General Characteristics
- Spiral-shaped (spirochete), slender (0.1–0.2 μm) and flexible, 10–13 μm long
- Motile via endoflagella (axial filaments within periplasmic space)
- Cannot be seen with standard Gram stain – best visualized by:
- Dark-field microscopy
- Silver stain
- Fluorescent antibody techniques
- Cannot be cultured on artificial media
- Structurally complex – central protoplasmic cylinder bounded by a cytoplasmic membrane and cell wall of similar structure to that of Gram (–) bacteria
- Cultivated in rabbit testis for diagnostic strains:
- Nichol’s strain (pathogenic, used for diagnostic antigen production)
- Reiter’s strain (nonpathogenic, used for serologic testing)
Transmission
- Sexual contact through microabrasions
- Transplacental (congenital syphilis)
- Blood transfusion (rare)
- Occupational (healthcare workers – non-venereal)
Stages of Syphilis
- Gummatous syphilis (soft granulomas)
- Cardiovascular syphilis (aortic aneurysm)
- Neurosyphilis: Tabes dorsalis, general paresis, dementia
Congenital , Transplacental. Signs: saddle nose, Hutchinson’s teeth, deafness, blindness, keratitis.
Laboratory Diagnosis
1. Microscopy
- Dark-field or phase-contrast microscopy of fluid from chancre
- Silver staining (Levaditi) in tissue biopsies
Serology
A. Nontreponemal Tests (screening)
⚠️ False positives: HIV, pregnancy, autoimmune diseases, malaria, endocarditis