Implications of coordinated cell-body rotations for Leptospira motility

Machine learning-based motion tracking reveals an inverse correlation between adhesivity and surface motility of the leptospirosis spirochete

Bacterial motility is often a crucial virulence factor for pathogenic species. A common approach to study bacterial motility is fluorescent labeling, which allows detection of individual bacterial cells in a population or in host tissues. However, the use of fluorescent labeling can be hampered by protein expression stability and/or interference with bacterial physiology. Here, we apply machine learning to microscopic image analysis for label-free motion tracking of the zoonotic bacterium Leptospira interrogans on cultured animal cells. We use various leptospiral strains isolated from a human patient or animals, as well as mutant strains. Strains associated with severe disease, and mutant strains lacking outer membrane proteins (OMPs), tend to display fast mobility and reduced adherence on cultured kidney cells. Our method does not require fluorescent labeling or genetic manipulation, and thus could be applied to study motility of many other bacterial species.

Motility of the Zoonotic Spirochete Leptospira: Insight into Association with Pathogenicity

If a bacterium has motility, it will use the ability to survive and thrive. For many pathogenic species, their motilities are a crucial virulence factor. The form of motility varies among the species. Some use flagella for swimming in liquid, and others use the cell-surface machinery to move over solid surfaces. Spirochetes are distinguished from other bacterial species by their helical or flat wave morphology and periplasmic flagella (PFs). It is believed that the rotation of PFs beneath the outer membrane causes transformation or rolling of the cell body, propelling the spirochetes. Interestingly, some spirochetal species exhibit motility both in liquid and over surfaces, but it is not fully unveiled how the spirochete pathogenicity involves such amphibious motility. This review focuses on the causative agent of zoonosis leptospirosis and discusses the significance of their motility in liquid and on surfaces, called crawling, as a virulence factor.

Role of the major determinant of polar flagellation FlhG in the endoflagella-containing spirochete Leptospira

Spirochetes can be distinguished from other bacteria by their spiral-shaped morphology and subpolar periplasmic flagella. This study focused on FlhF and FlhG, which control the spatial and numerical regulation of flagella in many exoflagellated bacteria, in the spirochete Leptospira. In contrast to flhF which seems to be essential in Leptospira, we demonstrated that flhG- mutants in both the saprophyte L. biflexa and the pathogen L. interrogans were less motile than the wild-type strains in gel-like environments but not hyperflagellated as reported previously in other bacteria. Cryo-electron tomography revealed that the distance between the flagellar basal body and the tip of the cell decreased significantly in the flhG- mutant in comparison to wild-type and complemented strains. Additionally, comparative transcriptome analyses of L. biflexa flhG- and wild-type strains showed that FlhG acts as a negative regulator of transcription of some flagellar genes. We found that the L. interrogans flhG- mutant was attenuated for virulence in the hamster model. Cross-species complementation also showed that flhG is not interchangeable between species. Our results indicate that FlhF and FlhG in Leptospira contribute to governing cell motility but our data support the hypothesis that FlhF and FlhG function differently in each bacterial species, including among spirochetes.

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 [Formula: see text] 17 pN of force, which is [Formula: see text] 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 [Formula: see text] 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.

In Situ Structural Analysis of Leptospira spp. by Electron Cryotomography

Spirochetes such as Treponema, Borrelia, and Leptospira species can rotate their bodies to swim in liquid environments by rotating periplasmic flagella or endoflagella, which are present inside the cell. Electron cryotomography (ECT) is an imaging technique that directly provides three-dimensional (3D) structures of cells and molecular complexes in their cellular environment at nanometer resolution. Here, I present a general protocol of ECT that covers the sample preparation, data collection, tilt series alignment, and tomographic reconstruction for visualization of intact periplasmic flagella in Leptospira spp. This protocol is capable of determining protein structures at resolutions high enough to visualize their individual domains and secondary structures in their cellular environment.

Measurement of the Cell-Body Rotation of Leptospira

Leptospira spp. swim in liquid and crawl on surfaces with two periplasmic flagella. The periplasmic flagella attach to the protoplasmic cylinder via basal rotary motors (flagellar motors) and transform the ends of the cell body into spiral or hook shape. The rotations of the periplasmic flagella are thought to gyrate the cell body and rotate the protoplasmic cylinder for propelling the cell; however, the motility mechanism has not been fully elucidated. Since the motility is a critical virulence factor for pathogenic leptospires, the kinematic insight is valuable to understand the mechanism of infection. This chapter describes microscopic methodologies to measure the motility of Leptospira, focusing on rotation of the helical cell body.

Tree of motility - A proposed history of motility systems in the tree of life

Motility often plays a decisive role in the survival of species. Five systems of motility have been studied in depth: those propelled by bacterial flagella, eukaryotic actin polymerization and the eukaryotic motor proteins myosin, kinesin and dynein. However, many organisms exhibit surprisingly diverse motilities, and advances in genomics, molecular biology and imaging have showed that those motilities have inherently independent mechanisms. This makes defining the breadth of motility nontrivial, because novel motilities may be driven by unknown mechanisms. Here, we classify the known motilities based on the unique classes of movement‐producing protein architectures. Based on this criterion, the current total of independent motility systems stands at 18 types. In this perspective, we discuss these modes of motility relative to the latest phylogenetic Tree of Life and propose a history of motility. During the ~4 billion years since the emergence of life, motility arose in Bacteria with flagella and pili, and in Archaea with archaella. Newer modes of motility became possible in Eukarya with changes to the cell envelope. Presence or absence of a peptidoglycan layer, the acquisition of robust membrane dynamics, the enlargement of cells and environmental opportunities likely provided the context for the (co)evolution of novel types of motility.

Spirochete Flagella and Motility

Spirochetes can be distinguished from other flagellated bacteria by their long, thin, spiral (or wavy) cell bodies and endoflagella that reside within the periplasmic space, designated as periplasmic flagella (PFs). Some members of the spirochetes are pathogenic, including the causative agents of syphilis, Lyme disease, swine dysentery, and leptospirosis. Furthermore, their unique morphologies have attracted attention of structural biologists; however, the underlying physics of viscoelasticity-dependent spirochetal motility is a longstanding mystery. Elucidating the molecular basis of spirochetal invasion and interaction with hosts, resulting in the appearance of symptoms or the generation of asymptomatic reservoirs, will lead to a deeper understanding of host-pathogen relationships and the development of antimicrobials. Moreover, the mechanism of propulsion in fluids or on surfaces by the rotation of PFs within the narrow periplasmic space could be a designing base for an autonomously driving micro-robot with high efficiency. This review describes diverse morphology and motility observed among the spirochetes and further summarizes the current knowledge on their mechanisms and relations to pathogenicity, mainly from the standpoint of experimental biophysics.

FcpB Is a Surface Filament Protein of the Endoflagellum Required for the Motility of the Spirochete Leptospira

The spirochete endoflagellum is a unique motility apparatus among bacteria. Despite its critical importance for pathogenesis, the full composition of the flagellum remains to be determined. We have recently reported that FcpA is a novel flagellar protein and a major component of the sheath of the filament of the spirochete Leptospira. By screening a library of random transposon mutants in the spirochete Leptospira biflexa, we found a motility-deficient mutant harboring a disruption in a hypothetical gene of unknown function. Here, we show that this gene encodes a surface component of the endoflagellar filament and is required for typical hook- and spiral-shaped ends of the cell body, coiled structure of the endoflagella, and high velocity phenotype. We therefore named the gene fcpB for flagellar-coiling protein B. fcpB is conserved in all members of the Leptospira genus, but not present in other organisms including other spirochetes. Complementation of the fcpB^- mutant restored the wild-type morphology and motility phenotypes. Immunoblotting with anti-FcpA and anti-FcpB antisera and cryo-electron microscopy of the filament indicated that FcpB assembled onto the surface of the sheath of the filament and mostly located on the outer (convex) side of the coiled filament. We provide evidence that FcpB, together with FcpA, are Leptospira-specific novel components of the sheath of the filament, key determinants of the coiled and asymmetric structure of the endoflagella and are essential for high velocity. Defining the components of the endoflagella and their functions in these atypical bacteria should greatly enhance our understanding of the mechanisms by which these bacteria produce motility.

The mechanism of two-phase motility in the spirochete Leptospira: Swimming and crawling

Many species of bacteria are motile, but their migration mechanisms are considerably diverse. Whatever mechanism is used, being motile allows bacteria to search for more optimal environments for growth, and motility is a crucial virulence factor for pathogenic species. The spirochete Leptospira, having two flagella in the periplasmic space, swims in liquid but has also been previously shown to crawl over solid surfaces. The present motility assays show that the spirochete movements both in liquid and on surfaces involve a rotation of the helical cell body. Direct observations of cell-surface movement with amino-specific fluorescent dye and antibody-coated microbeads suggest that the spirochete attaches to the surface via mobile, adhesive outer membrane components, and the cell body rotation propels the cell relative to the anchoring points. Our results provide models of how the spirochete switches its motility mode from swimming to crawling.