Treponema pallidum

It is a spirochaete, has endoflagella for motility and is arranged in regular spirals. They do not grow in culture media. It has cardiolipin antigen. The antibody against cardiolipin is used for diagnosing Treponemal infections. It is the causative agent of syphilis which is an STD and is a notifiable disease. Cases are steadily rising in the USA.

Syphilis: Syphilis presents as primary, secondary and tertiary forms. Primary form is characterised by painless, indurated, ulcer called chancre typically seen at the point of entry of the bacteria which may be genital area or mouth. This is accompanied by regional, rubbery lymphadenopathy. It is contagious. Following haematogenous dissemination there is a stage of secondary syphilis characterised by maculopapular rash including the palms and soles, condyloma lata in the genital area, patchy alopecia, constitutional symptoms, generalized lymphadenopathy, hepatitis, eye and bone lesions etc. Tertiary syphilis follows years later with complications such as aortic aneurysms, chronic granulomas called “gumma”, tabes dorsalis, general paralysis of the insane etc. Typically these lesions are not infectious. Latent syphilis can be seen in the primary or secondary stage. They will not have disease manifestations but will be positive on serological tests for syphilis. Congenital syphilis can be acquired across the placenta after the first trimester of pregnancy. It presents with hepatosplenomegaly, skin and bone lesions.

For diagnosis of syphilis, T. pallidum can be observed in samples taken from chancres or condylomas by dark field microscopy or direct fluorescent antibody test. Gram stain is not used as they cannot be seen well. In biopsy samples, special stains like silver impregnation by Levaditi method or immunofluorescence can be used.

There are reagin or non specific as well as specific tests done for detection of syphilis. The reagin tests rely on detection of antibodies to nontreponemal antigens like cardiolipin. They are VDRL (venereal disease research laboratory) and RPR (rapid plasma reagin) tests. Development of floccules is a positive test. A titre of 1:8 or more is considered significant. They are positive in primary and secondary syphilis. Antibody titre decreases after effective treatment. Drawback is false positive results in the presence of leprosy, SLE, malaria, infectious mononucleosis, hepatitis B etc. Remember that for CSF, VDRL test is preferred over RPR as it gives more false negative results than VDRL. When antibody titers are very high “prozone phenomenon” may be seen which may give a false negative result. That can be overcome by diluting the initial sample. The specific tests for syphilis are FTA-ABS which is an immunofluorescence test, TPHA and MHA-TP which are haemagglutination assays.