BACTERIA · Spirochete
Borrelia burgdorferi
Borrelia burgdorferi — Dark-field microscopy
Stain: Dark-field microscopyMorphology: Loosely coiled spirochete with wavy, irregular spiralsYield: HIGHDifficulty: MEDIUM
Image: Wikimedia Commons · File:Borrelia_dark_field.jpg (CC BY 2.0)
Key facts
**Pathogenesis**: Transmitted by Ixodes scapularis (deer tick); reservoir is white-footed mouse. OspA/OspC outer surface proteins switch during tick → mammal transition. Causes Lyme disease with three stages. **Diagnostic clue**: Clinical erythema migrans is diagnostic without serology; otherwise two-tier ELISA → Western blot. Often co-infected with Anaplasma or Babesia. **Virulence**: Antigenic variation of VlsE lipoprotein, immune evasion via molecular mimicry.
Boards buzzwords
- Ixodes tick
- erythema migrans / bull's-eye rash
- Bell palsy
- AV block
- Lyme arthritis
- white-footed mouse
Associated diseases
- Early localized Lyme (erythema migrans)
- Early disseminated (multiple EM, Bell palsy, AV block, meningitis)
- Late Lyme (chronic monoarticular knee arthritis, encephalopathy)
Treatment
Doxycycline × 10–21 days first-line (amoxicillin or cefuroxime in children/pregnancy); IV ceftriaxone for high-grade AV block, meningitis, or late neurologic disease
Related organisms
Listeria monocytogenes
Gram-positive rod (facultative intracellular)
Mycobacterium tuberculosis
Acid-fast bacillus (mycolic acid cell wall)
Nocardia spp.
Gram-positive filamentous, partially acid-fast
Treponema pallidum
Spirochete
Mycobacterium leprae
Acid-fast bacillus (obligate intracellular)
Neisseria meningitidis
Gram-negative diplococci