Lyme disease is caused by a spirochete, a distinctive type of spiral-shaped bacterium called Borrelia burgdorferi. The pathogen is so closely associated with this bacterial group that its official common name in genetic databases is simply “Lyme disease spirochete.” Understanding what makes spirochetes unique helps explain why Lyme disease behaves the way it does, from the way it spreads through your body to why it can be tricky to diagnose and treat.
What Makes a Spirochete Different
Spirochetes are a group of bacteria defined by their long, thin, spiral or wavy cell bodies. What sets them apart from other bacteria isn’t just their shape. Most bacteria that can swim use external flagella, tiny whip-like tails that stick out and push against surrounding fluid. Spirochetes hide their flagella inside the cell, sandwiched between the inner cell wall and an outer membrane. These internal flagella, called periplasmic flagella, rotate beneath the surface and cause the entire cell body to roll or undulate, almost like a corkscrew turning through liquid.
This design gives spirochetes a critical advantage. They can move efficiently through thick, gel-like substances that would slow or stop other bacteria. The Lyme disease spirochete is about 20 micrometers long, has a flat-wave shape, and contains 7 to 11 internal flagella that extend from each end and overlap in the middle of the cell. These flagella don’t just power movement; they also act like an internal skeleton, maintaining the bacterium’s characteristic wavy form. Without them, the cell loses its shape entirely.
Why the Shape Matters for Infection
The spirochetal design is directly tied to how Lyme disease spreads through your body. After a tick delivers the bacteria into your skin, the spirochete’s corkscrew-like motility allows it to burrow through connective tissue in ways that ordinary bacteria cannot. This specialized swimming ability lets it penetrate into joints, heart tissue, and even the nervous system.
Spirochetes as a group share a tendency to target specific organs. Lyme disease, syphilis (caused by the spirochete Treponema pallidum), and leptospirosis (caused by Leptospira species) all show a preference for skin, the nervous system, and the heart and arteries. All three diseases begin with localized infection and then disseminate to distant organs through tissues, blood, and other fluids. In Lyme disease, this shows up as the characteristic bull’s-eye rash at the bite site, followed weeks to months later by potential problems in the joints, heart, or brain.
Other Diseases Caused by Spirochetes
Lyme disease belongs to a family of spirochetal infections that have caused significant human disease for centuries. Syphilis, a sexually transmitted infection, is the most historically notorious. Leptospirosis spreads through contact with water contaminated by animal urine and can cause organ failure in severe cases. Relapsing fever, caused by other Borrelia species, produces recurring episodes of high fever and is transmitted by ticks or lice.
These infections share more than just their corkscrew-shaped culprits. The earliest visible sign of syphilis is a painless skin sore called a chancre, while Lyme disease typically announces itself with the expanding erythema migrans rash. Both represent the spirochete establishing itself at the point of entry before spreading deeper. The neurotropism of spirochetes, their tendency to invade the nervous system, is a common thread across all of these diseases.
How the Spirochete Evades Treatment
The Lyme spirochete is sensitive to common antibiotics. Doxycycline, which blocks the bacterium’s ability to build proteins, and amoxicillin, which disrupts its cell wall assembly, are the two most frequently used treatments for Lyme disease. When caught early, these antibiotics are highly effective.
However, the Lyme spirochete has a remarkable ability to change form under stress. When exposed to antibiotics or a hostile immune environment, it can shift from its normal spiral shape into round bodies, L-form bacteria, microcolonies, or biofilm-like clusters. These alternative forms, collectively called persisters, are tolerant to multiple drugs. They exist in low numbers, are difficult to detect, and can convert back into active, motile spirochetes when conditions improve. This shape-shifting capacity is one reason researchers continue to study why some patients experience prolonged symptoms. Even aggressive antibiotic treatment may not eliminate every morphological form, and biofilm-like aggregates show particularly high tolerance to standard antibiotics.
Diagnosis Challenges Tied to the Spirochete
Detecting the Lyme spirochete directly is difficult because the bacteria are present in very low numbers in blood and tissue. Instead, diagnosis relies on detecting your immune system’s response to the infection. The CDC recommends a two-step blood testing process: an initial screening test (enzyme immunoassay), and if that result is positive or borderline, a confirmatory second test (Western blot). Both steps must be positive for the overall result to count as positive. If the first test comes back negative, no further testing is recommended.
The catch is timing. Your immune system takes days to weeks to produce detectable antibodies against the spirochete. Testing too early, particularly in the first few days after a tick bite, often produces a false negative. This is why doctors frequently diagnose early Lyme disease based on symptoms and the presence of the bull’s-eye rash rather than waiting for lab confirmation.
Lyme Disease by the Numbers
Approximately 30,000 cases of Lyme disease are reported to the CDC each year through traditional surveillance. In 2022, an updated case definition expanded the count to roughly 63,000 reported cases, though the actual number of infections is widely believed to be higher since many cases go unreported or undiagnosed. The disease remains concentrated in the northeastern, mid-Atlantic, and upper midwestern United States, where the blacklegged tick that carries the spirochete is most common.