Core-sheath coupling controls flagellar curvature and motility in Leptospira.

Spirochaete pathogens are among the most invasive bacteria known, causing syphilis, Lyme disease, and leptospirosis. Their tissue penetration depends on periplasmic flagellar filaments that, unlike other bacterial flagella, are encased in a spirochaete-specific multi-protein sheath and deform the cell body into motile waves. How these filaments achieve the mechanical properties needed for invasive motility has remained unclear. Here we determine complete atomic structures of the Leptospira endoflagellar filament, revealing an elaborate sheath of 9 to 12 distinct asymmetrically arranged proteins. We show that the flagellin variant forming the filament core determines sheath composition, producing curvatures ranging from ~3.5 µm-1 to ~5.6 µm-1. The lower-curvature architecture, employed by pathogenic Leptospira interrogans, proves essential for motility in viscous environments and during infection. Thus, Leptospira achieves environment-specific motility through modular core-sheath coupling, linking atomic-scale structural plasticity to large-scale changes in swimming behaviour. Conservation of key sheath components suggests this mechanism may extend across spirochaetes.