Endoflagella are unique structures found exclusively in the bacterial phylum known as Spirochetes, a group that includes several organisms responsible for significant human diseases. Unlike the external whip-like appendages on many other bacteria, endoflagella are internal, residing between the cell’s membranes. This distinctive location gives them their name, inner flagella, and confers specialized functions. The unique design of the endoflagellum is directly responsible for the spirochetes’ characteristic spiral shape and their highly effective, burrowing mode of movement.
The Unique Architecture of the Endoflagellum
The endoflagellum, also referred to as a periplasmic flagellum, is situated entirely within the periplasmic space, the compartment located between the inner cytoplasmic membrane and the outer membrane of the bacterium. This internal positioning is a defining feature that sets spirochetes apart from most other motile bacteria. The flagellum itself is structurally similar to the external flagella of other bacteria, being composed of a basal body, a hook, and a filament. The basal body functions as the rotary motor, anchoring the entire apparatus to the inner membrane and generating the torque necessary for rotation. A short, flexible hook connects the basal body to the long, helical filament, which extends along the length of the cell.
The endoflagella do not extend out into the environment; instead, they originate near one pole of the cell and wrap back around the protoplasmic cylinder, often overlapping with the filaments originating from the opposite pole. Multiple filaments typically arise from each end of the cell, forming a ribbon-like bundle that spirals around the central body of the spirochete. The number of filaments can vary significantly between species. This arrangement gives the entire cell its characteristic helical or flat-wave morphology. The overall structure, including the presence of a spirochete-specific “collar” in the motor, is adapted to function within the confines of the periplasm.
Mechanism of Corkscrew Motility
The motor of the endoflagellum rotates, much like the motor of an external flagellum, to generate movement. However, because the filaments are contained within the periplasmic space, their rotation does not directly propel the cell through the surrounding fluid. Instead, the turning of the internal flagellar bundle applies a twisting force to the cell body itself. This internal rotation causes the entire helical cell body to twist and undulate, resulting in a unique corkscrew or drilling motion. The counter-rotation of the rigid cell body relative to the flagellar bundle is what drives the bacterium forward.
This physical mechanism is highly effective in environments that are thick or highly viscous, such as mucus layers or dense tissue matrices. In less viscous fluids, spirochetes may exhibit flexing and rotation without much forward movement. This ability to navigate highly viscous media is a significant functional advantage over bacteria with external flagella, which can become tangled or ineffective in such environments.
How Endoflagella Contribute to Infection
The distinctive motility generated by the endoflagella is intimately linked to the ability of spirochetes to cause disease. This mechanical penetration is a highly effective strategy for invading the host. This invasive motility allows spirochetes to quickly disseminate from the initial site of infection into deeper tissues and organs. The bacteria can actively migrate through the connective tissue and can cross barriers, such as the endothelial layers of blood vessels, facilitating systemic spread throughout the body. Loss of motility, even in genetically modified strains, significantly attenuates the bacteria’s ability to establish an infection.
Beyond mechanical invasion, the internal location of the endoflagella is a major factor in immune system evasion. By residing in the periplasm, the flagellar proteins, which would otherwise be strong antigens, are shielded from the host’s antibodies and complement system components. This unique design contributes to the persistence of spirochetal infections, allowing the bacteria to establish deep-seated, chronic conditions.
Notable Spirochete Diseases
The unique, invasive motility of endoflagella is directly responsible for the progression and severity of several diseases caused by spirochetes. Treponema pallidum, the organism that causes Syphilis, utilizes its corkscrew motion to penetrate mucous membranes or abraded skin during the initial infection. This motility allows the organism to rapidly enter the bloodstream and lymphatic system, leading to systemic dissemination early in the disease process.
Borrelia burgdorferi, the causative agent of Lyme disease, also relies heavily on its endoflagellar motility to establish infection. Once transmitted by a tick bite, the drilling motion enables the spirochete to migrate away from the inoculation site, causing the characteristic bullseye rash known as erythema migrans. This same motility facilitates the bacteria’s spread from the skin into joints, the heart, and the nervous system, which are features of later-stage Lyme disease. Another important example is Leptospira, which causes Leptospirosis, a zoonotic disease. The mobility provided by its endoflagella allows this pathogen to rapidly move through blood vessel walls and infect organs like the kidneys and liver.