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

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