The lack of a method for demonstrating the presence of Treponema pallidum by growth necessitates the use of alternative methods. Traditionally, these methods are divided into direct detection methods (animal inoculation, dark-field microscopy, etc.) and serologic tests for the presence of patient antibody against T. pallidum. Serologic methods are further divided into two classes. One class, the nontreponemal tests, detects antibodies to lipoidal antigens present in either the host or T. pallidum; examples are the Venereal Disease Research Laboratory and rapid plasma reagin and tests. Reactivity in these tests generally indicates host tissue damage that may not be specific for syphilis. Because these tests are easy and inexpensive to perform, they are commonly used for screening, and with proper clinical signs they are suggestive of syphilis. The other class of test, the treponemal tests, uses specific treponemal antigens. Confirmation of infection requires a reactive treponemal test. Examples of the treponemal tests are the microhemagglutination assay for antibodies to T. pallidum and the fluorescent treponemal antibody absorption test. These tests are more expensive and complicated to perform than the nontreponemal tests. On the horizon are a number of direct antigen, enzyme-linked immunosorbent assay, and PCR techniques. Several of these techniques have shown promise in clinical trials for the diagnosis of congenital syphilis and neurosyphilis that are presently difficult to diagnose.
A sensitive and specific PCR method to detect Treponema pallidum in clinical specimens was developed. PCR primers were designed based on two unique features of the DNA polymerase I gene (polA). The first distinctive characteristic is that the region codes for a high cysteine content and has low homology with similar regions of DNA polymerase I gene from known microorganisms. The second unique feature is the presence of four insertions in the gene. PCR tests using primers designed on the basis these regions reacted with various pathogenic T. pallidum subspecies but did not react with nonpathogenic treponemal species or other spirochetes. An additional 59 species of bacteria and viruses, including those that cause genital ulcers, tested negative. This PCR method is extremely robust and sensitive. The detection limit is about 10 to 25 organisms when analyzed on gel. However, the analytic sensitivity can be increased by at least 1 log, to a detection limit of a single organism, when the ABI 310 Prism Genetic Analyzer is used to detect fluorescence-labeled amplicons. We further used this test in a clinical setting and compared the results with results from a previously reported multiplex-PCR test (for T. pallidum, Haemophilus ducreyi, and herpes simplex virus). We tested 112 genital ulcer specimens by the polA PCR, obtaining a sensitivity of 95.8% and a specificity of 95.7%. These results suggest that the polA PCR is applicable as a routine clinical diagnostic test for syphilis.
BackgroundSince the first recorded epidemic of syphilis in 1495, controversy has surrounded the origins of the bacterium Treponema pallidum subsp. pallidum and its relationship to the pathogens responsible for the other treponemal diseases: yaws, endemic syphilis, and pinta. Some researchers have argued that the syphilis-causing bacterium, or its progenitor, was brought from the New World to Europe by Christopher Columbus and his men, while others maintain that the treponematoses, including syphilis, have a much longer history on the European continent.Methodology/Principal FindingsWe applied phylogenetics to this problem, using data from 21 genetic regions examined in 26 geographically disparate strains of pathogenic Treponema. Of all the strains examined, the venereal syphilis-causing strains originated most recently and were more closely related to yaws-causing strains from South America than to other non-venereal strains. Old World yaws-causing strains occupied a basal position on the tree, indicating that they arose first in human history, and a simian strain of T. pallidum was found to be indistinguishable from them.Conclusions/SignificanceOur results lend support to the Columbian theory of syphilis's origin while suggesting that the non-sexually transmitted subspecies arose earlier in the Old World. This study represents the first attempt to address the problem of the origin of syphilis using molecular genetics, as well as the first source of information regarding the genetic make-up of non-venereal strains from the Western hemisphere.
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