Yes, Lyme disease is caused by a spirochete, a distinctive type of corkscrew-shaped bacterium called Borrelia burgdorferi. Spirochetes are a unique group of bacteria defined by their coiled, wave-like shape and an unusual internal motor system that lets them bore through thick tissues other bacteria can’t penetrate. Lyme disease is one of several major human infections caused by spirochetes, alongside syphilis, leptospirosis, and relapsing fever.
What Makes a Spirochete Different
Spirochetes aren’t just oddly shaped versions of ordinary bacteria. They belong to their own distinct order (Spirochaetales) and have a body plan unlike anything else in the bacterial world. Most bacteria that can swim use whip-like tails called flagella on their outer surface. Spirochetes hide their flagella inside, sandwiched between the cell body and an outer membrane. These internal flagella, called endoflagella, wrap around the cell like a ribbon beneath the skin.
When these internal flagella rotate, they cause the entire cell body to twist and undulate, producing a corkscrew-like motion. This design gives spirochetes a major advantage in thick, gel-like environments. Most bacteria actually slow down in viscous fluids, but spirochetes speed up. Think of it like the difference between pushing a stick through mud versus drilling a screw into it. That screw-like motion is precisely what makes spirochetes so effective at burrowing through the dense tissues of the human body.
The Lyme Spirochete Up Close
Borrelia burgdorferi cells are extremely thin and elongated, roughly 10 to 20 micrometers long but only about 0.33 micrometers wide. For perspective, a human red blood cell is about 7 micrometers across, so the Lyme spirochete is longer than a red blood cell but far thinner. This slender shape is part of what makes it so difficult to detect. Standard light microscopy struggles to pick up something this narrow, which is one reason Lyme disease historically relied on specialized techniques like darkfield microscopy or fluorescent labeling to visualize the bacteria directly.
Under the microscope, B. burgdorferi appears as a flat, wavy ribbon rather than a tight corkscrew. Its internal flagella don’t bundle together the way external flagella do on other bacteria. Instead, they form a flat ribbon that wraps around the cylindrical cell body in a right-handed spiral, giving the organism its characteristic undulating shape.
How That Shape Helps It Spread
The spirochete’s body plan isn’t just a biological curiosity. It directly explains why Lyme disease can affect so many different parts of the body, from skin to joints to the nervous system. After a tick deposits B. burgdorferi into the skin, the bacteria need to travel through the dense connective tissue of the dermis, enter blood vessels, and eventually reach distant organs.
Research using gelatin models that mimic human tissue has shown that these spirochetes can force their way through pores significantly smaller than their own diameter. They do this by generating enough internal pressure (at least 5 kilopascals) to maintain their shape while squeezing through tight spaces. Their movement through tissue isn’t smooth or continuous, though. Researchers have observed spirochetes alternating between several distinct behaviors: translating steadily forward for seconds to minutes, wriggling in place without making progress, and lunging with one end anchored while the rest of the body bends and flexes as if trying to break free. Once they detach from whatever is holding them, they resume moving forward until they get stuck again.
This stop-and-go pattern helps explain why Lyme disease spreads gradually through the body rather than all at once. The spirochete’s ability to push through tissue barriers that would stop most other bacteria is a direct consequence of its unique internal motor system.
Other Diseases Caused by Spirochetes
Lyme disease is far from the only spirochetal infection. The same basic body plan shows up across a range of human diseases:
- Syphilis is caused by Treponema pallidum, a spirochete that shares B. burgdorferi’s ability to disseminate widely through the body and evade the immune system for years.
- Leptospirosis comes from Leptospira species, typically contracted through water contaminated with animal urine. It can cause kidney and liver damage.
- Relapsing fever is caused by other Borrelia species (closely related to the Lyme pathogen) and is transmitted by soft ticks or body lice.
- Periodontal disease involves oral spirochetes like Treponema denticola that thrive in the low-oxygen environment of deep gum pockets.
What these infections share is a pathogen whose corkscrew motility lets it penetrate tissues that act as barriers to most bacteria. The spirochete body plan has evolved independently to exploit dense, viscous environments across very different ecological niches.
Why the Spirochete Shape Matters for Treatment
B. burgdorferi has a relatively simple cell membrane with a thin layer of peptidoglycan, the rigid structural material that gives bacterial cells their shape. Notably, its outer membrane lacks the lipopolysaccharides found in most other types of bacteria with a similar double-membrane structure. This unusual cell wall composition is part of why certain antibiotics work well against it. Drugs that target cell wall construction (like amoxicillin) and drugs that block protein production (like doxycycline) are both effective, particularly when treatment starts early.
The Lyme spirochete actually belongs to a larger group called the B. burgdorferi sensu lato complex, which contains more than 23 distinct species. At least five of these are known to cause disease in humans, and at least eight have been isolated from patients. Different species predominate in different parts of the world, which partly explains why Lyme disease can look somewhat different in North America versus Europe. But they’re all spirochetes, and they all share the fundamental corkscrew body plan that defines the group.