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 their flagella, called endoflagella, or periplasmic flagella, which are sometimes called axial filaments.[8][9] Endoflagella are anchored at each end (pole) of the bacterium within the periplasmic 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 asexual transverse binary fission. Most spirochaetes are free-living and anaerobic, but there are numerous exceptions. Spirochaete bacteria are diverse in their pathogenic capacity and the ecological niches that they inhabit, as well as molecular characteristics including guanine-cytosine content and genome size.[11][12]
Many organisms within the Spirochaetota phylum cause prevalent diseases. Pathogenic members of this phylum include the following:
Salvarsan, the first partially organic synthetic antimicrobial drug in medical history, was effective against spirochaetes and primarily used to cure syphilis. Additionally, oral spirochaetes are known to play a significant role in the pathogenesis of human periodontal disease.[17]
The class currently consists of 14 validly named genera across 4 orders and 5 families.[18][19][20] The orders Brachyspirales, Brevinematales and Leptospirales each contain a single family, Brachyspiraceae, Brevinemataceae and Leptospiraceae, respectively. The Spirochaetales order harbours two families, Spirochaetaceae and Borreliaceae. Molecular markers in the form of conserved signature indels (CSIs) and CSPs have been found specific for each of the orders, with the exception of Brevinimetales, that provide a reliable means to demarcate these clades from one another within the diverse phylum.[19] Additional CSIs have been found exclusively shared by each family within the Spirochaetales. These molecular markers are in agreement with the observed phylogenetic tree branching of two monophyletic clades within the Spirochaetales order.[19] CSIs have also been found that further differentiate taxonomic groups within the Borreliaceae 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 fever Borrelia 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 3 amino acid insert in the flagellar basal 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, the Spirochaetales.[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 a synapomorphic 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]
Carroll KC, Hobden JA, Miller S (2019). "Spirochetes and Other Spiral Microorganisms". Jawetz, Melnick, & Adelberg’s Medical Microbiology. McGraw-Hill Education. Retrieved 9 May 2021.
Paster 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.
Paster 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.
