Treponema pallidum
Also known as: syphilis bacterium
Treponema pallidum is the spiral-shaped bacterium that causes syphilis. It is too thin to be seen on Gram stain, cannot be grown on standard laboratory media, and produces a disease that progresses through primary, secondary, latent, and tertiary stages if untreated.
*Treponema pallidum* is a spirochete — a slender, helically coiled bacterium that moves using axial filaments, or endoflagella, wound within its outer sheath. It is too thin to be resolved by light microscopy after Gram staining and cannot be cultured on artificial media, so it is visualized instead by darkfield microscopy of lesion exudate or by immunofluorescent staining. Transmission is sexual, through direct contact with an infectious lesion, or transplacental from mother to fetus.
Untreated infection moves through defined stages. Primary syphilis produces a chancre: a single, firm, painless ulcer with indurated edges at the site of inoculation, appearing weeks after exposure and healing on its own. Secondary syphilis follows as the organism disseminates, producing a diffuse maculopapular rash that characteristically involves the palms and soles, along with fever, lymphadenopathy, mucous patches, and condylomata lata — broad, moist, highly infectious lesions. Latent syphilis is asymptomatic and detectable only by serology. Tertiary syphilis appears years later in a minority of untreated patients and includes gummas, cardiovascular syphilis with ascending aortitis and aneurysm from vasa vasorum endarteritis, and neurosyphilis with tabes dorsalis, Argyll Robertson pupils, and general paresis. Congenital syphilis causes saddle nose, Hutchinson teeth, mulberry molars, and saber shins.
Because the organism cannot be cultured, diagnosis rests on serology in two tiers. Nontreponemal tests — VDRL and RPR — are sensitive and inexpensive, useful for screening and for following treatment response, but they produce false positives in viral infection, rheumatic fever, lupus, and pregnancy. Treponemal tests such as FTA-ABS confirm a reactive screen and stay positive for life. Penicillin G remains the treatment of choice at every stage; a Jarisch-Herxheimer reaction of fever, chills, and myalgia can follow the first dose as killed organisms release their contents.
USMLE Step 1 tests *T. pallidum* in both microbiology and pharmacology. The reliably examined points are darkfield visualization, the stage-specific findings — especially the painless chancre and the palm-and-sole rash — the two-tier serologic strategy, and penicillin as the drug of choice.
Key takeaways
- Treponema pallidum is a spirochete that causes syphilis and cannot be Gram stained or cultured on standard media.
- Darkfield microscopy of lesion exudate visualizes the organism directly.
- Primary syphilis produces a painless chancre; secondary syphilis produces a rash involving the palms and soles plus condylomata lata.
- Tertiary syphilis causes gummas, aortitis with aneurysm, and neurosyphilis years after infection.
- Diagnosis uses nontreponemal screening (VDRL, RPR) confirmed by treponemal testing (FTA-ABS); penicillin G is the treatment of choice.