Treponema pallidum, formerly known as Spirochaeta pallida, is a microaerophillic spirochaete bacterium with subspecies that cause the diseases syphillis, bejel (also known as endemic syphillis) and yaws, which are morphologically and serologically indistinguishable. It is transmitted only among humans. It is often described as Gram negative but its outer membran lacks LPS which is characteristic of other Gram negative bacteria. 

Lifecycle and Symptoms:

The clinical features of syphilis, yaws, and bejel occur in multiple stages that affect the skin. The skin lesions observed in the early stage last for weeks or months. The skin lesions are highly infectious, and the spirochetes in the lesions are transmitted by direct contact. The lesions regress as the immune response develops against T. pallidum. The latent stage that results can last a lifetime in many cases. In a few cases, the disease exits latency and enters a tertiary phase, in which destructive lesions of skin, bone, and cartilage ensue. Unlike yaws and bejels, syphilis in its tertiary stage often affects the heart, eyes, and nervous system, as well.

The etiological agent of syphilis is Treponema pallidum. Syphilis has diverse clinical manifestations and shares many clinical features with other treponemal and nontreponemal diseases. Therefore, it is mandatory that the clinical diagnosis is always supported by appropriate laboratory tests and that the test results are interpreted with reference to the patient’s history and physical examination findings. Syphilis progresses through distinct primary, secondary, latent and tertiary stages. The ulcers that appear in primary and secondary syphilis are rich in treponemes; venereal transmission occurs through direct contact with these lesions. The stage of the disease at which the patient presents has implications for diagnosis and treatment. In some stages, the disease may be asymptomatic, and there are problems in diagnosing very early syphilis, neurosyphilis, asymptomatic congenital syphilis and syphilis in intravenous drug users and persons coinfected with serologically cross-reacting agents and HIV.  See Ratnam “The laboratory diagnosis of syphilis”

Detection of T pallidum:

T pallidum, the etiological agent of syphillis, can not be seen using standard microscopic techniques. Instead, diagnosis is by darkfield microscopy or serology. 

In North America, many unsuspected cases are discovered by laboratory testing. The etiological agent, Treponema pallidum, cannot be cultured, and there is no single optimal alternative test. Serological testing is the most frequently used approach in the laboratory diagnosis of syphilis. Serological tests fall into two categories: nontreponemal tests for screening, and treponemal tests for confirmation. All nontreponemal tests measure both immunoglobulin (Ig) G and IgM antiphospholipid antibodies formed by the host in response to lipoidal material released by damaged host cells early in infection and lipid from the cell surfaces of the treponeme itself. All treponemal tests use T pallidum or its components as the antigen. If lesion exudate or tissue is available, direct examination is performed, followed by a nontreponemal serology test. A reactive nontreponemal test is then confirmed by a treponemal test. A confirmed serological test result is indicative of the presence of treponemal antibodies but does not indicate the stage of disease and, depending on the test, may not differentiate between past and current infection. Despite their shortcomings and the complexity of interpretation, serological tests are the mainstay in the diagnosis and follow-up of syphilis. Latent syphilis can only be diagnosed by serological tests. In fact, in North America, the majority of syphilis cases are identified at the latent stage by serological tests. The sensitivity and specificity of serological tests vary depending on the type of test and stage of the disease. See Ratnam “The laboratory diagnosis of syphilis”

One serological test is the Rapid Plasma Reagin (RPR) test, used to detect antilipid antibodies (reagin) present in the serum or plasma of persons with syphilis. Treponema pallidum, the etiological agent of syphilis, induces the production of at least two types of antibodies in human infection: anti-treponemal antibodies that can be detected by FTA-ABS antigen and anti-nontreponemal antibodies (reagin) that can be detected by RPR antigen.

For the test, the RPR antigen is mixed with unheated or heated serum on a plastic-coated card. If antibodies are present, they combine with the lipid particles of the antigen, causing them to agglutinate. The charcoal particles in an antigen suspension coagglutinate with the antibodies and show up as black clumps against the white card. If no antibodies are present, the test mixture is uniformly gray.  

Characteristic clumping ranging from marked and intense to slight from marked and intense to slight definitive clumping is Reactive. If the test is reactive, a semi-quantitative test to determine the highest titer giving a positive result. Titers are reported as 1:1, 1:2, 1:4, 1:8, 1:16, 1:32, etc. The higher the titer, the more antibodies present in the blood.

Treatment: 

T pallidum is sensitive to penicillin but more difficult to treat in patients with HIV infections.