Spirochetes: Morphology, Classification, Disease

Spirochetes are long, thin, flexible, helical bacteria. These motile, gram-negative organisms are classified into several genera 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 Borrelia which is poorly gram stained.

Features of Spirochetes

Three features make spirochetes a distinctive group and explain how they cause disease and how they are diagnosed:

• They are too thin to see on ordinary stains. Most spirochetes are so slender they cannot be seen by routine light microscopy or Gram stain, so they are studied by dark-field microscopy, silver stains, or immunofluorescence, and often diagnosed by serology rather than by seeing the organism. (Borrelia is the exception, thick enough to show on a Giemsa-stained blood smear.)
• They move with internal flagella. Their flagella run inside the cell (endoflagella, in the periplasm), not outside, which gives spirochetes their characteristic corkscrew and flexing motility and lets them burrow through tissue and viscous fluid. This motility is a genuine virulence feature.
• Only three genera cause human disease: Treponema (syphilis), Borrelia (Lyme disease and relapsing fever), and Leptospira (leptospirosis). The rest are harmless environmental organisms. Learning spirochetes really means learning these three.

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, Spirochaeta, Cristispira
• Family 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 comes from flagella anchored at each pole. Unlike typical bacterial flagella, these do not project outside the cell. They fold back along the protoplasmic cylinder and stay within the periplasm, which is why they are called endoflagella. In addition, both endoflagella and the protoplasmic cylinder are surrounded by a flexible membrane called the outer sheath.

Figure: Endoflagella of spirochetes

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. Spirilla do not have the spirochete's outer sheath or endoflagella. Instead they have typical external bacterial flagella.

Two common confusions

Weil's disease is not the Weil-Felix test. Weil's disease is the severe form of leptospirosis (a spirochete infection). The Weil-Felix test is an unrelated test for rickettsial infection that uses Proteus antigens. They share only the name "Weil."

Not every "coiled" bacterium is a spirochete. Spirilla (like Campylobacter and Helicobacter) are curved or spiral but have ordinary external flagella and no outer sheath. True spirochetes have internal flagella (endoflagella) inside an outer sheath. The internal flagella are what define a spirochete.

Morphological differences between Treponema, Borrelia and Leptospira

Feature	Treponema	Borrelia	Leptospira
Size	6–14 μm × 0.2 μm	10–30 μm × 0.2–0.5 μm	6–20 μm × 0.1 μm
Spirals (in number)	6-12	3-10	Numerous and tightly coiled with hooked ends
Wavelength	1 μm	3 μm	0.5 μm
Amplitude of spiral	1-1.5 μm	Up to 2 μm	0.1 μm
Endoflagella of each pole	3-4	7-11	1

Diseases caused by various genera of spirochetes

Spirochetes	Disease	Transmission
T. pallidum subsp. pallidum	Syphilis	Sexual contact
T. pertenue	Yaws	Direct contact
T. carateum	Pinta	Direct contact
B. burgdorferi	Lyme disease	Tick-borne
B. recurrentis	Relapsing fever	Louse-borne
L. interrogans	Leptospirosis	Contact with rodent urine

How to remember

Three pathogens: Treponema, Borrelia, Leptospira. Of all the spirochetes, only these three cause human disease: Treponema (syphilis), Borrelia (Lyme and relapsing fever), Leptospira (leptospirosis).

Flagella on the inside. What makes a spirochete a spirochete is its internal flagella (endoflagella), running inside an outer sheath rather than sticking out. This gives the corkscrew motility and lets it drill through tissue. Inside flagella = spirochete.

Too thin to stain, except Borrelia. Most spirochetes are too slender for the Gram stain and need dark-field, silver stain, or serology. Borrelia is the exception: fat enough to see on a Giemsa blood smear. So Borrelia is the one you can catch on a routine smear.

Match the genus to how you diagnose it. Treponema (can't culture, diagnose by serology), Borrelia (see it on a blood smear in relapsing fever; serology for Lyme), Leptospira (culture is slow; MAT serology; test blood early, urine late).

Key exam facts

Fact	Detail
What they are	Long, thin, coiled bacteria with internal flagella
Motility	Endoflagella (periplasmic) → corkscrew/flexing movement
Staining	Too thin for Gram stain; dark-field, silver stain, immunofluorescence (except Borrelia, seen on Giemsa)
Human pathogens	Treponema, Borrelia, Leptospira (only these three)
Treponema	Syphilis (T. pallidum), yaws, pinta, bejel
Borrelia	Lyme disease (B. burgdorferi), relapsing fever (B. recurrentis and others)
Leptospira	Leptospirosis (L. interrogans); severe form = Weil's disease
Endoflagella per pole	Treponema 3–4; Borrelia 7–11; Leptospira 1
Spirochete vs spirillum	Spirochetes have internal flagella + outer sheath; spirilla have external flagella
Disambiguation	Weil's disease (leptospirosis) ≠ Weil-Felix test (rickettsia)

Where students get confused

Only three genera cause human disease. Most spirochetes are harmless environmental organisms. The human pathogens are Treponema, Borrelia, and Leptospira. Knowing which three matters more than the full taxonomy.

Spirochetes are not the same as spirilla. Both look coiled, but spirochetes have internal flagella inside an outer sheath, while spirilla (like Campylobacter) have ordinary external flagella. The endoflagella are the defining feature.

Most spirochetes cannot be seen on a Gram stain. They are too thin. Diagnosis uses dark-field microscopy, silver staining, or serology. Borrelia is the exception, thick enough to see on a stained blood smear.

Weil's disease vs the Weil-Felix test. Weil's disease is severe leptospirosis. The Weil-Felix test detects rickettsial infection. Same name, unrelated.

References

1. Tille, P. M. (2022). Bailey & Scott's Diagnostic Microbiology (15th ed.). Elsevier.
2. Procop, G. W., et al. (2017). Koneman's Color Atlas and Textbook of Diagnostic Microbiology (7th ed.). Wolters Kluwer.
3. Murray, P. R., Rosenthal, K. S., & Pfaller, M. A. (2020). Medical Microbiology (9th ed.). Elsevier.