Treponema Pallidum An Overview

Treponema pallidum Overview

Treponema pallidum is caused by the spirochaetes bacteria, and the treponema pallidum species caused syphilis. Syphilis infection commonly caused by sexualities or sexual contact Spirochaetes are much more complex than other bacterias.

Treponema Pallidum Morphology

It is thin, delicate spirochaete with tapering ends, having about ten regular spirals. It is about 10 um long and 0.1-0.2 um wide. It is actively motile, showing rotation around the long axis, backward and forward movements, and flexion of the whole body.

It does not take ordinary bacterial stains and cannot be seen under the light microscope in wet films. Because of the thinness of the spirals, Treponema pallidum cannot be seen by a light microscope. However, its morphology and motility can be seen by dark ground microscopy or phase-contrast microscopy.

It can be stained with silver impregnation methods. The treponemes reduce silver nitrate to a metallic silver that is deposited on the surface enlarging the diameter of the organism. Three or four endoflagella are present.

Culture Of Treponema Pallidum

Pathogenic Treponema pallidum cannot be grown in artificial culture media but is maintained
by subculture in susceptible animals. The Nichol’s strain is a virulent treponema pallidum

Cultivable treponemes such as treponema phagedenic (Reiter’s treponeme) and
treponema refiringens (Noguchi strain) are non-pathogenic. They can be grown under strictly anaerobic conditions.

Disease Caused By Treponema Pallidum

Venereal syphilis is acquired by sexual contact. The incubation period is about a month (range 10-90 days). There are three clinical stages of the disease in an untreated case primary, secondary, and tertiary. The treponemes can cross the placental barrier. Thus congenital syphilis occurs.

Laboratory Diagnosis Of Treponema Pallidum

The diagnosis of syphilis consists of demonstration of treponemes and detection of antibodies by serological tests

Demonstration Of Treponema Pallidum

1. Dark Ground Microscopy

Treponema pallidum appears as a slender, spiral organism showing rotational as well as flexion and extension movements

2 Direct Fluorescent Antibody Staining for Treponema Pallidum (DFA-TP)

The smear is stained with a fluorescent-labeled monoclonal antibody against treponema pallidum. The treponemes exhibit an apple-green fluorescence.

Serological Tests

These tests form the mainstay of laboratory diagnosis. Two types of antibodies are produced in syphilis, non-specific antibody (reagin-antibody), and specific anti-treponemal antibody. Depending upon the antigen used, serological tests for syphilis are divided into non-treponemal tests (cardiolipin or lipoidal antigen is used) and treponemal tests (treponemes are used as the antigen).

Non-Treponemal Tests

In the Standard Tests for Syphilis (STS), reagin antibodies are detected by cardiolipin antigen. The STS includes the Venereal Disease Research Laboratory (VDRL) test, Rapid Plasma Reagin (RPR), Kahn test, and Wassermann reaction. All these tests are flocculation tests except Wassermann reaction which is a complement fixation test (CFT). The Wassermann reaction and the Kahn test are no longer in use.

Flocculation Tests for Treponema Pallidum

Cardiolipin antigen reacts with reagin syphilitic serum resulting in the formation of visible clumps or floccules. Results can be read in a few minutes DRL and RPR tests are equally sensitive.

VDRL (Venereal Disease Research Laboratory) Test

It is the most widely used simple and rapid serological test. It can be used to detect antibodies in serum as well as in cerebrospinal fluid (CSF). It is a slide flocculation test. The VDRLantigen (cardiolipin antigen) is used. The results are reported as reactive’, weak reactive’, or ‘non-reactive’. ‘Reactive’ means
positive while ‘non-reactive’ is negative.

RPR (Rapid Plasma Reagin) Test

It is almost similar to the VDRL test. Finely particles are added to Cardiolipin.

Advantages of RPR test over VDRL Test

Unheated serum or plasma can be used. A fingerprick sample of blood is sufficient It does not require a microscope and can be done in the field. It is available commercially as a kit. VDRL and RPR tests are useful in surveys because of their low cost.

Disadvantage Of Standard Test

The antigen (cardiolipin) is non-specific and hence may react with the sera of patients who may not have syphilis. This accounts for the biological false positive (BFP) reactions. These are not
caused by technical faults. They represent non-treponemal cardiolipin antibody responses.

Conditions In Which BFP Reactions Occur.

  • Leprosy
  • Malaria
  • Relapsing fever
  • Infectious mononucleosis
  • Tropical eosinophilia

Test Using Treponema Pallidum

Treponema Pallidum Immobilisation (TPI) Test. This test employs live T. pallidum.
The test serum is mixed with actively motile Nichol’s strain of T. pallidum and incubated anaerobically. If antibodies are present, the treponemes are immobilized (rendered non-motile), when examined under dark ground illumination.

Although TPI is the most specific test for syphilis, it is infrequently used, as live treponemes are required from infected animals and it is technically difficult to
perform the test.

Treatment Of Treponema Pallidum

  • Benzathine benzylpenicillin.
  • Alternatively, doxycycline may be used.
  • In certain cases, treatment with penicillin may lead to a Jarisch-Hercheimer reaction.
  • It is a systemic response that consists of fever, chills, myalgia, and hypotension.
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