Treponema pallidum !

Bacterium
Does not Gram-stain
Exam priority: high

The syphilis spirochete: it spreads through sex and from mother to baby in pregnancy; penicillin cures it.

Updated 2026-11-01

Also called: syphilis spirochete

At a glance

Gram: Does not Gram-stain
Shape: Thin, tightly coiled spirochete
Oxygen: Microaerophilic
Family: Spirochaetaceae
Stain: Not seen on Gram: dark-field, direct immunofluorescence or silver stain
Size: Very thin (about 0.15 µm wide)

Morphology

Fine spiral with endoflagella and corkscrew motility
Outer membrane with very few exposed proteins: evades the immune system
Not cultured on routine media

Transmission

Sexual contact (vaginal, anal or oral) with an active lesion
Transplacental at any stage of pregnancy: congenital syphilis
Rarely by transfusion; not by objects such as toilet seats

Reservoir: Humans only

Isolation precautions

What the care team adds to standard precautions.

Standard

Virulence factors

Crosses mucosa and microabraded skin and spreads through the blood very early
Obliterative endarteritis: the basis of vascular lesions and aortitis
Few surface proteins: evades antibodies and persists in latency for years

Diseases it causes

Primary, secondary, latent and tertiary syphilis; neurosyphilis; gestational and congenital syphilis Syphilis →

Clinical picture

Primary: painless, indurated chancre with regional lymphadenopathy
Secondary: rash on palms and soles, condylomata lata, patchy alopecia
Tertiary: gummas, aortitis, tabes dorsalis, Argyll Robertson pupil
Congenital: stillbirth, rhinitis, hepatosplenomegaly; late: Hutchinson teeth, saddle nose, deafness

Diagnosis

Non-treponemal tests (VDRL, RPR): screening and treatment follow-up by titre
Treponemal tests (FTA-ABS, TP-PA, rapid tests): confirm and stay positive for life
Both types are needed; some labs use the reverse algorithm (treponemal first)
Dark-field of chancre fluid; CSF VDRL for neurosyphilis

Treatment

Parenteral penicillin is the treatment of choice (benzathine penicillin; aqueous crystalline penicillin for neurosyphilis). In pregnancy it is the only treatment effective for the fetus: allergic patients are desensitised. Alternatives outside pregnancy: doxycycline or ceftriaxone. There is no penicillin resistance; there is azithromycin resistance.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

Screening at prenatal visits and immediate treatment of the pregnant woman and her partner
Condoms; screening of higher-risk groups
Notification and treatment of sexual contacts

Test yourself

Think of the answer, then tap the question to see it.

Which serological test type tracks the response to syphilis treatment?

A non-treponemal test (VDRL or RPR), because its titres fall with cure.

A pregnant woman with syphilis is allergic to penicillin. What is done?

She is desensitised and treated with penicillin, the only drug proven to protect the fetus.

Where does the secondary syphilis rash typically appear?

On the trunk and, characteristically, on the palms and soles.

Which late signs suggest congenital syphilis?

Hutchinson teeth, saddle nose, sabre shins, interstitial keratitis and deafness.

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Syphilis

← Back to the group: Spirochetes

Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: Syphilis (World Health Organization (WHO), 2025); Syphilis and Congenital Syphilis - PAHO/WHO | Pan American Health Organization (Pan American Health Organization (PAHO), 2025); Syphilis - STI Treatment Guidelines (Centers for Disease Control and Prevention (CDC), 2021); About Syphilis | Syphilis | CDC (Centers for Disease Control and Prevention (CDC), 2025).

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Representative model, for educational purposes only.