Syphilis Microbiology

Organism Structure and Morphology

• Morphology: A corkscrew-shaped spirochete with 5-20 μm length, characterized by 4-14 regular spiral coils that resemble a gentleman bowing (Sherris & Ryan's Medical Microbiology)
• Motility: Actively motile with three characteristic movements - axial projection, rotation on its long axis, and side-to-side bending/twisting - while maintaining precise spiral uniformity
• Gram staining: Not visible on routine Gram stain; requires darkfield microscopy, immunofluorescence, or silver impregnation for visualization
• Outer membrane structure: Unique gram-negative structure but crucially lacks lipopolysaccharide (LPS) and contains extremely few surface proteins - only about 1% the protein content of E. coli (Andrews' Diseases of the Skin)

Metabolic Limitations

• Genome: Contains only about one-quarter the number of genes of most bacteria
• Metabolic deficiency: Lacks catalase, oxidase, and efficient ATP-producing pathways (tricarboxylic acid cycle and electron transport chain)
• Growth rate: Extremely slow with a mean generation time exceeding 30 hours
• Culture: Cannot be cultured in vitro; sustained growth only achieved in animal models (rabbit tests), which serve as the sole source of bacteria for diagnostic reagents
• Environmental sensitivity: Dies rapidly on drying and is readily killed by detergents and disinfectants

Pathogenesis

1. Inoculation: 10-90 days incubation (average 3 weeks)
2. Primary syphilis: Painless ulcer (chancre) with regional lymphadenopathy
3. Hematogenous dissemination: Organisms reach bloodstream within hours and multiple organs (including brain) within 18 hours of infection
4. Secondary syphilis: 3-10 weeks after chancre appearance; diffuse maculopapular rash teeming with spirochetes; highly infectious
5. Latency: 3-12 weeks to years; asymptomatic but spirochetes persist in tissues
6. Tertiary syphilis: 2-20+ years later; involves cardiovascular system (aortitis, aortic aneurysm) and central nervous system (neurosyphilis, dementia), with usually sterile lesions

Immune Evasion Mechanisms

• Low antigenic mass: Each tissue contains extremely few organisms, possibly below the "critical antigenic mass" required for effective immune recognition
• Minimal surface antigens: Expresses only about 1% the antigenic targets of E. coli on its outer membrane
• Antigenic variation: Outer membrane proteins (OMPs) undergo rapid mutation during infection, generating antigenically distinct subpopulations that evade antibody responses
• Slow multiplication: The extremely slow growth rate (>30 hour generation time) may allow the organism to remain undetected
• Imperfect immunity: Clinical observations show immunity develops slowly, appearing only in early latency; even then, immunity is incomplete, with approximately one-third of infected individuals eventually clearing infection while others progress to tertiary disease

Transmission and Epidemiology

• Primary transmission: Direct sexual contact with active primary or secondary lesions (>50% transmission rate with infected partners)
• Other routes: Nongenital contact (e.g., lip lesions), sharing of contaminated needles, transplacental transmission to fetus
• Infectivity: Only primary and secondary stages are infectious; tertiary syphilis is noninfectious
• Host: Exclusively human pathogen under natural conditions
• Global burden: Approximately 12 million persons infected annually worldwide; 2 million are pregnant women

Diagnostic Methods

• Darkfield microscopy: Most specific for fresh lesion exudates; shows characteristic corkscrew motility
• Fluorescent antibody testing: Direct or indirect methods for confirmation

• Nontreponemal tests (VDRL, RPR): Usually positive early but may be negative in advanced disease; can yield false positives
• Treponemal tests (FTA-ABS): Become positive later in primary syphilis and remain positive indefinitely; more specific

Pathology

• Primary and secondary lesions: Demonstrate proliferative endarteritis (inflammation of small blood vessel linings) and plasma cell-rich inflammatory infiltrate
• Tertiary lesions (gummata): Central area of necrosis surrounded by lymphoplasmacytic infiltrates, activated macrophages, and fibrosis; usually bacteria-free despite severe tissue destruction

Treatment

• Early syphilis: 2.4 million units intramuscularly as single dose
• Late latent or unknown duration: 2.4 million units weekly for 3 weeks
• Penicillin concentration of 0.003 U/mL shows definite treponemicidal activity