Morphology and Cultivation
Treponema pallidum has a characteristic structure: a slender spiral rod whose motility apparatus consists of three periplasmic fibrils at each end. These enable simultaneous rotational and translational movement. Pathogenic species are microaerophiles regarding their oxygen requirements.
The primary challenge in studying virulent strains is their inability to be cultured in vitro (they do not grow on standard artificial media). To propagate biomass, bacteria are maintained in vivo by intratesticular inoculation of laboratory rabbits. Non-virulent strains can be grown on media containing kidney or brain tissue under anaerobic conditions at 35 °C, but this leads to a loss of virulence and altered antigenic structure.
The bacterium takes up aniline dyes poorly (staining a pale pink with the Romanowsky-Giemsa stain, which gave rise to its species name pallidum, meaning 'pale'). Optimal visualization techniques include dark-field microscopy, phase-contrast microscopy, or silver impregnation. Under adverse environmental conditions, the pathogen can transform into cysts or L-forms (with cell wall defects).
Pathogenesis and Stages of Syphilis
The infection is transmitted primarily via sexual contact, and less frequently through direct contact, transplacental (congenital) transmission, or blood transfusion. Once inside the body, the bacterium adheres to host cells using adhesins (expressed exclusively in vivo), while its lipoproteins trigger immunopathological cascades.
The infection follows a strictly cyclical progression:
- Incubation period: Lasts 3–4 weeks.
- Primary stage (6–7 weeks): A hard chancre (a painless ulcer with indurated borders) develops at the inoculation site (genitals, oral mucosa). Regional lymphadenopathy occurs as the pathogen actively multiplies within lymph nodes.
- Secondary stage: Characterized by hematogenous dissemination. Manifests as generalized rashes (papules, pustules, roseola). These lesions contain high concentrations of bacteria, rendering the patient maximally contagious. This stage lasts for years and is characterized by relapses.
- Tertiary stage: Appears 2–4 years (or decades) later. Gummas (granulomas) form in internal organs due to delayed-type hypersensitivity. Their breakdown leads to extensive tissue destruction.
- Quaternary stage (Neurosyphilis): In the absence of treatment, manifestations such as tabes dorsalis and general paresis develop.
Antigenic Structure and Diagnostics
Treponema pallidum possesses two main antigen types: specific (thermolabile protein) and nonspecific (lipoidal). The latter is identical to cardiolipin extracted from bovine heart tissue.
Immunity in syphilis is non-sterile: the presence of antibodies does not protect against reinfection. Diagnostics rely on direct methods (dark-field microscopy of chancre exudate in early stages) and a two-step serological algorithm:
- Screening tests: Rapid plasma reagin (RPR), VDRL, and Wasserman-type precipitation reactions. These utilize nonspecific cardiolipin antigens. They are well suited for mass population screening and monitoring treatment efficacy (as "reagin" titers decline). A limitation is false-positive results in diabetes, malignancies, and collagen vascular diseases.
- Confirmatory tests: FTA-ABS, TPHA, TPPA, and ELISA. These use treponemal protein antigens and are applied to verify diagnoses. They exhibit high specificity but are unsuitable for monitoring post-treatment cure because specific antibodies persist for years, creating a "serological scar."
Treatment and Prevention
Targeted antimicrobial therapy relies on penicillin-class antibiotics, with bismuth-containing preparations as alternatives. Specific prophylaxis (vaccination) is unavailable. Nonspecific prevention involves early case detection, mandatory serological screening of blood donors, pregnant women, and hospitalized patients, as well as monitoring high-risk populations.