Spirochaete

Spirochaete
Treponema pallidum, a spirochaete which causes syphilis
Scientific classification
Domain:	Bacteria
(unranked):	Gracilicutes
Phylum:	Spirochaetes Garrity & Holt 2001
Class:	Spirochaetia Paster 2011
Orders & Families
Brachyspirales Brachyspiraceae Brevinematales Brevinemataceae Leptospirales Leptospiraceae Spirochaetales Borreliaceae Spirochaetaceae
Synonyms
Spirochaetae Cavalier-Smith 2002 Spirochaetaeota Oren et al. 2015

Cross section of a spirochaete cell

Endoflagella 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.

A spirochaete (/ˈspaɪroʊˌkiːt/)^[1] or spirochete is a member of the phylum Spirochaetes (/-ˈkiːtiːz/),^[2] which contains distinctive diderm (double-membrane) gram-negative bacteria, most of which have long, helically coiled (corkscrew-shaped or spiraled, hence the name) cells.^[3] Spirochaetes are chemoheterotrophic in nature, with lengths between 3 and 500 μm and diameters around 0.09 to at least 3 μm.^[4]

Spirochaetes are distinguished from other bacterial phyla by the location of their flagella, called endoflagella which are sometimes called axial filaments.^[5] 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.^[6] These cause a twisting motion which allows the spirochaete to move about. When reproducing, a spirochaete will undergo asexual transverse binary fission. Most spirochaetes are free-living and anaerobic, but there are numerous exceptions. Spirochaetes 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.^[7][8]

Pathogenicity

Many organisms within the Spirochaetes phylum cause prevalent diseases. Pathogenic members of this phylum include the following:

• Leptospira species, which causes leptospirosis^[9]
• Borrelia burgdorferi, B. mayonii, B. bissettiae, B. garinii, B. afzelii, B. spielmanii, B. lusitaniae, which cause Lyme disease.^[10]
• Borrelia recurrentis, which causes relapsing fever^[11]
• Treponema pallidum subspecies which cause treponematoses such as syphilis and yaws.
• Brachyspira pilosicoli and Brachyspira aalborgi, which cause intestinal spirochaetosis^[12]

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.^[13]

Taxonomy and molecular signatures

The class currently consists of 14 validly named genera across 4 orders and 5 families.^[14][15][16] 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.^[15] 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.^[15] 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.).^[17] However, this study has been criticized, and other studies using different approaches do not support the proposed split.^[18] The new naming system for the Lyme and relapsing fever Borrelia has not been adopted by the scientific literature.^[18]

A CSI has also been found exclusively shared by all Spirochaetes species.^[15] 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 Spirochaetes species.^[19] 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 Spirochaetes species.^[15][19]

Historically, all families belonging to the Spirochaetes phylum were assigned to a single order, the Spirochaetales.^[7][8] 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.^[15]

Phylogeny

The phylogeny is based on 16S rRNA-based LTP release 132 by The All-Species Living Tree Project.^[20]

Taxonomy

The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN)^[21] and National Center for Biotechnology Information (NCBI).^[22]

• Phylum Spirochaetes Garrity & Holt 2001 [Spirochaetae Cavalier-Smith 2002; Spirochaetaeota Oren et al. 2015]
  • Class Spirochaetae Cavalier-Smith 2002 ["Protozoobacteriales"; "Spirochaetia" Paster 2011]
    • Order Leptospirales Gupta et al. 2014
      • Family Leptospiraceae Hovind-Hougen 1979 emend. Levett et al. 2005
        • Genus Leptonema Hovind-Hougen 1983
        • Genus Leptospira Noguchi 1917 emend. Faine and Stallman 1982
        • Genus Turneriella Levett et al. 2005 ["Turneria" Hookey, Bryden & Gatehouse 1993 non-Forel 1895]
    • Order Brachyspirales corrig. Gupta et al. 2014
      • Family Brachyspiraceae Paster 2012
        • Genus Brachyspira Hovind-Hougen et al. 1982 [Serpula Stanton et al. 1991 non Linnaeus 1758; Serpulina Stanton 1992 non Zborzevski 1834]
    • Order Brevinematales Gupta et al. 2014
      • Family Brevinemataceae Paster 2012
        • Genus Brevinema Defosse et al. 1995
    • Order Spirochaetales Buchanan 1917 emend. Gupta et al. 2013
      • Genus Exilispira Imachi et al. 2008
      • Genus Alkalispirochaeta Sravanthi et al. 2016
      • Genus Oceanispirochaeta Subhash & Lee 2017b
      • Genus Pleomorphochaeta Arroua et al. 2016
      • Genus Sediminispirochaeta Shivani et al. 2016
      • Genus Sphaerochaeta Ritalahti et al. 2012 emend. Miyazaki et al. 2014
      • Family Borreliaceae Gupta et al. 2014
        • Genus Borrelia Swellengrebel 1907 emend. Adeolu & Gupta 2014, emend. Margos et al. 2018 (relapsing fever Borrelia, Lyme borreliosis Borrelia, reptile-associated Borrelia and Echidna-associated Borrelia)
        • Genus Cristispira pectinis Gross 1910
      • Family Spirochaetaceae Swellengrebel 1907
        • Genus ?Clevelandina reticulitermitidis ♦ Bermudes et al. 1988
        • Genus ?Diplocalyx calotermitidis ♦ (ex Gharagozlou 1968) Bermudes et al. 1988
        • Genus ?Hollandina pterotermitidis ♦ (ex To et al. 1978) Bermudes et al. 1988
        • Genus ?Pillotina calotermitidis ♦ (ex Hollande and Gharagozlou 1967) Bermudes et al. 1988
        • Genus Marispirochaeta Shivani et al. 2017
        • Genus Spirochaeta Ehrenberg 1835 emend. Pikuta et al. 2009 ["Ehrenbergia" Gieszczykiewiez 1939 non-Spreng.; incl. Salinispira Ben Hania et al. 2015]
        • Genus Treponema Schaudinn 1905 emend Abt et al. 2013 ["Spironema" Vuillemin 1905 non-Klebs 1892; "Microspironema" Stiles & Pfender 1905; incl. Rectinema Koelschbach et al. 2017]

Notes:
♦ Type strain lost or not available
♪ Prokaryotes where no pure (axenic) cultures are isolated or available, i.e. not cultivated or can not be sustained in culture for more than a few serial passages
♠ Strains found at the National Center for Biotechnology Information (NCBI) but not listed in the List of Prokaryotic names with Standing in Nomenclature (LSPN)

See also

References

External links

• Introduction to the Spirochetes University of California Museum of Paleontology (UCMP)