Structural basis for curvature generation and functional specialization in spirochete flagella

Spirochetes are spiral-shaped bacteria whose curved periplasmic flagella drive corkscrew-like motility through viscous environments. Despite their clinical importance, molecular mechanisms underlying flagellar assembly remain poorly understood. Here, we combine cryo–EM of flagellar filaments natively isolated from oral pathogen Treponema denticola , with proteomics and comparative genomics to characterise the molecular organisation. We resolve two filament types to sub-3 Å resolution: thin FlaB3-only cores, and thick filaments with a FlaB1/2 core asymmetrically decorated by a sheath comprising FlaA1/2/3 and lattice proteins FlaL1/2. We show sheath asymmetry imposes differential axial compaction on the FlaB1/2 core, generating convex and concave filament faces and establishing curvature. FlaB sequence and glycosylation differences at sheath–core interfaces correlate with selective incorporation into distinct filament types, supporting a mechanism for regulated filament architecture. Phylogenetic analysis reveals FlaL conservation across spirochete lineages, suggesting modular sheath lattices provide an adaptable mechanism for tuning flagellar mechanics across diverse pathogenic lifestyles.