Spirochetes: Morphology, Classification, Disease

Spirochetes are long, thin, flexible, spirally coiled helical bacilli. These motile, gram-negative bacilli are classified into eight genera primarily on the basis of habitat, pathogenicity, phylogeny, morphological and physiological characteristics.

These long slender bacteria are only a fraction of a micron in diameter but 5 to 250 microns long. They are tightly coiled, and so look like miniature springs or telephone cords. Spirochetes do not take up ordinary stains but can be stained only by silver impregnation stains, except Borreliawhich is poorly gram stained.

Most of the genera of spirochetes are saprophytes and are widespread in aquatic environments and in animals. Only three genera Treponema, Borrelia, and Leptospira contain organisms pathogenic for humans. Spirochetes cause several diseases, notably syphilis, Lyme disease, relapsing fever, leptospirosis, yaws, pinta, etc.

The order Spirochaetales includes two families of spiral bacteria, Spirochaetaceae and Leptospiraceae.

• Order Spirochaetales

Family Spirochaetaceae: Four genera Treponema, Borrelia, Spirocheta, CristispiraFamily Leptospiraceae: Two genera Leptospira, Leptonema

Differentiation of the genera within the family is based on the number of axial fibrils, the number of insertion disks present and biochemical and metabolic features.

Structure

Spirochete motility is conferred by flagella that emerge from each pole. However, unlike typical bacterial flagella, spirochete flagella fold back from each pole onto the protoplasmic cylinder itself and remain in the periplasm of the cell; because of this, they have been called endoflagella. In addition, both endoflagella and the protoplasmic cylinder are surrounded by a flexible membrane called the outer sheath.

Figure: Endoflagella of spirochetes (Image source:crondon)

Cell wall

Cell wall of spirochetes is similar to that of gram-negative bacteria but differs by bearing endoflagella. It is more complex, consisting of:

• Outer membrane
• Periplasmic space containing flagella
• Peptidoglycan layer
• Inner (cytoplasmic) membrane

Endoflagella

Endoflagella are present in the periplasmic space between the outer membrane and the peptidoglycan layer. They attach to the membrane only at the pole. The number of periplasmic flagella varies from species to species.

Endoflagella are responsible for the motility of spirochetes. Motility may be of various types, such as:

• Flexion-extension type
• Corkscrew type rotatory movement
• Translatory type

Spirochetes vs. Spirilla

Though spirochetes and spirilla differ phylogenetically, spirilla with many turns can superficially resemble spirochetes. In addition, spirilla do not have spirochetes’ outer sheath and endoflagella but instead contain typical bacterial flagella.

Morphological differences between Treponema, Borrelia and Leptospira

Diseases caused by various genus of Spirochetes

References and further readings

1. Bacterial Motility, Spirochaetes. Retrieved June 8, 2021, from https://cronodon.com/BioTech/Bacteria_motility3.html
2. Bailey & Scott’s Diagnostic Microbiology, Forbes, 11th edition