Spirochaetes are spiral or flat-wave-shaped Gram-negative bacteria that have periplasmic flagella between the peptidoglycan layer and outer membrane. Rotation of the periplasmic flagella transforms the cell body shape periodically, allowing the cell to swim in aqueous environments. Because the virulence of motility-deficient mutants of pathogenic species is drastically attenuated, motility is thought to be an essential virulence factor in spirochaetes. However, it remains unknown how motility practically contributes to the infection process. We show here that the cell body configuration and motility of the zoonotic spirochaete Leptospira changes depending on the viscosity of the medium. Leptospira swim and reverse the swimming direction by transforming the cell body. Motility analysis showed that the frequency of cell shape transformation was increased by increasing the viscosity of the medium. The increased cell body transformation induced highly frequent reversal of the swimming direction. A simple kinetic model based on the experimental results shows that the viscosity-induced increase in reversal limits cell migration, resulting in the accumulation of cells in high-viscosity regions. This behaviour could facilitate the colonization of the spirochaete on host tissues covered with mucosa.

Rotation of a helical cell body and mobility of cell-surface adhesins result in surface motility of the spirochete Leptospira.

The motility of bacteria is an important factor in their infectivity. In this study, the motility of Leptospira, a member of the spirochete family that causes a zoonotic disease known as leptospirosis, was analyzed in different viscous or osmotic conditions. Motility assays revealed that both pathogenic and saprophytic strains increase their swimming speeds with increasing viscosity. However, only pathogenic Leptospira interrogans maintained vigorous motility near physiological osmotic conditions. This suggests that active motility in physiological conditions is advantageous when Leptospira enters hosts and when it migrates toward target tissues.

Chemotaxis allows bacterial cells to migrate towards or away from chemical compounds. In the present study, we developed a microscopic agar-drop assay (MAA) to investigate the chemotactic behaviour of a coiled spirochete, Leptospira biflexa. An agar drop containing a putative attractant or repellent was placed around the centre of a flow chamber and the behaviour of free-swimming cells was analysed under a microscope. MAA showed that L. biflexa cells gradually accumulated around an agar drop that contained an attractant such as glucose. Leptospira cells often spin without migration by transformation of their cell body. The frequency at which cells showed no net displacement decreased with a higher glucose concentration, suggesting that sensing an attractive chemical allows these cells to swim more smoothly. Investigation of the chemotactic behaviour of these cells in response to different types of sugars showed that fructose and mannitol induced negative chemotactic responses, whereas xylose and lactose were non-chemotactic for L. biflexa. The MAA developed in this study can be used to investigate other chemoattractants and repellents.

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 loaddependent 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. Motility has been identified as a crucial virulence factor for pathogenic bacteria 1. For example, a motility-deficient mutant of Vibrio cholerae is attenuated due to the decreased invasion efficiency of the epithelium 2. In some flagellated bacteria, both motility and flagella are considered essential as an adhesin. For example, Salmonella enterica attaches to the host tissue via peritrichous flagella, which results in colonization and clinical outcomes 3. Although spirochetes, such as Borrelia burgdorferi (the Lyme disease) 4 and Brachyspira hyodysenteriae (swine dysentery) 5 , also utilize motility during infection, their flagella exist beneath the outer membrane, which is known as the periplasmic flagella (PFs), and spirochetal flagella are not directly involved in pathogenicity. Instead, the improvement of swimming ability 6 and diverse adherence 7 in viscoelastic environments is believed to be responsible for their colonization and dissemination within hosts. The genus Leptospira is a member of spirochetes, and these pathogenic species have been found to cause a worldwide zoonosis known as leptospirosis. Pathogenic Leptospira cells are maintained in the proximal renal tubules of rodents as a reservoir. When the hosts urinate, they spread the spirochetes into the environment; as a result, many mammals, including humans, are percutaneously or transmucosally infected by contact with the contaminated soil and water 8,9. Leptospira spp. have a right-handed spiral cell body and exhibit curvatures at both ends (Fig. 1A). Spirochetes can swim in liquid and crawl on surfaces using two PFs (one PF/cell end) (Fig. 1B). The morphology of the cell ends frequently changes between a spiral and a hook shape; and there is an asymmetri...