Treponema pallidum and Syphilis: Stages, Pathogenesis, and Diagnosis

A young adult notices a single firm, painless ulcer on the genitals. Because it does not hurt and heals on its own within a few weeks, it is easy to ignore. But the ulcer was the first sign of syphilis, and the organism that caused it, Treponema pallidum, has already spread through the bloodstream. Weeks later comes a rash on the palms and soles; years later, if still untreated, the infection can damage the heart, brain, and nerves.

Syphilis is called "the great imitator" because its signs mimic many other diseases at every stage. This page is about the organism behind it, how the infection moves through the body in distinct stages, and how it is diagnosed, using an organism so thin and delicate it cannot even be grown in the laboratory.

Treponema pallidum is a spirochete, a thin, tightly coiled, corkscrew-shaped bacterium, and the cause of syphilis, a sexually transmitted infection. Its name captures its biology: trepo (turn) and nema (thread), a turning thread. It belongs to the wider group of spirochetes; the spirochetes overview covers the family, and this page focuses on T. pallidum and syphilis.

What Treponema pallidum looks like, and why you can't culture it

T. pallidum has features that both define it and make it uniquely difficult to work with.

It is extremely thin and can't be seen on ordinary microscopy. The cells are long (6 to 14 μm) but only about 0.2 μm wide, too thin to be seen with a normal light microscope and too thin to take up ordinary stains. They are viewed instead by dark-field microscopy, or by silver staining or immunofluorescence. The demonstration of the organism by dark-field microscopy is covered separately.

It cannot be grown on artificial media. This is the single most important practical fact. T. pallidum is so dependent on the host that it cannot be cultured on any laboratory medium; it has historically been maintained only by inoculation into animals such as rabbits. Because you cannot culture it, diagnosis relies on seeing the organism directly (dark-field) or, far more often, on serology (antibody tests), not on culture. Everything about how syphilis is diagnosed follows from this one limitation.

It is delicate and dies quickly outside the body. It is rapidly killed by heat, drying, and disinfectants, which is why transmission needs close, direct contact.

Humans are the only host. There is no animal reservoir for syphilis.

The pathogenic treponemes

Four treponemes cause human disease, and they are distinguished mainly by the disease and the region rather than by laboratory tests (they look identical and share antigens):

• T. pallidum subsp. pallidum: venereal syphilis (the subject of this page).
• T. pallidum subsp. pertenue: yaws (a tropical skin and bone infection).
• T. pallidum subsp. endemicum: endemic syphilis (bejel).
• T. carateum: pinta (a skin infection).

The last three are non-venereal, spread by close skin contact, and occur mainly in specific tropical regions. All four are indistinguishable in the laboratory, so the diagnosis depends on the clinical and geographic picture.

Virulence factors and how Treponema pallidum causes disease

T. pallidum is unusual: it causes enormous damage over years, yet it produces no classic toxins. Its success comes from invasiveness, immune evasion, and the host's own inflammatory response.

Motility and invasiveness. Its corkscrew motility, driven by internal flagella (endoflagella), lets it burrow through mucous membranes and tissue. Within hours of entering through intact or slightly abraded mucosa, it reaches the lymphatics and bloodstream and spreads throughout the body. This early, silent dissemination is why syphilis is a systemic disease from the very start, even when the only visible sign is a single genital sore.

Enzymes that aid spread. Hyaluronidase, produced by virulent treponemes, helps break down tissue and spread around blood vessels (perivascular infiltration).

Immune evasion. T. pallidum has a remarkably protein-poor outer membrane, giving the immune system few targets, and it coats itself with host proteins such as fibronectin, which helps disguise it from phagocytes. This stealth is why the organism can persist for years and progress through latent stages.

The damage is mostly immune-mediated. Crucially, T. pallidum makes no significant toxin. The tissue destruction of syphilis, the ulcers, the rash, the gummas of late disease, comes largely from the host's own inflammatory and immune response to the organism, not from a bacterial poison. Understanding this explains why the disease is chronic and relapsing: it is a long struggle between the immune system and a persistent, stealthy organism.

Putting it together

The organism enters through mucosa or broken skin, uses its motility and enzymes to invade and spread through the blood within hours, and evades the immune system with its bare, host-coated surface. The immune response to it produces the lesion at each stage: the chancre where it first multiplies, the widespread rash when it disseminates, and the destructive granulomas (gummas) of late disease. Because it hides so well, it can lie latent for years and re-emerge, which is why untreated syphilis unfolds in stages over a lifetime.

How syphilis is transmitted

Acquired syphilis spreads mainly by sexual contact, through direct contact with an infectious lesion (the chancre or secondary lesions, which are rich in organisms) during vaginal, anal, or oral sex. Because the organism is so delicate, it needs this close, direct contact. It can also spread rarely through infected blood (transfusion or shared needles).

Congenital syphilis occurs when T. pallidum crosses the placenta from an infected mother to the fetus, or when the baby contacts lesions during birth. The risk to the fetus is highest when the mother has early (more infectious) syphilis. Congenital syphilis is preventable by screening and treating pregnant women, which is why it is a routine part of antenatal care.

Roughly a third of people exposed to an infectious partner develop syphilis.

The stages of syphilis

Untreated syphilis moves through four stages over months to years. Each stage has a characteristic picture, and knowing the sequence is the key to the whole disease.

Primary syphilis: the chancre

About 2 to 4 weeks after infection, a single chancre appears at the site where the organism entered, usually the genitals, but also the anus, mouth, or cervix. The classic chancre is a painless, firm, indurated ulcer with a clean base, teeming with spirochetes and therefore highly infectious. Regional lymph nodes become enlarged, firm, and painless. The chancre heals on its own in a few weeks, which is exactly why it is dangerous: the person may think the problem has resolved when the infection has in fact spread throughout the body. In HIV-infected people, chancres may be multiple, painful, or atypical.

Secondary syphilis: the disseminated stage

About 4 to 8 weeks after the chancre heals, the widely disseminated organism produces secondary syphilis. The hallmark is a maculopapular rash that characteristically involves the palms and soles, an important clue, because few rashes do. There may be fever, malaise, sore throat, generalized painless lymphadenopathy, and patchy hair loss. In warm, moist areas, broad, wart-like lesions called condylomata lata form; these are highly infectious. Secondary syphilis resolves on its own, and the infection then enters latency.

Latent syphilis: the hidden stage

After secondary syphilis resolves, the infection becomes latent: no symptoms, but positive serology. It is divided into early latent (within the first year, still potentially infectious) and late latent (after one year). A latent infection may persist for life, resolve, or progress to tertiary disease. During latency the disease is not spread by ordinary contact, but a pregnant woman can still transmit it to her fetus.

Tertiary (late) syphilis: the destructive stage

Years to decades later, a minority of untreated people develop tertiary syphilis, the destructive stage. It takes three main forms:

• Gummatous syphilis: granulomatous, destructive lesions (gummas) in skin, bone, and other tissues.
• Cardiovascular syphilis: classically an aneurysm of the ascending aorta and aortic regurgitation, from damage to the vessel wall.
• Neurosyphilis: involvement of the nervous system, which can appear as meningitis, meningovascular disease, general paresis (a dementia), and tabes dorsalis (degeneration of the spinal cord's dorsal columns, causing sensory loss and an unsteady gait).

In the antibiotic era, the gummatous and cardiovascular forms have become uncommon because people usually receive antibiotics for other reasons along the way. Neurosyphilis remains the most important late form, partly because many antibiotics penetrate the central nervous system poorly.

Congenital syphilis

When T. pallidum crosses the placenta, it can cause miscarriage, stillbirth, or a liveborn infant with congenital syphilis. Features are grouped by timing:

• Early congenital syphilis (first two years): resembles severe secondary syphilis, with a rash and desquamation of the palms and soles, and a characteristic persistent rhinitis ("snuffles") with an infectious, sometimes blood-tinged nasal discharge.
• Late congenital syphilis (after two years) and its permanent stigmata: interstitial keratitis (corneal damage), eighth-nerve deafness, and Hutchinson's teeth (notched, peg-shaped upper incisors). The combination of interstitial keratitis, eighth-nerve deafness, and Hutchinson's teeth is the classic Hutchinson's triad.

How syphilis is diagnosed

Because T. pallidum cannot be cultured, diagnosis rests on two approaches: seeing the organism directly, and (far more often) detecting antibodies. The logic is more useful to learn than the individual procedures, which are covered on their own pages.

Direct detection

In early syphilis with a lesion, the organism can be seen directly from the chancre or secondary lesion by dark-field microscopy, or detected by PCR. This works only when an active, organism-rich lesion is present.

Serology: the screen-then-confirm logic

Most diagnosis is serological, and it uses two kinds of antibody test together, because each covers the other's weakness. This is the key concept to understand.

Non-treponemal tests: VDRL and RPR. These detect antibodies against a lipid (cardiolipin) released from damaged host cells, not against the organism itself. They are cheap, fast, and good for screening and for following treatment, because their titer falls as the patient improves. Their weakness is false positives (in pregnancy, other infections, autoimmune disease), so a positive result must be confirmed.

Treponemal tests: TPHA and FTA-ABS. These detect antibodies against T. pallidum itself, so they are specific and used to confirm a positive screen. Their weakness is that they usually stay positive for life even after cure, so they cannot be used to judge treatment response or reinfection.

The logic in one line: screen with a non-treponemal test (VDRL/RPR), confirm with a treponemal test (TPHA/FTA-ABS), and follow treatment with the non-treponemal titer. The full principle and procedure of each test, and the complete interpretation of result combinations, are covered in the laboratory diagnosis of syphilis article and the individual test pages linked above.

Treatment

The key fact is simple and has not changed in over 70 years: T. pallidum remains highly sensitive to penicillin, which is the drug of choice at every stage. There is no significant penicillin resistance in syphilis. For patients allergic to penicillin, doxycycline is an alternative in some situations, though penicillin is so preferred that pregnant women with allergy are usually desensitized rather than given a substitute.

Two points worth knowing. Neurosyphilis needs a regimen that penetrates the central nervous system, because many drugs reach the CNS poorly. And when treatment begins, a Jarisch-Herxheimer reaction can occur: a brief fever and worsening of symptoms as large numbers of spirochetes are killed and release their contents. It is self-limited and is not an allergy to penicillin. Specific doses and durations are clinical decisions and are not covered here.

How to remember

Can't stain it, can't grow it. T. pallidum is too thin for an ordinary stain and cannot be cultured on any medium. That double limitation is why diagnosis relies on dark-field microscopy and, mostly, on antibody tests rather than culture. If you remember "you can't culture syphilis," the whole diagnostic approach follows.

Painless is the danger. The primary chancre is painless and heals by itself. That is exactly why it is dangerous: the person stops worrying just as the organism spreads through the body. Painless sore, silent spread.

Palms and soles. The secondary syphilis rash characteristically involves the palms and soles, unusual for a rash, so a palm-and-sole rash should make you think of secondary syphilis (and a few other conditions). One image anchors the stage.

Screen with the cheap one, confirm with the specific one. Non-treponemal (VDRL/RPR) is the cheap, titer-falling screen that also follows treatment. Treponemal (TPHA/FTA-ABS) is the specific confirmer that stays positive for life. Screen, then confirm; follow treatment with the non-treponemal titer.

Penicillin, still, always. After more than 70 years, T. pallidum has never become resistant to penicillin. It remains the drug of choice at every stage. One of the few infections where the first-choice drug has not changed.

Key exam facts in one table

Where students get confused

You cannot culture Treponema pallidum. Unlike most bacteria, it will not grow on any laboratory medium. This is why diagnosis is by dark-field microscopy or serology, never routine culture. Students expecting a culture result are stuck.

Why two different antibody tests. Non-treponemal tests (VDRL/RPR) are cheap screens whose titer falls with treatment but which can give false positives. Treponemal tests (TPHA/FTA-ABS) are specific confirmers but stay positive for life. You need both: one to screen and follow treatment, one to confirm. They are not interchangeable.

The painless chancre heals, but the disease has not gone. The primary sore disappears on its own, which wrongly reassures the patient. The organism has already spread. Spontaneous healing of the chancre is not cure.

Treponemal tests stay positive after cure. A positive TPHA or FTA-ABS years after successful treatment does not mean active infection or treatment failure; these tests usually remain positive for life. Treatment response is judged by the falling non-treponemal (VDRL/RPR) titer, not the treponemal test.

The damage is immune-mediated, not toxin-mediated. T. pallidum makes no significant toxin. The lesions come from the body's own inflammatory response. This is why syphilis is a slow, staged, relapsing disease rather than an acute toxin illness.

Jarisch-Herxheimer is not penicillin allergy. The fever and worsening that can follow the first dose of penicillin is the reaction to dying spirochetes, not an allergic reaction. Mislabeling it as allergy can wrongly stop the one drug that works.

References

1. Tille, P. M. (2022). Bailey & Scott's Diagnostic Microbiology (15th ed.). Elsevier.
2. Procop, G. W., et al. (2017). Koneman's Color Atlas and Textbook of Diagnostic Microbiology (7th ed.). Wolters Kluwer.
3. Radolf, J. D., Deka, R. K., Anand, A., Šmajs, D., Norgard, M. V., & Yang, X. F. (2016). Treponema pallidum, the syphilis spirochete: making a living as a stealth pathogen. Nature Reviews Microbiology, 14(12), 744–759. https://doi.org/10.1038/nrmicro.2016.141
4. Centers for Disease Control and Prevention. (current). Syphilis. https://www.cdc.gov/std/syphilis/