Leveraging a multidisciplinary approach that combines high-resolution cryo-electron microscopy with rigorous genetic, biochemical, and phylogenetic analyses, the study deciphers auxiliary structural components that endow C. jejuni’s motor with enhanced stability and torque-generation capacities. Central to their findings is the identification of a novel ‘E ring’ composed of 17 homodimers of the protein FlgY, which assembles circumferentially around the MS ring, a pivotal basal body structure. This E ring is not a mere architectural curiosity but appears critical for anchoring additional scaffolding proteins and stabilizing the entire rotary apparatus under the mechanical stresses of high-speed rotation.

In addition to the E ring, the researchers describe a sophisticated cage-like network involving proteins FcpM, FcpN, FcpO, and PflD. This cage forms an encompassing lattice around the motor’s stator complexes—energy transducing units responsible for torque generation. Intriguingly, these stator units are not randomly dispersed but preserved in a symmetric assembly coinciding with the unique 17-subunit E ring, suggesting a finely tuned coupling between these accessory components that maximizes motor efficiency and durability.

Bridging the E ring and the surrounding cage, the study highlights spoke–rim formations mediated by the interactions between PflA and PflB proteins. This structural spoke arrangement, mimicking a wheel’s spokes, strengthens the motor’s scaffolding network and further stabilizes the stator complexes. The extraordinary modularity and architectural complexity of these assemblies underscore evolutionary adaptations that likely confer enhanced motility advantages to C. jejuni in its ecological niches, where precise and powerful motility can mean the difference between colonization success and failure.

Moving beyond structural characterization, the team utilized phylogenetic methodologies to explore the evolutionary trajectory of these complex motor components. Their analyses reveal that the E ring and spoke structures are not anomalies but ancestral features with a surprisingly widespread distribution across diverse bacterial lineages. This suggests an ancient origin predating the divergence of multiple phyla—a finding that dramatically extends our understanding of flagellar motor evolution, beyond the oversimplified models based on enteric bacteria.

Moreover, the study uncovers compelling evidence for the evolutionary co-option of type IV pilus components into the flagellar motor machinery in the Campylobacterota phylum, to which C. jejuni belongs. This molecular repurposing event presumably equipped these bacteria with the elaborate scaffolds needed to support extraordinary torque and stability demands, highlighting an elegant example of evolutionary innovation through modular assembly and protein function reassignment.

The functional consequences of these complex motor architectures are profound. Compared to simpler bacterial motors, the C. jejuni motor can sustain higher torque output and rotational speeds, thereby enhancing its chemotactic abilities under diverse environmental challenges. These adaptive benefits possibly underpin the pathogenic success of C. jejuni, which relies on aggressive motility to traverse mucus layers and establish infection within host organisms.

From a structural biology perspective, the revelation of 17-fold symmetry in FlgY homodimer arrangements and their direct physical and functional linkages to other motor elements pushes the envelope in molecular microbial nanomachinery. It challenges previously held notions regarding the stoichiometric and spatial organization of flagellar motor complexes and invites re-examination of torque generation mechanisms under novel structural constraints.

The interdisciplinary strategy adopted in this investigation exemplifies how integrated structural, genetic, and computational tools can uncover hidden complexities in well-known biological machines. The synergy of advanced cryo-EM imaging with detailed protein interaction assays, coupled with evolutionary genomics, constructs a vivid narrative of both the assembly and the long evolutionary dance that sculpted the C. jejuni flagellar motor into its present form.

This study also raises intriguing questions about the dynamics and regulation of such complex flagellar systems. For example, how do the multiple interacting scaffolds assemble in vivo during flagellar biogenesis, and what molecular signals coordinate the integration of these auxiliary components with core motor elements? Understanding these processes could pave the way for the development of novel antibacterial strategies aimed at disrupting motility — a key factor in bacterial pathogenesis.

Beyond microbiology, these findings inspire biomimetic engineering pursuits. The sophisticated organization and interplay of modular scaffolds in flagellar motors embody principles of nanoscale mechanical design that could inform the creation of synthetic nanomachines or microrobots, offering revolutionary applications in medicine and technology.

This comprehensive structural elucidation of Campylobacter’s complex flagellar motor thus represents a paradigm shift, blending evolutionary biology with cutting-edge molecular insights while laying fertile ground for translational innovations. As the field progresses, further exploration of bacterial flagellar diversity promises to unlock even deeper understanding of microbial motility, adaptation, and evolution.

In conclusion, the work spearheaded by Feng, Tachiyama, He, and colleagues delivers an unprecedented window into the structural sophistication and evolutionary ingenuity of bacterial flagellar motors, especially within the enigmatic Campylobacterota lineage. By delineating the roles of novel scaffold components and their integration into highly symmetric molecular assemblies, the study enriches a classic model with fresh complexity and evolutionary perspective. It is a compelling reminder that the microscopic engines driving bacterial life remain a continual source of scientific wonder and technological inspiration.

Subject of Research: Structural, functional and evolutionary characterization of complex bacterial flagellar motors in Campylobacter jejuni.

Article Title: Structural insights into the assembly and evolution of a complex bacterial flagellar motor.

Article References:
Feng, X., Tachiyama, S., He, J. et al. Structural insights into the assembly and evolution of a complex bacterial flagellar motor. Nat Microbiol (2026). https://doi.org/10.1038/s41564-025-02248-5

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41564-025-02248-5

Recent groundbreaking research spearheaded by scientists from Heinrich Heine University Düsseldorf (HHU), the University of Tübingen, along with contributions from Gießen University, Freiburg University, the Max Planck Institute for Biology Tübingen, the US National Institutes of Health, and the New Jersey Medical School, has brought to light a previously uncharacterized family of signaling proteins. These proteins are ubiquitously distributed throughout the bacterial kingdom and are pivotal in regulating the dynamic behavior of bacterial motility mechanisms and DNA uptake.

Published in the prestigious journals Cell Discovery and Proceedings of the National Academy of Sciences (PNAS), this research elucidates the intricate molecular dialogue governed by second messenger molecules, specifically the cyclic dinucleotides c-di-GMP and c-di-AMP. At the heart of this regulatory nexus lies the newly identified receptor protein ComFB. This receptor mediates the bacterial response to cyclic dinucleotides, thereby finely tuning both motility and DNA competence processes. The findings underscore the sophistication of bacterial intracellular signaling networks and their evolutionary conservation.

The discovery of ComFB reveals a versatile mechanism through which bacteria perceive and integrate environmental and cellular signals. This receptor protein’s ability to bind both c-di-GMP and c-di-AMP with high specificity indicates a dual regulatory role, modulating flagellar and pilus activity as well as the uptake of extracellular DNA — a process known as natural competence. Natural competence is vital for genetic diversity and horizontal gene transfer, allowing bacteria to quickly adapt to new stressors such as antibiotics, thereby exacerbating the challenge of antimicrobial resistance.

PhD student Sherihan Samir, leading the experimental investigations, highlights that the ComFB proteins from cyanobacteria, Bacillus subtilis, and the notorious pathogen Vibrio cholerae demonstrate remarkable precision in sensing second messenger molecules and translating these signals into functional changes in bacterial behavior. This interplay represents a paradigm shift in our understanding of how bacteria coordinate complex physiological functions using minimalistic molecular tools.

Professor Khaled Selim, the principal investigator, explains that c-di-AMP belongs to a novel class of cyclic dinucleotide second messengers whose roles had remained largely elusive until now. Their research successfully connects c-di-AMP signaling with the regulation of natural competence, mediated by ComFB proteins. This regulatory axis not only governs motility but also impacts the integration of foreign DNA into bacterial genomes, influencing traits such as virulence and resistance.

Understanding bacterial natural competence through the lens of ComFB and cyclic dinucleotide signaling has profound implications. The capacity for bacteria to uptake and incorporate genetic material facilitates rapid genetic diversification, offering a survival advantage in fluctuating environments. Moreover, it accentuates the threat posed by multi-drug resistant bacteria, as genes conferring resistance can disseminate swiftly across bacterial populations and species boundaries via competence-driven DNA uptake.

The breadth of bacteria employing this signaling mechanism remains an open question. Researchers are keen to explore whether clinically significant pathogens utilize ComFB-mediated signaling pathways. Such insights could herald novel antimicrobial strategies, targeting the molecular machinery of bacterial competence and motility, thereby mitigating the spread of antibiotic resistance.

The contractile pilus machinery, a critical motility organelle involved in DNA uptake, is evidently regulated by ComFB in response to cyclic dinucleotides. Visualization and structural studies, enhanced by innovative AI-generated imaging, have elucidated the conformational dynamics underpinning this mechanism. This detailed molecular understanding paves the way for targeted interventions that disrupt pathogenic bacterial communication and movement.

Furthermore, the interdisciplinary collaboration across multiple universities and research institutions underscores the complexity and significance of this discovery. Integrating microbiology, biochemistry, structural biology, and advanced imaging techniques, the research exemplifies how multidisciplinary efforts can unravel fundamental biological processes with far-reaching implications.

As scientific inquiry advances, the potential to exploit second messenger systems like c-di-AMP and c-di-GMP in clinical settings becomes increasingly tangible. By manipulating bacterial signaling pathways, it may become feasible to attenuate virulence, inhibit DNA uptake, and ultimately stem the tide of antibiotic resistance — one of the most pressing global health challenges of the 21st century.

These findings not only deepen our molecular comprehension of bacterial adaptability and pathogenicity but also open new horizons for translational research. The identification of ComFB as a central node in bacterial signal transduction emphasizes the intricate balance between motility and genetic exchange, a balance that sustains bacterial evolution and ecological success.

In conclusion, this seminal work expands the frontier of bacterial cell biology, spotlighting the intricate signaling mechanisms that govern critical aspects of bacterial life. As researchers continue to probe the depths of microbial communication and competence, the insights gained from the ComFB-cyclic dinucleotide axis promise to inspire innovative therapeutic strategies and enhance our ability to combat infectious diseases effectively.

Subject of Research: Regulation of bacterial motility and DNA uptake by cyclic dinucleotide second messengers via the ComFB receptor protein.

Article Title: ComFB, a new widespread family of c-di-NMP receptor proteins

News Publication Date: 18-Sep-2025

Web References:

• PNAS Article DOI: 10.1073/pnas.2513041122
• Cell Discovery Article DOI: 10.1038/s41421-025-00816-x

References:
Samir S, Elshereef AA, Alva V, et al. ComFB, a new widespread family of c-di-NMP receptor proteins. Proc Natl Acad Sci USA. 2025;122(38).
Samir S, Doello S, Enkerlin AM, et al. The second messenger c-di-AMP controls natural competence via ComFB signaling protein. Cell Discov. 2025;11(65).

Image Credits: HHU / Khaled Selim

Keywords: Cellular physiology, Flagella, Cyclic dinucleotides, Bacterial motility, DNA uptake, Natural competence, Antibiotic resistance, Signal transduction, ComFB receptor, c-di-GMP, c-di-AMP, Bacterial pathogenicity

Bacterial flagella are among nature’s most remarkable molecular machines, functioning as rotary engines that propel bacteria through their environments. The flagellar motor, embedded in the bacterial cell envelope, transduces the electrochemical gradient of ions into rotational motion, spinning the helical flagellum at remarkable speeds. Despite decades of research, the precise structural basis for the motor’s high torque output and adaptability to different environmental stresses has remained elusive. The current study bridges this gap by resolving the flagellar motor’s architecture while it operates in its native cellular context, confirming long-standing hypotheses and revealing novel structural motifs that underpin enhanced torque generation.

By employing in situ cryo-electron tomography, the researchers preserved bacterial cells in a near-native state, vitrifying them rapidly to prevent structural distortion. This approach allowed for the visualization of the intact flagellar motor within the cellular milieu, avoiding artifacts associated with purification and isolation. The subnanometre resolution achieved—on the order of less than one nanometre—unveils remarkable details of the motor’s protein complexes, including the stator units, rotor components, and interfaces critical for torque transmission. These insights extend beyond previous crystallographic or isolated structural studies by capturing the dynamic conformational states the motor adopts during operation.

The study’s findings highlight several key structural adaptations that enhance torque production. One pivotal discovery relates to the arrangement and density of the stator units, which form the torque-generating complexes embedded in the cytoplasmic membrane. The researchers observed that increased stator unit occupancy results in a larger rotational force, confirming the direct correlation between stator number and motor power. Remarkably, the stator units displayed conformational flexibility allowing them to respond dynamically to changes in load, thereby fine-tuning torque output in real time. This dynamic remodeling suggests a sophisticated regulatory mechanism enabling bacteria to adapt their motility force to diverse environmental conditions.

Moreover, the precise spatial configuration of rotor components revealed novel protein-protein interaction networks. These interactions contribute to the rigidity and stability of the rotor while permitting the flexible rotation necessary for continuous motor function. The subnanometre resolution also exposed the presence of previously unidentified accessory proteins that act as molecular braces, reinforcing the rotor-stator interface. These braces likely enhance the durability of the motor under high torque conditions, preventing structural failure during rapid or forceful rotation.

The elucidation of the proton or sodium ion pathways was another critical aspect of this research. The flagellar motor’s energy source, the transmembrane electrochemical gradient, drives ion flow through stator complexes, inducing conformational changes that generate torque. The study mapped these ion channels with exceptional clarity, showing how the pathways are structurally optimized to maximize ion flux efficiency while minimizing leakage. This optimization directly translates to higher torque output and improved energy conversion efficiency, offering novel targets for disrupting motility in pathogenic bacteria.

In addition to structural revelation, the study delivers fresh insights into the evolutionary adaptations of flagellar motors across bacterial species. By comparing motors from different species with varying environments and lifestyles, the researchers identified conserved and divergent features linked to torque optimization. For instance, pathogens inhabiting viscous bodily fluids display distinct architectural enhancements that support sustained high-torque rotation, compared to environmental bacteria in dilute aqueous habitats. These findings provide a framework to understand how natural selection sculpts molecular machines to meet ecological challenges.

The implications of this research extend far beyond microbiology. Understanding the mechanical principles underlying the bacterial flagellar motor informs the design of synthetic nanomachines. Bioengineers can draw inspiration from the natural architecture to develop artificial rotary motors that replicate the efficiency, durability, and adaptability observed in bacteria. Such biomimetic devices hold promise for applications ranging from targeted drug delivery to environmental sensing at the nanoscale.

Furthermore, the detailed knowledge of flagellar motor structure opens new avenues for antimicrobial intervention. Because motility is essential for infection, particularly in enabling bacterial dissemination and biofilm formation, selectively targeting key structural components or their assembly could incapacitate pathogenic bacteria without affecting human cells. The study’s revelation of specific protein interaction sites and ion conduction pathways offers precise molecular targets for next-generation antibacterial agents designed to impair motility.

The technical achievements enabling this discovery are themselves remarkable. The integration of state-of-the-art cryo-electron tomography with advanced image processing algorithms, including subtomogram averaging and machine learning-enhanced denoising, allowed the authors to overcome inherent challenges linked to sample thickness and heterogeneity. The seamless coordination between experimental imaging and computational reconstruction was essential to deduce the high-resolution structure of a functioning bacterial flagellar motor in situ, a feat previously deemed unattainable.

Complementing structural insights, the authors conducted functional assays to correlate morphological features with motor performance. Using mutant bacterial strains with altered stator compositions or accessory protein deletions, they demonstrated corresponding changes in torque output and swimming behavior. These experiments validate the structural interpretations and establish causative relationships between motor architecture and mechanical function, reinforcing the robustness of the conclusions.

Looking ahead, these findings set the stage for in-depth exploration of flagellar motor dynamics during bacterial behavior such as chemotaxis, surface adhesion, and host invasion. Combining high-resolution structural snapshots with real-time biophysical measurements may illuminate how conformational shifts govern directional switching and speed modulation. Such integrative approaches promise to unravel the full complexity of bacterial motility control at the molecular level.

In summary, the work by Drobnič, Cohen, Calcraft, and collaborators stands as a milestone in the field of molecular microbiology and nanotechnology. By revealing the in situ flagellar motor structure at subnanometre resolution with unprecedented detail, they have uncovered the molecular adaptations that empower bacteria with formidable torque generation. This comprehensive structural and functional characterization not only advances fundamental understanding but also paves the way for innovative applications in medicine, biotechnology, and synthetic biology.

As the quest to decode biological nanomachines continues, this study exemplifies the power of marrying cutting-edge experimental techniques with computational rigor. It underscores the intricate elegance of microbial machinery and reaffirms the value of investigating life’s fundamental processes at the finest scales. The bacterial flagellar motor, a paradigm of evolutionary engineering, remains a source of endless inspiration and scientific discovery.

Subject of Research: In situ structural analysis of bacterial flagellar motors with a focus on molecular adaptations for enhanced torque generation.

Article Title: In situ structure of a bacterial flagellar motor at subnanometre resolution reveals adaptations for increased torque.

Article References:
Drobnič, T., Cohen, E.J., Calcraft, T. et al. In situ structure of a bacterial flagellar motor at subnanometre resolution reveals adaptations for increased torque. Nat Microbiol (2025). https://doi.org/10.1038/s41564-025-02012-9

Image Credits: AI Generated

Bacterial flagella are among the most sophisticated biological motors known, enabling microorganisms to swim, navigate harsh environments, and colonize diverse habitats. Composed of thousands of protein subunits, the flagellum extends from the bacterial cell surface into the extracellular milieu, functioning as a rotary propeller powered by a basal motor embedded in the cell membrane. Despite decades of study, the precise structural organization of the extracellular portion and the exact molecular choreography by which the filament elongates remained elusive until now.

The researchers employed cutting-edge cryo-electron microscopy combined with advanced computational modeling to capture the entire extracellular flagellum at near-atomic resolution. This comprehensive visualization revealed a finely tuned helical arrangement of flagellin proteins, each adopting subtly different conformations according to its position along the filament. The study depended on integrating data from multiple bacterial species, allowing the team to infer conserved structural features critical for function and assembly.

Central to the investigation is the revelation of the mechanism of flagellin incorporation. Flagellin monomers are synthesized inside the cell and transported through the narrow central channel within the growing flagellum, a process akin to threading a needle from the interior outwards. The new structural data revealed an intricate gating system at the filament base that ensures only correctly folded flagellin subunits are incorporated, preventing malformed proteins from disrupting filament integrity. This molecular gate likely functions through conformational changes triggered by the flagellin subunits themselves, reflecting an elegant feedback mechanism that synchronizes assembly and stability.

Moreover, the filament’s extracellular environment was found to influence flagellin polymerization. The flagellin subunits undergo a series of conformational adjustments once outside the cell, transitioning from flexible monomers into the crystalline lattice that defines the filament’s stiffness and helical pitch. This transition is critical, enabling the flagellum to withstand the mechanical stresses generated during high-speed rotation without compromising its structural integrity. The discovery of these conformational checkpoints opens exciting possibilities for synthetic biology, where engineered flagellins might be designed to self-assemble into customizable filaments for nanoscale applications.

The study also addressed the enigmatic role of accessory proteins interfacing with the flagellum’s external surface. These proteins were observed to act as molecular chaperones, stabilizing the nascent filament and protecting it from enzymatic degradation or environmental damage. Their presence underscores the complex orchestration behind flagellum biogenesis, which extends far beyond the simplistic notion of a linear polymerization process. The findings challenge long-standing assumptions about extracellular protein assembly, highlighting a coordinated network of interactions that maintain flagellar function under diverse conditions.

Intriguingly, the completed extracellular flagellum is not a static entity. The research sheds light on its dynamic nature, revealing that filament subunits can be exchanged and repaired in situ, allowing bacteria to rapidly adapt their motility machinery in response to environmental stimuli. This remodeling capacity implies a hitherto unappreciated level of structural plasticity, which confers resilience against damage incurred during motility or host immune defenses. Such plasticity could be a universal feature of extracellular protein complexes, inviting further investigation into other microbial appendages.

The implications of these discoveries extend beyond basic microbiology. Understanding the precise molecular mechanics governing flagellin incorporation offers new avenues for targeting bacterial motility in medical and industrial settings. By disrupting key steps in filament assembly, novel antimicrobial compounds might incapacitate motility-dependent virulence factors, reducing infection severity without relying on traditional bactericidal approaches that promote resistance. Additionally, engineered flagellar systems could serve as inspiration for designing synthetic motile devices or responsive biomaterials.

This research also contributes fundamental insights into the evolution of protein export and assembly systems in bacteria. The flagellum is evolutionarily related to the type III secretion system, which injects virulence proteins into host cells. The elucidation of a gating mechanism for extracellular protein assembly suggests common principles underlying diverse bacterial secretion and motility machineries, enlightening our understanding of microbial adaptation and pathogenesis. Comparative analyses with these related systems could reveal novel targets for therapeutic intervention.

Technically, the success of this study hinged on technical breakthroughs in cryo-EM data acquisition and image processing, which overcame previous limitations posed by the flagellum’s length and flexibility. The team developed innovative sample preparation protocols to isolate intact extracellular flagella while preserving native conformations. In combination with machine learning algorithms capable of sorting heterogeneous conformational states, these tools allowed the construction of highly accurate three-dimensional reconstructions sparking a leap forward in structural microbiology.

Given the flagellum’s role as a model for self-assembling protein nanomachines, these findings will likely inspire a flurry of bioengineering efforts. Synthetic peptides mimicking flagellin or its assembly intermediates could be harnessed to build tailor-made nanostructures, potentially useful in drug delivery, biosensing, or materials science. Moreover, the concept of a molecular gate controlling filament assembly represents a fascinating paradigm for regulating complex macromolecular architectures, inviting efforts to emulate such mechanisms in artificial systems.

In conclusion, the comprehensive structural elucidation of the complete extracellular bacterial flagellum and the underlying mechanism of flagellin incorporation represent a monumental advance in microbiology. The integration of state-of-the-art structural biology with functional insights illuminates a process critical for bacterial motility and survival, with vast implications for biotechnology and medicine. As the scientific community digests these transformative insights, the bacterial flagellum continues to stand out as a profound natural example of molecular ingenuity and evolution’s craftsmanship.

Subject of Research: Structure and assembly mechanism of the bacterial flagellum, specifically the extracellular filament and flagellin incorporation process.

Article Title: The structure of the complete extracellular bacterial flagellum reveals the mechanism of flagellin incorporation.

Article References:
Einenkel, R., Qin, K., Schmidt, J. et al. The structure of the complete extracellular bacterial flagellum reveals the mechanism of flagellin incorporation. Nat Microbiol (2025). https://doi.org/10.1038/s41564-025-02037-0

Image Credits: AI Generated