Insights into the structure and diversity of the bacterial flagellum from cryo-EM studies

The bacterial flagellum is a large proteinaceous macromolecular machine that enables motility, host interaction, and virulence across bacterial species. Cryo-electron microscopy (cryo-EM) and cryo-electron tomography (cryo-ET) have reshaped our understanding of flagellar architecture, revealing near-atomic detail of core subcomplexes and extensive structural diversity beyond the canonical Salmonella system. Here we review the structure of the prototypical flagellum, including the basal body, export apparatus, motor complex, and extracellular hook-filament assembly, derived from recent cryo-EM studies. We compare species-specific adaptations such as sheathed and periplasmic flagella, extended basal body scaffolds, and variable stator arrangements that enable high-torque motility in different environments. We also synthesize the cryo-EM/cryo-ET experimental strategies used to stablish our current model of hierarchical flagellar assembly, emphasizing the coordination between secretion specificity switching, molecular rulers, and chaperone-mediated substrate targeting. Finally, we outline the major gaps in our understanding of motor mechanics, length control, regulatory factors governing flagellar number and localization, and the functional roles of accessory structures. Collectively, these advances illustrate how cryo-EM and cryo-ET guided structural biology is advancing our understanding of flagellar architecture, assembly, function and diversity.