Structural features and locomotor apparatus
Spirochetes possess a complex multi-layered organization. The basic cell structure includes a protoplasmic cylinder (cytoplasm surrounded by the cytoplasmic membrane) and a thin outer membrane covering it.
The main feature of spirochetes is their motility apparatus. Unlike many other bacteria, the flagella do not protrude outward. They are represented by periplasmic fibrils concealed within the cell wall thickness (in the periplasmic space between the cylinder and the outer membrane).
- The fibrils are composed of the contractile protein flagellin and are analogs of flagella.
- One end of the fibril is firmly anchored at the pole of the cylinder (analogous to the blepharoplast), while the other remains free.
- The fibrils are directed toward each other, wrap around the cylinder, and form the helical cell shape—primary coils.
- The total number of fibrils is a strict species-specific trait (ranging from 2 to over 100 bundles at both ends).
Contraction of the fibrils provides three types of movement: translational, rotational around their own axis, and bending. During active body bending, loops are formed—secondary coils.
Comparative characteristics of pathogenic spirochetes
Among the many species of spirochetes, three genera are distinguished that cause severe human diseases. They are clearly differentiated by morphology (shape and number of coils).
- Genus Treponema
They appear as thin threads twisted like a corkscrew. They have 8 to 12 small, regular coils.
- T. pallidum — the causative agent of syphilis.
- T. pertenue — the causative agent of yaws (a tropical disease).
- Saprophytic species: non-pathogenic, inhabit water bodies and the oral cavity.
- Genus Borrelia
Unlike treponemes, their coils are larger but sparser (3 to 8 per cell).
- B. recurrentis — the causative agent of relapsing fever.
- B. burgdorferi — the causative agent of borreliosis (Lyme disease).
- Genus Leptospira
They are distinguished by frequent and shallow coils, making the cell look like a tightly twisted rope. The ends of leptospires are thickened and bent into hook shapes. Due to secondary coils, the entire cell takes the shape of the Latin letters S or C.
- L. interrogans — the causative agent of leptospirosis. The infection is transmitted alimentarily (via contaminated water or food) and clinically manifests with hemorrhages and jaundice.
Physiology and microscopy methods
By type of nutrition, spirochetes are chemoorganotrophs (obtaining energy from carbohydrates, amino acids, or lipids depending on the genus). Their relation to oxygen is extremely diverse: spirochetes include aerobes, microaerophiles, as well as facultative and strict anaerobes.
Cultivation of pathogenic spirochetes is difficult. They grow slowly and are extremely demanding regarding nutrient media (requiring the addition of serum and tissue extracts). They reproduce by transverse fission, but upon unfavorable conditions, they can enter a persistent state: form L-forms, cysts, or disintegrate into separate "granules".
Research methods: Due to their minimal thickness (0.1–0.3 µm), the detection of spirochetes has certain specific features.
- Native (live) preparations are studied using phase-contrast or dark-field microscopy.
- Fixed preparations are stained using the Romanowsky–Giemsa method or silver impregnation is applied.
To easily remember the shape of the three pathogenic genera: Treponemes — like a Precise corkscrew (small, frequent coils), Borrelia — Big and rare waves, Leptospires — Latin letters S and C (with hooks at the ends).
Class and exam questions
What is the main difference between spirochetes and other coiled bacteria (spirilla, helicobacters)?
In the arrangement of flagella. In spirochetes, flagella (periplasmic fibrils) are located inside the cell — between the protoplasmic cylinder and the outer membrane, whereas in other bacteria such as Proteobacteria, they emerge into the surrounding environment.
How do spirochetes survive under adverse conditions?
They are capable of changing their shape and entering a state of persistence: forming L-forms (cells without a cell wall), protective cysts, or disintegrating into small granules.
What is the difference between primary and secondary coils?
Primary coils determine the basic helical shape of the bacterium (formed by the wrapping of fibrils around the cylinder). Secondary coils are bends and loops of the entire cell that appear only at the moment of active movement.
What else to study on this topic
- Detailed mechanisms of persistence: how exactly L-forms and cysts are formed
- Energy metabolism of various spirochete genera
- Differences between the Romanowsky-Giemsa stain and the silver impregnation method
- Symbiotic and commensal human spirochete species (oral microflora)
- Pathogenesis of leptospirosis: why jaundice and hemorrhages occur
Study the full topic in the «Microbiology» course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Spirochetes" and 4,383 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 141 video lectures on the subject
Other topics in the section "Morphology and Classification of Microbes"
Free video lessons: microbiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.