Phylum Spirochaetes

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Taxonomy

  • Phylum Spirochaetes
    • Class Spirochaetes
      • Order Spirochaetales
        • Family Spirochaetaceae
          • Genus Spirochaeta
          • Genus Borrelia
          • Genus Brevinema
          • Genus Clevelandina
          • Genus Cristispira
          • Genus Diplocalyx
          • Genus Hollandina
          • Genus Pillotina
          • Genus Treponema
        • Family Serpulinaceae
          • Genus Serpulina
          • Genus Brachyspira
          • Genus Serpula
        • Family Leptospiraceae
          • Genus Leptospira
          • Genus Leptonema
          • Genus Turneriella

About this phylum

Diversity

Most of familiar organisms in this group (the pathogens and their relatives) are closely-related members of the family Spirochaetaceae. Many species that were originally thought to be members of this group, even species of the genera Spirochaeta and Treponema have more recently been shown by phylogenetic analysis to constitute a separate group, the Family Serpulinaceae. This Family is known primarily for animal and human intestinal parasites. The environmental species are more diverse, and because they are typically difficult to grow, even most of the conspicuous species have not been grown or studied in culture, and so their phylogenetic affiliations are generally unknown.

Metabolism

The spirochaetes are uniformly heterotrophs and generally microaerophilic or anaerobic and saccharolytic, although some ferment amino acids (e.g. Treponema denticola, a member of the normal flora of human teeth and gingiva). H2 and CO2 are the main products of this fermentation, although some can convert these two waste products to acetate. In contrast, the leptospiras are generally aerobic degraders of fatty acids.

Morphology

Spirochaete morphology
Spirochaete morphlogy - the periplasmic flagella (axial fibers) are labeled "PF"
The prokaryotes, pp3540, attributed to EM Walker

The spirochaetes share a common body plan. The long, thin body of the cell is generally helical or a two-dimensional wave. Polar flagella (typically one at each end, but more are present in the larger species) are anchored subterminally in the cytoplasmic membrane but do not emerge from the outer membrane; they are contained in the periplasm. These flagella wind their way along the curved or helical shape of the cell body, usually overlapping in the medial part of the cell. Flagellar rotation causes the body of the cell to rotate within the outer membrane. In flat wavy species, this will cause the cell to move across a surface or through a liquid environment similar to the way a snake moves along the ground or in the water. This also works for helical species because the viscosity of their surroundings retard rotation of the outer membrane surface; the shape of the cell therefore rotates like a corkscrew, providing propulsive force.

Habitat

Spirochaetes are common sediment inhabitants, especially in those rich in decomposing plant material, in which they are involved in the decomposition of cellulose and other polysaccharides. The best-studied spirochaetes, however, are symbionts or parasites of the gastrointestinal tract of animals, including humans. A few parasites (e.g. Treponema pallidum) invade the tissues of the host, but most inhabit the surface of the mucosa. The richest, most readily available source of samples containing large numbers of spirochaetes are the hindguts (homologous to the colon of vertebrates) of wood-eating insects, especially termites.

Are Spirochaetes the progenitors of eukaryotic flagella?

termites
A termite (top), and it's GI tract. The enlarged posterior half is the hindgut. A sample of this magnified is shown below, showing free-living spirochaetes and the protist Trichonympha : John Breznak, Michigan State University

Termites and other wood-eating insects subsist primarily on a diet of cellulose. The degradation of cellulose and the generation of the nutrients required by the insect is an involved process carried out by a complex population of symbiotic Bacteria (including spirochaetes), Archaea (methanogens), and eukaryotes (protists). Some of the protists involved harbor symbiotic spirochaetes, which are attached to the surface of the protists at one end of their wavy or helical cells. Movement of the this “collective” is driven by movement by the spirochaetes, but apparently directed by the protist. The spirochaetes greatly resemble flagella or long cilia, to the extent that in many cases it is difficult to distinguish normal flagella born by the protist from symbiotic spirocheates attached to the same creature. This similarity in form and function lead to the suggestion that eukaryotic flagella, and the associated structures including cilia, basal bodies, and the spindle apparatus, all hallmarks of the eukaryotic cell structure, may have originated by symbiosis of a progenitor eukaryote with a spirochaete. We know this is the case for the origin of mitochondria (by symbiosis with an α-proteobacterium) and plastids (by symbioses with cyanobacteria). However, unlike mitochondria or plastids, neither flagella nor their associated cellular structures contain DNA, nor is their evidence for the transfer of significant numbers of spirochaete genes into the nuclear genome. Neither do the cytoskeletal structures of spirochetes nor their periplasmic flagella resemble in structure, mechanism, or molecular sequence the tubulin/microtubule structures of eukaryotic flagella or their associated structures.

Spirochaetes

The familiar genera in this Family are Spirochaeta (free-living species), Treponema (animal symbionts and pathogens) and Borrelia (human pathogens with arthropod vectors). These species are anaerobic, microaerophilic or facultatively aerobic. The cultivated Spirochaeta and Borrelia are saccharolytic. Cultivated Treponema are also generally saccharolytic, but a few can grow on fatty acids or amino acids and most have not been grown in pure culture and so their growth substrates are not known. Linear (rather than circular) genomes and plasmids are common in this group.

Example : Treponema denticola

Treponema
"Coloured scanning electron micrograph (SEM) of bacteria and plaque particles found in a decaying tooth. Yellow and green bacteria are seen amongst brown plaque." Magnifi cation: x1,500 at 6 x 6 cm size. COVER IMAGE copyright EYE OF SCIENCE/SCIENCE PHOTO LIBRARY

Although this genus is best known for the important pathogen Treponema pallidum, treponemes are common in the mouth and GI tract of healthy humans and other animals. The pathogenicity of T. denticola is not clear; it is found in dental plaque and gingiva of healthy human teeth (and those of other primates), but are more abundant in people and teeth affected by gingivitis. Although usually described as helical in shape, this specie is probably a 2-dimensional wave like its close relative, T. pallidum. Unlike most spirochaetes and treponemes, T. denticola is strongly proteolytic, and so presumably is capable of fermentation of amino acids in its natural environment.

Example : Borrelia recurrentis

Borrelia
Borrelia recurrentis : http://www.techmicrobio.net/html/systematique/GramNegatif/Spirochetes/Spirochetes.html (UNATTRIBUTED)

Although the best known Borellia is B. burgdorferi, the causative agent of Lyme disease, a wide variety of Borrelia species cause similar zoonotic infections. B. recurrentis is the cause of louse-borne relapsing fever. This relapsing fever is caused by the organisms use of antigenic variation to evade elimination by the hosts immune response. Like other species of this genus, the ends of the cells are tapered to fine points. Cells are relatively open helices, and contain 8-10 periplasmic flagella. Although it has not been tested in the case of this specie, other species of Borellia have linear genomes.

Leptospiras

In contrast to the other spirochaetes, the leptospiras are obligately aerobic and use fatty acids as their growth substrate. Leptospiras are common in the aquatic environments and in association with mammals, including humans. Human pathogenesis is zoonotic; humans are apparently not effective hosts or carriers. Animals carriers harbor the parasites in their kidneys and the transmit the infection via urine. Human infections are primarily those with direct contact with wild or domestic mammals. Leptospirosis is easily confused with other fevers, especially yellow fever. Only two periplasmic flagella are present, and the cells a very thin and tightly coiled, usually with bent or curved ends.

Example : Leptospira biflexa

Leptospira
Leptospira biflexa : http://www.pasteur.fr/recherche/RAR/RAR2007/Spiro-en.html (unattributed)

L. biflexa is a non-pathogenic aquatic specie that is very closely related to several pathogenic and opportunistically pathogenic species, and so serves as a model system for spirochaete virulence. It is also a model system for examining motility in spirochaetes. When moving, the trailing end of the cell is bent in a hook shape, and the leading end forms an open left-handed spiral (over and above the usual right-handed helical cell shape). Both of these rotate as the cell progresses forward. Although rotation of the helical cell body is sufficient to propel the organism through low viscosity environments, the rotation of the hook and spiral ends allow the organism to bore its way through viscous environments efficiently.

Leptospira motility
Motility in Leptospira : Goldstein & Charon 1988 Cell Motility & the Cytoskeleton 9:101-110