H+ and Na+ are involved in flagellar rotation of the spirochete Leptospira

•This is a study on input energy for motility in the spirochete Leptospira.•Leptospira biflexa exhibited active motility in acidic to alkaline pH.•Both H+ and Na+ are involved in flagellar rotation in Leptospira.•H+ is a primary energy source, but Na+ can secondarily enhance motility.

Leptospira is a spirochete possessing intracellular flagella. Each Leptospira flagellar filament is linked with a flagellar motor composed of a rotor and a dozen stators. For many bacterial species, it is known that the stator functions as an ion channel and that the ion flux through the stator is coupled with flagellar rotation. The coupling ion varies depending on the species; for example, H+ is used in Escherichia coli, and Na+ is used in Vibrio spp. to drive a polar flagellum. Although genetic and structural studies illustrated that the Leptospira flagellar motor also contains a stator, the coupling ion for flagellar rotation remains unknown. In the present study, we analyzed the motility of Leptospira under various pH values and salt concentrations. Leptospira cells displayed motility in acidic to alkaline pH. In the presence of a protonophore, the cells completely lost motility in acidic to neutral pH but displayed extremely slow movement under alkaline conditions. This result suggests that H+ is a major coupling ion for flagellar rotation over a wide pH range; however, we also observed that the motility of Leptospira was significantly enhanced by the addition of Na+, though it vigorously moved even under Na+-free conditions. These results suggest that H+ is preferentially used and that Na+ is secondarily involved in flagellar rotation in Leptospira. The flexible ion selectivity in the flagellar system could be advantageous for Leptospira to survive in a wide range of environment.