situ cryo-electron tomography (cryo-ET) studies revealed unprecedented details of the interactions between stator unit and rotor. In this review, we summarize recent advances in our understanding of the structure and function of the flagellar stator unit, torque generation, and directional switching of the motor.

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.

Recent Advances in the Bacterial Flagellar Motor Study

The bacterial flagellum is a supramolecular motility machine that allows bacterial cells to swim in liquid environments [...].

Structural basis of assembly and torque transmission of the bacterial flagellar motor

The bacterial flagellar motor is a supramolecular protein machine that drives rotation of the flagellum for motility, which is essential for bacterial survival in different environments and a key determinant of pathogenicity. The detailed structure of the flagellar motor remains unknown. Here we present an atomic-resolution cryoelectron microscopy (cryo-EM) structure of the bacterial flagellar motor complexed with the hook, consisting of 175 subunits with a molecular mass of approximately 6.3 MDa. The structure reveals that 10 peptides protruding from the MS ring with the FlgB and FliE subunits mediate torque transmission from the MS ring to the rod and overcome the symmetry mismatch between the rotational and helical structures in the motor. The LP ring contacts the distal rod and applies electrostatic forces to support its rotation and torque transmission to the hook. This work provides detailed molecular insights into the structure, assembly, and torque transmission mechanisms of the flagellar motor.

Structural basis of bacterial flagellar motor rotation and switching

The bacterial flagellar motor, a remarkable rotary machine, can rapidly switch between counterclockwise (CCW) and clockwise (CW) rotational directions to control the migration behavior of the bacterial cell. The flagellar motor consists of a bidirectional spinning rotor surrounded by torque-generating stator units. Recent high-resolution in vitro and in situ structural studies have revealed stunning details of the individual components of the flagellar motor and their interactions in both the CCW and CW senses. In this review, we discuss these structures and their implications for understanding the molecular mechanisms underlying flagellar rotation and switching.

The Structure, Composition, and Role of Periplasmic Stator Scaffolds in Polar Bacterial Flagellar Motors

In the bacterial flagellar motor, the cell-wall-anchored stator uses an electrochemical gradient across the cytoplasmic membrane to generate a turning force that is applied to the rotor connected to the flagellar filament. Existing theoretical concepts for the stator function are based on the assumption that it anchors around the rotor perimeter by binding to peptidoglycan (P). The existence of another anchoring region on the motor itself has been speculated upon, but is yet to be supported by binding studies. Due to the recent advances in electron cryotomography, evidence has emerged that polar flagellar motors contain substantial proteinaceous periplasmic structures next to the stator, without which the stator does not assemble and the motor does not function. These structures have a morphology of disks, as is the case with Vibrio spp., or a round cage, as is the case with Helicobacter pylori. It is now recognized that such additional periplasmic components are a common feature of polar flagellar motors, which sustain higher torque and greater swimming speeds compared to peritrichous bacteria such as Escherichia coli and Salmonella enterica. This review summarizes the data available on the structure, composition, and role of the periplasmic scaffold in polar bacterial flagellar motors and discusses the new paradigm for how such motors assemble and function.