Treponema pallidum

Morphology and Cultivation Features

Treponema pallidum has a characteristic structure: a thin spiral rod whose motor apparatus is represented by three periplasmic fibrils at each pole, providing simultaneous rotational and translational movement. Pathogenic species are microaerophiles in terms of respiration type.

The main challenge in studying virulent strains is their inability to be cultured in vitro (they do not grow on artificial nutrient media). For biomass accumulation, bacteria are cultured 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 loss of virulence and alteration of their antigenic structure.

The bacterium takes up aniline dyes poorly (it turns pale pink upon Romanowsky-Giemsa staining, hence its name). For its detection, native dark-field microscopy, phase-contrast microscopy, or silver impregnation are optimal. Under adverse environmental conditions, the pathogen transforms into cysts and L-forms (with cell wall defects).

Pathogenesis and Stages of Syphilis

The disease is transmitted primarily through sexual contact, less commonly via direct contact, transplacentally, or through blood transfusion. Upon entering the organism, the bacterium attaches to cells using adhesins (their synthesis is induced only in vivo), and its lipoproteins trigger immunopathological processes.

The infection proceeds strictly in cycles:

• Incubation period: lasts 3–4 weeks.
• Primary period (6–7 weeks): a hard chancre (ulcer with indurated margins) forms at the site of entry (genitals, oral mucosa). Regional lymphadenopathy develops, and the pathogen actively multiplies in the lymph nodes.
• Secondary period: hematogenous dissemination occurs. Manifests as generalized rashes (papules, pustules, roseolae). The rash elements contain a huge number of bacteria, making the patient maximally contagious. The period lasts for years and is recurrent in nature.
• Tertiary period: develops after 2–4 years (or decades). Gummas (granulomas) form in internal organs due to hypersensitivity. Their breakdown causes extensive tissue destruction.
• Quaternary period (neurosyphilis): in the absence of treatment, tabes dorsalis (tabes dorsalis) and progressive paralysis develop.

Antigenic Structure and Diagnostics

Treponema pallidum has two main types of antigens: specific (thermolabile protein) and non-specific (lipoid). The latter is identical to cardiolipin extracted from bovine heart.

Immunity in syphilis is non-sterile: the presence of antibodies does not protect against reinfection. Diagnostics is based on direct methods (bacterioscopy of chancre smears in early stages) and a two-stage serological algorithm:

1. Screening tests: rapid plasma reagin (RPR) test, VDRL, RMP. They use a non-specific cardiolipin antigen. Excellent for mass population screening and monitoring treatment efficacy (the titer of " reagins" decreases). Drawback: may yield false-positive results in diabetes, malignancies, and collagenoses.
2. Confirmatory tests: ELISA, FTA-ABS, TPHA, TPI. Based on treponemal protein antigen. Used to verify the diagnosis. Characterized by high specificity, but unsuitable for monitoring cure, as specific antibodies persist for years, forming a "serological scar".

Treatment and Prevention

Etiotropic therapy is based on penicillin antibiotics, with bismuth-containing drugs as an alternative. Specific prophylaxis (vaccines) does not exist. Non-specific prevention includes early detection of patients, mandatory serological screening of donors, pregnant women, and hospitalized patients, as well as monitoring of high-risk groups.

Class and exam questions

What else to study on this topic

• Classification and clinical significance of other subspecies (agents of bejel, yaws, and pinta).
• Symbiosis of non-pathogenic treponemes (T. vincentii) with fusobacteria and the development of necrotizing Vincent's angina.
• Pathogenesis features: mechanisms of endarteritis and tissue necrosis development.
• The nature of "reagins": dynamics of the production of an IgM and IgG mixture against lipoid antigen.

Other topics from the section 'Spirochetes and spiral bacteria (Treponemes/syphilis, Borrelia/Lyme disease and relapsing fevers, Leptospira, Campylobacters, Helicobacters)'

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