Cross section of a spirochaete cellEndoflagella Components. Legend: Fig. 1: A cross-section of a typical spirochete cell showing endoflagella located in the periplasm between the inner cytoplasmic membrane and the outer membrane. Periplasm, consisting of a gel-like matrix, provides a semi-stable medium to secure endoflagella during rotation. The axial filament, indicated in red, is composed of bundles of endoflagella. Fig. 2: A side-view of a spirochete cell which shows two axial filaments in opposing motion. One axial filament rotates in a clockwise orientation; an adjacent axial filament rotates in a counter-clockwise orientation. Rotation of the endoflagella creates torsion and drives the corkscrew rotation of the cell. Fig. 3: An expanded view of the cellular membranes that surround endoflagellum. Both the inner and outer membrane contain a phospholipid bi-layer, with non-polar fatty acid chains in-ward of polar phosphorus heads. Peptidoglycan, the cell wall, provides structure in bacterial microorganisms. Axial filaments are superior to the peptidoglycan.
Spirochaetes are distinguished from other bacterial phyla by the location of theirflagella, calledendoflagella, orperiplasmic flagella, which are sometimes calledaxial filaments.[8][9] Endoflagella are anchored at each end (pole) of the bacterium within theperiplasmic space (between the inner and outer membranes) where they project backwards to extend the length of the cell.[10] These cause a twisting motion which allows the spirochaete to move. When reproducing, a spirochaete will undergo asexualtransverse binary fission. Most spirochaetes are free-living andanaerobic, but there are numerous exceptions. Spirochaete bacteria are diverse in theirpathogenic capacity and theecological niches that they inhabit, as well as molecular characteristics includingguanine-cytosine content andgenome size.[11][12]
The class currently consists of 14 validly named genera across four orders and five families.[18][19][20] The ordersBrachyspirales,Brevinematales andLeptospirales each contain a single family,Brachyspiraceae,Brevinemataceae andLeptospiraceae, respectively. TheSpirochaetales order harbours two families,Spirochaetaceae andBorreliaceae. Molecular markers in the form ofconserved signature indels (CSIs) and CSPs have been found specific for each of the orders, with the exception ofBrevinimetales, that provide a reliable means to demarcate these clades from one another within the diverse phylum.[19] AdditionalCSIs have been found exclusively shared by each family within theSpirochaetales. These molecular markers are in agreement with the observed phylogenetic tree branching of two monophyletic clades within theSpirochaetales order.[19] CSIs have also been found that further differentiate taxonomic groups within theBorreliaceae family that further delineate evolutionary relationships that are in accordance with physical characteristics such as pathogenicity (viz.Borrelia emend.Borreliella gen. nov.).[21] However, this study has been criticized, and other studies using different approaches do not support the proposed split.[22] The new naming system for the Lyme and relapsing feverBorrelia has not been adopted by the scientific literature.[22]
A CSI has also been found exclusively shared by all Spirochaetota species.[19] This CSI is a three-amino-acid insert in the flagellarbasal body rod protein FlgC which is an important part of the unique endoflagellar structure shared by Spirochaetota species.[23] Given that the CSI is exclusively shared by members within this phylum, it has been postulated that it may be related to the characteristic flagellar properties observed among Spirochaetota species.[19][23]
Historically, all families belonging to the Spirochaetota phylum were assigned to a single order, theSpirochaetales.[11][12] However, the current taxonomic view is more connotative of accurate evolutionary relationships. The distribution of a CSI is indicative of shared ancestry within the clade for which it is specific. It thus functions as asynapomorphic characteristic, so that the distributions of different CSIs provide the means to identify different orders and families within the phylum and so justify the phylogenetic divisions.[19]
^Madigan MT (2019).Brock biology of microorganisms (Fifteenth, Global ed.). NY, NY: Pearson. p. 519.ISBN9781292235103.
^abPaster BJ (2011). "Phylum XV. Spirochaetes Garrity and Holt.". In Brenner DJ, Krieg NR, Garrity GM, Staley JT (eds.).Bergey's Manual of Systematic Bacteriology. New York: Springer. p. 471.
^abPaster BJ (2011). "Family I. Sprochaetes Swellengrebel 1907, 581AL.". In Brenner DJ, Krieg NR, Garrity GM, Staley JT (eds.).Bergey's Manual of Systematic Bacteriology. New York: Springer. pp. 473–531.
^Wolcott KA, Margos G, Fingerle V, Becker NS (September 2021). "Host association of Borrelia burgdorferi sensu lato: A review".Ticks and Tick-Borne Diseases.12 (5): 101766.doi:10.1016/j.ttbdis.2021.101766.PMID34161868.
^Schwan TG (June 1996). "Ticks and Borrelia: model systems for investigating pathogen-arthropod interactions".Infectious Agents and Disease.5 (3):167–181.PMID8805079.
^Amat Villegas I, Borobio Aguilar E, Beloqui Perez R, de Llano Varela P, Oquiñena Legaz S, Martínez-Peñuela Virseda JM (January 2004). "[Colonic spirochetes: an infrequent cause of adult diarrhea]".Gastroenterol Hepatol (in Spanish).27 (1):21–3.doi:10.1016/s0210-5705(03)70440-3.PMID14718105.
^Adeolu M, Gupta RS (2014). "A phylogenomic and molecular marker based proposal for the division of the genus Borrelia into two genera: the emended genus Borrelia containing only the members of the relapsing fever Borrelia, and the genus Borreliella gen. nov. containing the members of the Lyme disease Borrelia (Borrelia burgdorferi sensu lato complex)".Antonie van Leeuwenhoek.105 (6):1049–1072.doi:10.1007/s10482-014-0164-x.PMID24744012.
