Treponema pallidum
One-Sentence Definition
Treponema pallidum subsp. pallidum is a thin spirochete that causes syphilis — a multistage STI diagnosed by serology (and PCR in selected specimens), still fully penicillin-responsive for most stages.
Taxonomy & Morphology
- Spirochaetaceae; spiral, highly motile; too thin for reliable light-microscopy Gram stain
- Visualized by dark-field (lesions) or immunostain historically
- Related: nonvenereal treponematoses (T. pallidum subsp. pertenue — yaws); Borrelia, Leptospira — other spirochetal diseases
Virulence Highlights
- Outer membrane sparse in proteins — immune evasion
- Dissemination early; can invade CNS and placenta
- No classic exotoxin; disease from invasion + host inflammation
- Cannot be routinely cultured on artificial media
Clinical Syndromes
- Primary: chancre (painless ulcer)
- Secondary: disseminated rash (palms/soles), condyloma lata, systemic symptoms
- Latent: seropositive, asymptomatic
- Tertiary: gummas, cardiovascular, neurosyphilis (can occur at various stages)
- Congenital syphilis — preventable with prenatal screening
Diagnosis
- Serology algorithm: nontreponemal (RPR/VDRL) + treponemal EIA/FTA-ABS/TP-PA — reverse or traditional sequences
- PCR on lesions/CSF in specialized contexts
- LP when neurosyphilis suspected
- Screen pregnancy and MSM / high-incidence networks
Treatment Notes (conceptual)
- Benzathine penicillin G — cornerstone by stage
- Neurosyphilis: IV aqueous penicillin regimens
- Penicillin allergy: desensitization preferred in pregnancy
- Jarisch–Herxheimer reaction possible after first dose
AMR
- Penicillin remains reliably active for syphilis — major contrast to Neisseria gonorrhoeae
- Azithromycin resistance (23S) documented — macrolides not relied on
- Surveillance focuses on reinfection and congenital cases more than MIC creep
Genomics & Computational Notes
- Extremely small genome; lab propagation historically in rabbits / limited systems
- WGS for outbreak clusters and macrolide resistance alleles
- Molecular clock studies of syphilis re-emergence (Phylodynamics)
Related Notes
- Neisseria gonorrhoeae · Chlamydia trachomatis · Genitourinary Infections · HIV (coinfection / risk synergy)
- MOCs: MOC - Bacteriology · MOC - Clinical Microbiology · MOC - Public Health & Epidemiology
Active Recall
- Primary vs secondary syphilis — classic clinical clues?
- Why pair nontreponemal with treponemal tests?
- Drug of choice for most stages of syphilis?