Syphilis: Laboratory Testing and Diagnosis
Treponema pallidum is the causative agent of syphilis. T. pallidum is a helical (coiled) shaped rod known as a spirochete that measures approximately 0.18 um in diameter and ranges from 6 to 20 um in length. Spirochetes have pointed ends and show rapid rotation about the axis (corkscrew motility) in fresh exudate from active sores.
Microscopic detection of T. pallidum
T. pallidum is difficult to culture: the easiest means of diagnosing syphilis is through direct microscopic detection of the organism. However, T. pallidum presents some challenges to microscopic detection. When unstained, T. pallidum cannot be seen with standard bright-field microscopy due to its small cell diameter. The spirochete is best seen with dark-field microscopy.
Dark-field microscopy allows a lower limit of resolution than bright-field microscopy (0.1 um versus 0.2 um) allowing spirochetes to be visualized. Like the terms suggest, objects are observed against a bright background in bright-field microscopy as light is focused directed on the specimen, and against a dark background in dark-field microscopy where directly transmitted light is excluded by a dark-field condenser allowing only scattered light to be focused on the specimen. With dark-field microscopy bacteria appear luminous against a dark background.
In addition to its small size, T. pallidum is difficult to stain with routine laboratory stains. Special stains must be used, either silver stains, or direct fluorescent antibody (DFA) stains.
Detection of helical rods showing pointed ends and the characteristic cork-screw motility on microscopic examination of fresh exudate from sores is diagnostic for syphilis when other treponeme species can be excluded as the cause of infection.
Direct fluorescent antibody staining does not require motile organisms and because the antibody conjugated to the stain is specific for pathogenic strains of T. pallidum, it can be used for oral and rectal lesion samples as well as genital samples. The technique requires a UV fluorescence microscope equipped with a dark-field condenser. Non-pathogenic strains of treponemes can be differentiated from pathogenic strains following staining of the specimen with fluorescein isothiocyanate (FITC) that is linked to anti-T. pallidum antibodies. Fluorescing spirochetes displaying typical helices with pointed ends are characteristic of T. pallidum
Molecular detection of T. pallidum
Molecular testing using nucleic acid sequences (primers or probes) targeting a unique segment of T. pallidum may also be used for detection. In contrast to microscopic examination, molecular diagnosis does not require living organisms; therefore length of time from sample collection to processing, and sample transport conditions are not as critical.
Blood testing
Blood testing for antibodies to T. pallidum is also suggestive of infection and is inexpensive and relatively simple to perform. A rapid plasma reagin (RPR) screening test is commonly performed to detect antibodies in the serum of individuals with syphilis infection. The regain screening tests detect IgM and IgG antibodies to lipid material released from damaged treponemes and host cells.
The anti-lipid antibodies may result from syphilis infection, treponeme infection or from other non-treponeme infections where cell damage results in release of lipid materials. More specific confirmatory tests must be performed when positive RPR tests result as approximately 1% of the population will have false positive results. Rapid plasma regain testing also lacks the sensitivity in early primary cases and late syphilis.
More specific blood tests to detect antibodies specific to T. pallidum (e.g. microhemagglutination assay [MHA-TP], fluorescent-treponemal-antibody absorption [FTA-ABS, enzyme-linked immunosorbent assay ELISA]). These tests are not intended for routine use. They are expensive and may lack sensitivity during the early stages of primary syphilis.
Read more about syphilis and other sexually transmitted infections:
Syphilis Infection: Sexually Transmitted Spirochete
Syphilis Symptoms: Primary, Secondary and Tertiary Syphilis
Spread of Syphilis: Historical Perspective
Sexually Transmitted Infection: STI and STD
Source:
Norris SJ, SA Larsen. Treponema and Other Host-Associated Spirochetes in Manual of Clinical Microbiology, Eighth Edition. ASM Press. 2003.
Judy Arbique - I graduated from Medical Laboratory Technology training in 1979 and since then obtained advanced recognition in clinical microbiology from ...
