Structural Features and Locomotor Apparatus
Spirochetes have a complex, multi-layered organization. The basic cell structure includes a protoplasmic cylinder (cytoplasm surrounded by a cytoplasmic membrane) enclosed by a thin outer membrane.
The defining feature of spirochetes is their motility apparatus. Unlike many other bacteria, flagella do not project outward. Instead, they are represented by periplasmic fibrils (axial filaments) hidden within the cell envelope (in the periplasmic space between the cylinder and the outer membrane).
- The fibrils are composed of the contractile protein flagellin and are analogs of bacterial flagella.
- One end of each fibril is firmly anchored near the pole of the cylinder (analogous to a basal body), while the other end remains free.
- The fibrils run toward each other, wrap around the cylinder, and establish the helical shape of the cell, known as 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 produces three types of movement: translational, rotational along the axis, and flexing. Active body bending creates loops known as secondary coils.
Comparative Characteristics of Pathogenic Spirochetes
Among the many species of spirochetes, three genera cause severe human diseases. They are clearly distinguished by their morphology (shape and number of coils).
- Genus Treponema
Appear as thin, corkscrew-like filaments with 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 bodies of water and the oral cavity.
- Genus Borrelia
Unlike treponemes, their coils are larger but fewer and more spread out (3 to 8 coils per cell).
- B. recurrentis — the causative agent of louse-borne relapsing fever.
- B. burgdorferi — the causative agent of Lyme disease (borreliosis).
- Genus Leptospira
Characterized by frequent, shallow coils, making the cell look like a tightly twisted rope. The ends of leptospires are thickened and hooked. Due to secondary bending, the entire cell takes the shape of the Latin letters S or C.
- L. interrogans — the causative agent of leptospirosis. The infection is transmitted via contaminated water or food and clinically manifests with hemorrhages and jaundice.
Physiology and Microscopy Methods
Nutritionally, spirochetes are chemoorganoheterotrophs (obtaining energy from carbohydrates, amino acids, or lipids depending on the genus). Their oxygen requirements vary widely: spirochetes include aerobes, microaerophiles, facultative anaerobes, and strict anaerobes.
Cultivating pathogenic spirochetes is difficult. They grow slowly and are extremely fastidious, requiring enriched media supplemented with serum and tissue extracts. They reproduce by transverse fission, but under unfavorable conditions, they can enter a persistent state: forming L-forms, cysts, or breaking down into individual "granules."
Research Methods: Due to their minimal thickness (0.1–0.3 µm), detecting spirochetes requires specialized techniques.
- Native (viable) preparations are examined using phase-contrast or dark-field microscopy.
- Fixed preparations are stained using the Romanowsky-Giemsa method or silver impregnation.