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BACTERIA · Spirochete

Treponema pallidum

Treponema pallidum — Microscopy (syphilis spirochete)

Stain: Electron microscopy / dark-field equivalentMorphology: Long, slender, tightly coiled corkscrew-shaped spirocheteYield: HIGHDifficulty: MEDIUM
Treponema pallidum microscopic image — Spirochete, Electron microscopy / dark-field equivalent
Image: Wikimedia Commons · File:Treponema_pallidum.jpg · CDC/PHIL (Public Domain)

Key facts

**Pathogenesis**: Cannot be Gram-stained or cultured — visualized only by dark-field microscopy or silver stain. Outer membrane is poorly antigenic → late disease driven by host immune response and obliterative endarteritis of vasa vasorum. **Diagnostic clue**: Dark-field of chancre exudate; nontreponemal screen (VDRL/RPR) confirmed with treponemal FTA-ABS or TP-PA. **Virulence**: Endoflagella for corkscrew motility, evasion via outer-membrane lipoproteins.

Boards buzzwords

  • corkscrew motility
  • painless chancre
  • snail-track ulcers
  • gummas
  • tabes dorsalis
  • Argyll Robertson pupil
  • VDRL/RPR + FTA-ABS

Associated diseases

  • Primary syphilis (painless chancre)
  • Secondary (palms/soles rash, condyloma lata)
  • Tertiary (gummas, aortitis, neurosyphilis — tabes dorsalis, paresis)
  • Congenital syphilis

Treatment

Benzathine penicillin G IM — single dose for primary/secondary/early latent; 3 weekly doses for late latent; IV aqueous penicillin G × 10–14 days for neurosyphilis; doxycycline if non-pregnant penicillin allergic; watch for Jarisch-Herxheimer

Related organisms

Listeria monocytogenes
Listeria monocytogenes
Gram-positive rod (facultative intracellular)
Mycobacterium tuberculosis
Mycobacterium tuberculosis
Acid-fast bacillus (mycolic acid cell wall)
Nocardia spp.
Nocardia spp.
Gram-positive filamentous, partially acid-fast
Mycobacterium leprae
Mycobacterium leprae
Acid-fast bacillus (obligate intracellular)
Neisseria meningitidis
Neisseria meningitidis
Gram-negative diplococci
Borrelia burgdorferi
Borrelia burgdorferi
Spirochete