Species in the phylum Spirochaetes (order: Spirochaetales) are thin, spiral‐shaped or wave‐like, highly motile bacteria that are best visualised by darkfield microscopy. Spirochaetes are Gram‐negative‐like, in that they possess inner and outer membranes separated by a peptidoglycan‐containing periplasmic space. Beyond these ultrastructural similarities, few parallels exist with conventional Gram‐negative bacteria. There have been more than 90 spirochaete species identified, including the well‐known pathogenic, disease‐causing species Treponema pallidum (syphilis), Leptospira sp. (leptospirosis) and Borrelia burgdorferi (Lyme disease). The phylogenetic relationship among spirochaetes is evident at the level of gross phenotypic characteristics, which include a distinctive morphology derived from the presence of periplasmic flagella. In this article, the authors describe the genomic, physiologic, structural and functional features that make Spirochaetes unique among the prokaryotes.

Key Concepts:

  • Spirochaetes are a genetically and morphologically distinct group of bacteria.

  • Spirochaetes are thin spiral‐shaped or wave‐like bacteria that are highly motile.

  • Endoflagella/periplasmic flagella provide spirochaetes with their characteristic corkscrew‐like motility.

  • Spirochaetes differ from typical Gram‐negative bacteria at the ultrastructural level.

  • Spirochaetes are chemo‐organotrophic, grow in diverse conditions and are comprised of both pathogenic and nonpathogenic members.

  • Complete genome sequencing has allowed for the characterisation and differentiation of spirochaetes at the genetic level.

  • Medically important spirochaetes belong to the three genera Borrelia, Leptospira and Treponema.

Keywords: periplasmic endoflagella; syphilis; leptospirosis; Lyme disease; Treponema; Leptospira; Borrelia

Figure 1.

Evolutionary relationships among representatives of Spirochaetales. The evolutionary history was inferred using the Neighbor‐Joining method. The percentage of replicate trees in which the associated taxa clustered together in the bootstrap test (1000 replicates) is shown next to the branches (Felsenstein, ). The tree is drawn to scale, with branch lengths computed using the p‐distance method (Nei and Kumar, ). The analysis involved 37 nucleotide sequences. All ambiguous positions were removed for each sequence pair. There were a total of 1492 positions in the final dataset. Evolutionary analyses were conducted in MEGA5 Tamura et al. ().

Figure 2.

Schematic diagram of the three representative genera Treponema (top image), Leptospira (middle image), and Borrelia (bottom image).

Figure 3.

Treponema pallidum has a flat‐wave morphology. Representative micrographs show the flat‐wave morphology of T. pallidum as revealed by darkfield microscopy (a and b) and by epifluorescence microscopy (c and d). Panels c and d show sequential images of the same treponeme; note how the leftward segment changes from helical to linear as it moves away from the focal plane. Panels e and f show darkfield images of Borrelia burgdorferi in two different orientations. Arrows and asterisks indicate regions of the spirochaetes that are parallel to the z axis or in the xy plane, respectively. Reproduced with permission of Izard et al. (). © American Society for Microbiology.

Figure 4.

The T. pallidum cell envelope architecture. The space between the outer and cytoplasmic membranes increases from approximately 23 nm to approximately 49 nm in regions containing the periplasmic flagella (a and c). The periplasmic flagella (blue line) remain closely associated with the cell cylinder following removal of the outer membrane through repeated centrifugation (b and d). The 4 nm lipid bilayer composition of the cytoplasmic membrane is visible in higher magnification views (e and g). The locations of the cytoplasmic filaments (red line), lipoprotein layer (purple circles) and peptidoglycan (orange line) are indicated (e). a, b and e are longitudinal views, while c and d are the cross‐section views. Most membrane proteins (coloured in orange) are anchored to the cytoplasmic membrane (CM) just underneath the thin layer of peptidoglycan (PG). A model of the T. pallidum cell envelope is shown in (f). Rare outer membrane proteins (coloured in purple) are exposed on the outer membrane (OM). Reproduced with permission from Liu et al. (). © Elsevier.



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Further Reading

Adler B, Lo M, Seemann T and Murrray GL (2011) Pathogenesis of leptospirosis: the influence of genomics. Veterinary Microbiology 153: 73–81.

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Samuels DS and Radolf JD (eds) (2010) Borrelia: Molecular Biology, Host Interaction and Pathogenesis. Norfolk: Caister Academic Press.

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Houston, Simon, Taylor, John S, and Cameron, Caroline E(Jul 2014) Spirochaetes. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000466.pub3]