Implications of back-and-forth motion and powerful propulsion for spirochetal invasion

The spirochete Leptospira spp. can move in liquid and on a solid surface using two periplasmic flagella (PFs), and its motility is an essential virulence factor for the pathogenic species. Mammals are infected with the spirochete through the wounded dermis, which implies the importance of behaviors on the boundary with such viscoelastic milieu; however, the leptospiral pathogenicity involving motility remains unclear. We used a glass chamber containing a gel area adjoining the leptospiral suspension to resemble host dermis exposed to contaminated water and analyzed the motility of individual cells at the liquid-gel border. Insertion of one end of the cell body to the gel increased switching of the swimming direction. Moreover, the swimming force of Leptospira was also measured by trapping single cells using an optical tweezer. It was found that they can generate ∼17 pN of force, which is ∼30 times of the swimming force of Escherichia coli . The force-speed relationship suggested the load-dependent force enhancement and showed that the power (the work per unit time) for the propulsion is ∼3.1×10 −16 W, which is two-order of magnitudes larger than the propulsive power of E. coli . The powerful and efficient propulsion of Leptospira using back-and-forth movements could facilitate their invasion.