Borrelia burgdorferi is a corkscrew-shaped bacterium, a spirochete, that causes Lyme disease, the most common tick-borne illness in the Northern Hemisphere. It lives in a cycle between ticks and small mammals, and when an infected tick bites a person, the spirochete can invade the skin, spread through the bloodstream, and settle into joints, the heart, and the nervous system. What makes this bacterium so effective is not brute-force virulence but a sophisticated set of tools for moving through tissue, dodging the immune system, and exploiting its tick vector in ways researchers are still working to fully understand.
A Spirochete With an Unusual Blueprint
B. burgdorferi looks nothing like the round or rod-shaped bacteria most people picture. It is a long, thin, tightly coiled spiral, roughly 20 to 30 micrometers in length but only about a third of a micrometer wide. Its shape is maintained by internal flagella that run between the outer membrane and the cell body, bundled together in what are called endoflagella. When these internal motors spin, the entire cell rotates and corkscrews forward, a mode of movement that lets it bore through viscous environments like connective tissue and the gel-like matrix between cells far more efficiently than a typical swimming bacterium could manage.1Frontiers in Immunology. Borrelia burgdorferi Keeps Moving and Carries on: A Review of Borrelial Dissemination and Invasion
Its genome is also unusual. Rather than carrying a single circular chromosome like most bacteria, B. burgdorferi has a linear chromosome plus more than 20 additional linear and circular plasmids, an extraordinarily fragmented genetic setup.2PubMed Central. Linear and circular plasmid content in Borrelia burgdorferi clinical isolates Many of those plasmids encode the surface proteins the bacterium swaps in and out depending on whether it sits in a tick gut or a mammalian bloodstream. Lose certain plasmids in the lab, and the organism loses its ability to infect.
Metabolically, B. burgdorferi is a minimalist. It cannot synthesize many of its own amino acids, nucleotides, or fatty acids, so it scavenges them from its host. Perhaps most strikingly, it has evolved with essentially no requirement for iron, a metal that almost all other known pathogens need. Instead, it accumulates high levels of manganese to power key enzymes, including a manganese-dependent superoxide dismutase that protects it from the oxidative burst immune cells use to kill invading microbes.3PubMed Central. A manganese-rich environment supports superoxide dismutase activity in a Lyme disease pathogen, Borrelia burgdorferi4Journal of Biological Chemistry. Characterization of the Manganese-dependent Superoxide Dismutase Involved in the Prevention of Oxidative Stress in Borrelia burgdorferi This is a major departure from conventional bacterial biology and may explain why the common immune strategy of starving pathogens of iron does not work against this organism.
The Tick-Mouse-Human Triangle
In the wild, B. burgdorferi cycles between hard-bodied ticks of the Ixodes genus and small mammals, primarily white-footed mice in North America. The spirochete does not harm its rodent reservoir in any measurable way. That is not an accident. The bacterium depends on mice staying mobile and healthy, because its tick vector is a sit-and-wait predator that only encounters hosts that wander past.5PLoS ONE. The Lyme Disease Pathogen Has No Effect on the Survival of Its Rodent Reservoir Host In the northeastern United States, there is a seasonal gap of several months between the time nymphal ticks infect mice and the time the next generation of larval ticks feeds on those same mice. B. burgdorferi has to persist in a healthy host for that entire window to complete its life cycle. Killing or debilitating the mouse would be self-defeating.
Humans are accidental dead-end hosts. We do not pass the infection on to other ticks in any meaningful way. The disease we experience is essentially collateral damage from an infection strategy that evolved to be invisible inside a mouse.6PubMed Central. Of ticks, mice and men: understanding the dual-host lifestyle of Lyme disease spirochaetes
How the Tick Delivers the Infection
Transmission from tick to mammal is not as simple as a bite injecting bacteria. Inside the unfed tick’s midgut, B. burgdorferi sits in a relatively dormant state, expressing one set of surface proteins suited to the tick environment. When the tick attaches and begins feeding, the influx of blood triggers the spirochete to change its outer coat. One critical shift is the upregulation of outer surface protein C (OspC), which the bacterium strictly requires to infect mammals but does not need inside the tick itself.7PubMed Central. Outer-surface protein C of the Lyme disease spirochete: a protein induced in ticks for infection of mammals This molecular costume change takes time, which is a major reason why a tick generally needs to be attached for at least 36 to 48 hours before transmission occurs. The spirochetes are not ready to go immediately.
Once activated, the bacteria migrate from the tick’s midgut to its salivary glands and are injected into the host’s skin along with tick saliva. That saliva turns out to be far more than a lubricant. Tick saliva contains a cocktail of proteins that actively suppress the host’s initial immune response at the bite site. One well-studied protein, Salp15, dampens the skin’s alarm signals by downregulating defensive molecules like antimicrobial peptides and chemokines that would otherwise recruit immune cells to the wound.8PubMed Central. Antialarmin effect of tick saliva during the transmission of Lyme disease9PubMed. Spitting image: tick saliva assists the causative agent of Lyme disease in evading host skin’s innate immune response Other tick salivary proteins, such as the serpin IxsS17, inhibit host enzymes involved in inflammation, pain sensing, blood clotting, and complement defenses, all of which the tick needs to suppress simply to feed successfully.10PLOS Pathogens. A tick saliva serpin, IxsS17 inhibits host innate immune system proteases and enhances host colonization by Lyme disease agent
The result is a local immunological blind spot: the bite site becomes a permissive zone where the bacteria can establish themselves with minimal resistance. B. burgdorferi essentially hitches a ride on the tick’s own feeding strategy.
Spreading Through the Body
Once deposited in the skin, B. burgdorferi does not stay put. Its corkscrew motility allows it to migrate outward through the skin’s connective tissue. As it moves, the immune system begins to react: macrophages and other immune cells arrive, releasing inflammatory signals that cause local blood vessel dilation. This inflammatory cascade produces the telltale erythema migrans rash, an expanding red ring at the bite site, which is usually the first visible sign of infection.11Biophysical Journal. Mathematical Model of the Spatiotemporal Dynamics of Erythema Migrans and Early Lyme Disease The bull’s-eye appearance occurs because the bacteria radiate outward from the center, producing inflammation at the expanding edge while the original bite site begins to clear.
From the skin, spirochetes can enter the bloodstream and reach distant organs. This dissemination step involves a remarkable process. Under the shear forces of flowing blood, B. burgdorferi interacts with the walls of small blood vessels in a way that mechanically resembles how white blood cells roll along vessel walls before exiting into tissue. The bacterial adhesin BBK32 binds to fibronectin and other molecules on the blood vessel lining, creating tethering and dragging interactions that slow the spirochete down and let it migrate across the vessel wall into surrounding tissue.12Cell Press (Cell Reports). Biomechanical Characterization of Borrelia burgdorferi-Vascular Interactions The forces involved are small enough that the bacterium’s own flagellar motors can still drive active movement even while tethered to the vessel wall. This mechanism helps explain how spirochetes seed themselves in joints, the heart, and the brain.
Dodging the Immune System
B. burgdorferi does not produce toxins. It does not destroy tissue directly. Nearly all of the damage in Lyme disease comes from the host’s own inflammatory response. But the bacterium is remarkably good at staying one step ahead of that response, using at least three distinct evasion strategies.
The first and arguably most important is antigenic variation through the VlsE system. VlsE is a surface-exposed protein that the immune system tries to target with antibodies. But the gene encoding VlsE sits next to a set of silent cassettes containing alternative sequence fragments. During mammalian infection, segments of these silent cassettes recombine into the active gene at high frequency, continuously reshuffling the portions of VlsE that face outward.13PLoS Pathogens. Detailed Analysis of Sequence Changes Occurring during vlsE Antigenic Variation in the Mouse Model of Borrelia burgdorferi Infection The crystal structure of VlsE reveals why this is so effective: the variable regions form loops on the outer surface of the protein, physically shielding the conserved regions underneath from antibody contact.14Journal of Biological Chemistry. Crystal Structure of Lyme Disease Variable Surface Antigen VlsE of Borrelia burgdorferi VlsE also acts as a physical shield for other surface proteins. Experiments have shown that when VlsE is present, antibodies directed against another surface antigen called Arp cannot bind to the spirochete and clear the infection, even though those antibodies work perfectly well when VlsE is absent.15Cell Reports. VlsE Enhances Lyme Disease Spirochete Survival by Shielding Surface Antigens from Host Antibody Recognition
The second major evasion strategy targets the complement system, one of the first immune defenses to activate against invading microbes. B. burgdorferi expresses surface proteins, including CspA and CspZ, that grab human complement regulators called factor H and FHL-1 and coat the bacterial surface with them.16PubMed Central. CspA-mediated binding of human factor H inhibits complement deposition and confers serum resistance in Borrelia burgdorferi17PubMed. Immune evasion of Borrelia burgdorferi by acquisition of human complement regulators FHL-1/reconectin and Factor H By hijacking the host’s own complement-control molecules, the spirochete prevents the cascade of reactions that would otherwise punch holes in its membrane and mark it for destruction.18PubMed Central. CspZ variant-specific interaction with factor H incorporates a metal site to support Lyme borreliae complement evasion
Third, the spirochete’s low surface-protein density and its ability to shed outer membrane blebs that act as decoys further complicate the immune system’s targeting. Taken together, these strategies mean that even a robust immune response often controls but does not fully eliminate the infection without antibiotic treatment.
Why the Inflammation Gets Out of Hand
If B. burgdorferi does not make toxins, what causes the joint swelling, nerve damage, and heart problems? The answer is the immune system itself, reacting to bacterial surface components it finds intensely provocative. The spirochete’s outer membrane is studded with lipoproteins, and these lipoproteins activate immune cells through a receptor called Toll-like receptor 2 (TLR2). When TLR2 on macrophages and other cells recognizes B. burgdorferi lipoproteins, it triggers a signaling cascade that produces a flood of inflammatory cytokines.19The Journal of Immunology. Cutting Edge: Inflammatory Signaling by Borrelia burgdorferi Lipoproteins Is Mediated by Toll-Like Receptor 220The Journal of Immunology. Toll-Like Receptor 2 Is Required for Innate, But Not Acquired, Host Defense to Borrelia burgdorferi
In the skin, this inflammation is usually manageable and produces the erythema migrans rash. But when spirochetes reach a joint, the same inflammatory reaction confined to a small, enclosed space can cause significant swelling, pain, and tissue damage. In the heart, it can disrupt electrical conduction and cause temporary heart block. In the nervous system, inflammation around nerve roots can produce facial palsy, meningitis, or shooting pains. The bacterium’s strategy of staying alive and mobile for extended periods keeps provoking this response, and because immune evasion prevents rapid clearance, the inflammation can become chronic in untreated individuals. Lyme disease is a wide-spectrum illness affecting the skin, joints, heart, and nervous system, all driven by this ongoing inflammatory loop.21PubMed Central. Lyme disease (Lyme borreliosis)
European and American Strains Are Not the Same
In North America, essentially all Lyme disease is caused by B. burgdorferi sensu stricto. In Europe, the picture is more complicated: disease can also be caused by B. afzelii and B. garinii, two related but distinct species. Even when comparing B. burgdorferi strains from both continents, American and European isolates turn out to be genetically distinct lineages with meaningful differences in how they behave. American strains are generally associated with more severe disease and provoke a stronger innate and Th1-type adaptive immune response in lab tests, while European strains of the same species tend to elicit different inflammatory profiles, with greater Th17-associated responses.22PubMed Central. Differences in Genotype, Clinical Features, and Inflammatory Potential of Borrelia burgdorferi sensu stricto Strains from Europe and the United States
These strain differences help explain some of the clinical variation doctors see between the two regions. In North America, Lyme arthritis is common, while in Europe, neurological involvement and a chronic skin condition called acrodermatitis chronica atrophicans show up more frequently, partly because of the different species involved.23PubMed Central. Comparison of Lyme Disease in the United States and Europe The practical upshot for patients is that “Lyme disease” is not a single clinical entity worldwide. The specific strain and species dictate not just severity but also which organs are most likely to be affected.
Alternative Forms and the Biofilm Question
Beyond its classic spirochete shape, B. burgdorferi can adopt other morphologies under stress. In laboratory conditions that mimic unfavorable environments, spirochetes can convert into round bodies (sometimes called cystic forms), shed blebs, and even form biofilm-like aggregates.24PLoS ONE. Characterization of Biofilm Formation by Borrelia burgdorferi In Vitro Transcriptomic profiling shows that these different morphological forms express distinct gene sets, suggesting they are not simply dead or dying cells but distinct physiological states.25PubMed Central. Pleomorphic Variants of Borreliella (syn. Borrelia) burgdorferi Express Evolutionary Distinct Transcriptomes
This is where the science gets contentious. Some researchers and patient advocates argue that these alternative forms explain why a subset of people continue to have symptoms after standard antibiotic treatment, the idea being that round bodies or biofilms resist antibiotics and re-emerge later. It is an attractive hypothesis, but hard evidence that these forms persist in human tissue and cause ongoing disease remains thin. Most mainstream Lyme disease researchers acknowledge the morphological plasticity in vitro but are cautious about extrapolating to clinical significance in vivo. The existence of these forms is well documented; their role in human disease is not settled.
Lingering Symptoms After Treatment
A fraction of patients treated for Lyme disease with recommended antibiotics continue to experience fatigue, pain, and cognitive difficulties for months or longer. This condition is often called post-treatment Lyme disease syndrome (PTLDS). The cause is debated, with hypotheses ranging from residual immune activation triggered by remnants of dead spirochetes, to autoimmune responses where the immune system continues to attack the body’s own tissues after the infection has cleared, to the possibility of low-level persistent infection.26SpringerLink. A Review of Post-treatment Lyme Disease Syndrome and Chronic Lyme Disease for the Practicing Immunologist
Controlled trials of extended antibiotic therapy for PTLDS have generally not shown lasting benefit, which argues against active infection as the primary driver in most cases. But the immune dysregulation hypothesis has some mechanistic plausibility given what we know about how strongly B. burgdorferi lipoproteins stimulate TLR2 and downstream inflammatory pathways. An immune system that has been intensely activated for weeks or months may not simply reset to baseline the moment the bacteria are gone. This remains one of the most clinically important open questions in Lyme disease research.
Diagnosis Still Relies on a Two-Step Blood Test
Diagnosing Lyme disease is straightforward when a patient presents with a classic erythema migrans rash and a history of tick exposure. The rash alone is considered diagnostic. Problems arise in later-stage disease or when the rash is missed. The standard lab approach uses a two-tier serological test: a screening immunoassay followed by a confirmatory immunoblot. The first test checks for antibodies against B. burgdorferi; if positive or borderline, the second test confirms the result with greater specificity.27PubMed. Concordance of four commercial enzyme immunoassay and three immunoblot formats for the detection of Lyme borreliosis antibodies in human serum
The limitation is timing. Because the tests detect the immune response rather than the bacterium itself, they can be negative in the first few weeks of infection before the body has made enough antibodies. A negative test during early disease does not rule out Lyme. Conversely, antibodies can persist for months or years after successful treatment, so a positive test in someone who was treated does not mean they are still infected. Updated protocols now allow two immunoassay tests instead of an immunoassay followed by a blot, which speeds up laboratory workflow without sacrificing accuracy.
Where Vaccines Stand
A human Lyme disease vaccine (LYMErix) was available in the late 1990s but was withdrawn from the market in 2002 amid poor sales and controversy about side effects, though post-market surveillance did not confirm serious safety problems. Since then, efforts have continued. A protein subunit vaccine targeting OspA, the same surface protein LYMErix used, is in late-stage clinical trials. Meanwhile, a newer approach uses mRNA technology similar to the COVID-19 vaccines. In mouse experiments, an mRNA vaccine encoding OspA delivered in lipid nanoparticles generated significantly higher antibody levels than protein-based formulations and achieved complete protection against tick-transmitted infection in one of the tested formulations.28Molecular Therapy: Nucleic Acids. Development of an mRNA-lipid nanoparticle vaccine against Lyme disease
The challenge with any OspA-based vaccine is that OspA is primarily expressed by B. burgdorferi inside the tick, not inside a mammal. Vaccine-induced antibodies circulating in the host’s blood are taken up by the tick during feeding and kill the spirochetes inside the tick’s gut before they can migrate to the salivary glands and be transmitted. It is, in effect, a vaccine that works inside the vector rather than inside the patient. This unusual mechanism is part of why it works well but also why it requires high sustained antibody levels, which in turn means booster doses. Whether the public will accept a multi-dose vaccine for a disease many still view as uncommon is a question of perception as much as immunology.