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.
Spirochaetes are distinguished from other bacterial phyla by the location of their flagella, sometimes called axial filaments, which run lengthwise between the bacterial inner membrane and outer membrane in periplasmic space. 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, the ecological niches that they inhabit, as well as molecular characteristics including guanine-cytosine content and genome size.
The class currently consists of 14 validly named genera across 4 orders and 5 families. 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. 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. 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.).
A CSI has also been found exclusively shared by all Spirochaetes species. 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. 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.
Historically, all families belonging to the Spirochaetes phylum were assigned to a single order, the Spirochaetales. However, the current taxonomic view is more connotative of accurate evolutionary relationships, where the distribution of conserved signature indels are indicative of shared ancestry within each respective clade for which they are specific. These synapomorphic characteristics justify the phylogenetic divisions and are a means to identify each order/family within the phylum.
^ abPaster BJ (2011) Phylum XV. Sprochaetes Garrity and Holt. In: Bergey’s Manual of Systematic Bacteriology, pp. 471. Eds D.J. Brenner, N.R.Krieg, G.M.Garrity, and J.T. Staley Springer-: New York.
^ abPaster BJ (2011) Family I. Sprochaetes Swellengrebel 1907, 581AL. In: Bergey’s Manual of Systematic Bacteriology, pp. 473-531. Eds D.J. Brenner, N.R.Krieg, G.M.Garrity, and J.T. Staley Springer-: New York.
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