Abstract
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.
Highlights
Motility systems of living organisms are currently classified into 18 types [1]
Spirochetes, which are members of a group of gram-negative bacteria with a spiral or flat-wave cell body, show flagella-dependent motility, but their flagella are hidden within the periplasmic space and are called periplasmic flagella (PFs)
Flagellated bacteria are propelled by direct interaction of flagella and fluid, whereas spirochetes swim by rolling or undulation of a cell body driven by PFs rotation beneath the outer membrane
Summary
Motility systems of living organisms are currently classified into 18 types [1]. Even when focusing on bacteria only, the motility is diverse when bacterial species are concerned [2]. A major motility form would be the flagella-dependent swimming well observed and described in Escherichia coli and Salmonella enterica, and these species have helical flagella extending to the cell exterior. Spirochetes, which are members of a group of gram-negative bacteria with a spiral or flat-wave cell body, show flagella-dependent motility, but their flagella are hidden within the periplasmic space and are called periplasmic flagella (PFs). This review article describes the motility of spirochetes while connecting it with the unique structures of their cell bodies and PFs. Taxonomically, the phylum Spirochaetae is classified into Leptospiraceae, Brachyspiraceae, Spirochaetaceae, and Brevinemataceae families, containing pathogenic species, for example, Leptospira interrogans (leptospirosis), Brachyspira hyodysenteriae (swine dysentery), Borrelia burgdorferi (Lyme disease), and Treponema pallidum (syphilis). The last part of this review discusses the involvement of motility in spirochetal pathogenicity
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