Spirochetes are a phylum of bacteria identified by their unique, long, and coiled morphology. The name, derived from Greek, means “coiled hair,” describing their characteristic helical or corkscrew shape. These bacteria are motile and thrive in diverse environments, including aquatic habitats and the tissues of living hosts.
Spirochetes are found globally in water, soil, and as symbionts or parasites within animals, including humans. Their structure enables them to penetrate tissues and evade immune responses, which is why several species cause serious, persistent diseases.
The Anatomy of a Spiral Bacterium
The defining feature of a spirochete is its flexible, elongated, helical cell body, measuring between 3 and 500 micrometers in length. Like other Gram-negative bacteria, the cell has a double-membrane structure: an inner cytoplasmic membrane and an outer membrane. The inner membrane encloses the protoplasmic cylinder, which contains the cell’s genetic material and cytoplasm.
The feature that sets spirochetes apart is their internal mechanism for locomotion, known as the axial filament or endoflagella. Unlike the flagella of most other bacteria, the endoflagella are located within the periplasmic space, nestled between the inner and outer membranes. These filaments are anchored at each pole and wrap around the protoplasmic cylinder.
The rotation of these enclosed endoflagella creates a torsional force, causing the cell body to rotate and flex in a corkscrew motion. This twisting movement allows the spirochete to burrow through viscous materials like mucus, connective tissue, and blood. This invasive capability allows them to spread throughout a host’s body, reaching organs like the brain, joints, and heart.
Diverse Habitats and Classification
Spirochetes are a large phylum, and while many species are free-living in aquatic environments, a few genera are medically important pathogens. Free-living species, such as those in the genus Spirochaeta, are commonly found in mud and water. These environmental species play a part in natural ecosystems but do not cause disease in humans.
Pathogenic spirochetes that affect human health primarily belong to three genera: Treponema, Borrelia, and Leptospira. These groups share the helical morphology and internal motility mechanism but differ in their habitats, transmission modes, and the diseases they cause. For example, Treponema species require direct host-to-host transmission, while Borrelia and Leptospira are often transmitted through vectors or contaminated environments.
Major Diseases Caused by Spirochetes
Syphilis
Syphilis is a sexually transmitted infection caused by Treponema pallidum subspecies pallidum. It is transmitted through sexual contact with infectious lesions or from an infected mother to her fetus. The disease progresses through distinct stages if left untreated.
The primary stage begins with a chancre, a firm, painless ulceration that develops at the site of entry. This sore heals spontaneously, but the infection progresses to the secondary stage. Secondary syphilis is characterized by the systemic spread of the bacteria, resulting in a diffuse, non-itchy rash often involving the palms and soles. This stage also commonly presents with flu-like symptoms, including fever and swollen lymph nodes.
Following the secondary stage, the infection enters the latent phase, defined by a lack of visible symptoms, which can last for years. Approximately one-third of untreated patients progress to tertiary syphilis, occurring years after the initial infection. Tertiary disease involves serious complications, such as gummas (soft, tumor-like growths), neurological issues, and damage to the cardiovascular system.
Lyme Disease
Lyme disease is the most common vector-borne illness in the United States, caused by Borrelia burgdorferi and related species. The bacteria are transmitted to humans through the bite of an infected hard tick, primarily the deer tick (Ixodes). Transmission typically requires the tick to be attached and feeding for a minimum of 24 to 48 hours.
The most recognizable sign of early localized infection is erythema migrans, an expanding red rash that often resembles a bullseye, appearing at the bite site. Early symptoms include flu-like complaints such as fever, headache, and fatigue. If untreated, the bacteria can disseminate, leading to severe conditions, including neurological issues like facial paralysis, heart problems such as Lyme carditis, and arthritis affecting the joints.
Leptospirosis
Leptospirosis is a zoonotic disease caused by bacteria in the genus Leptospira. Humans acquire the infection primarily through contact with water, soil, or food contaminated with the urine of infected animals, such as rodents, dogs, and farm animals. The bacteria enter the body through breaks in the skin or mucous membranes.
The disease often presents with a biphasic course, occurring in two phases. The initial phase is characterized by an abrupt onset of flu-like symptoms, including fever, severe headache, muscle aches, and conjunctival suffusion (redness of the eyes). In some cases, the disease progresses to a more severe second phase known as Weil’s disease. This severe form can result in jaundice, kidney failure, and internal bleeding.