Urogenital Tract Pathogens

Neisseria gonorrhoeae and Chlamydia trachomatis were among the first organisms to be targeted for detection in clinical specimens by molecular methods. The molecular methods are so well characterized for these two organisms that detection of the nucleic acid of N. gonor-rhoeae and C. trachomatis is the laboratory method used almost exclusively. Other sexually transmitted bacteria are considered good targets for the development of molecular-based methods because traditional laboratory methods of detection and identification for these organisms either lack sensitivity or are time-consuming. Table

12.4 summarizes the molecular-based tests that have been described for the bacteria that cause genital tract infections.

Neisseria gonorrhoeae and Chlamydia trachomatis

N. gonorrhoeae and C. trachomatis are the two most common causes of sexually transmitted disease. Disease caused by N. gonorrhoeae, called gonorrhea, is associated with dysuria and urethral discharge in men and cer-vicovaginal discharge in women. N. gonorrhoeae can also cause pharyngitis and anorectal infections. C. trachomatis causes a nongonococcal urethritis and is asymptomatic in 50%-66% of men and women. N. gonorrhoeae and C. trachomatis are often found in coinfec-tions, so it is prudent to rule out both organisms when considering that one is present.61

Traditional laboratory diagnosis of N. gonorrhoeae entails culture of endocervical or urethral swabs onto chocolate agar and selective, enriched media such as modified Thayer Martin. For male urethral swabs, Gram stain alone with the observation of gram-negative diplo-cocci is diagnostic by itself for N. gonorrhoeae (sensitivity = 90%-95%; specificity = 95%-100%).62 For female endocervical swabs or other specimen types from males and females, Gram stain alone is not diagnostic (sensitivity = 50%-70% for endocervical). N. gonorrhoeae is fastidious, and the specimen needs to be transported in a transport medium or plated directly onto media at the bedside. Delays in culturing the specimen are associated with false-negative cultures. Plates are examined daily for 72 hours for the presence of colonies resembling Neisseria and identified by biochemical testing.

Laboratory diagnosis of C. trachomatis is more problematic. C. trachomatis is an obligate intracellular pathogen; thus, when collecting specimens for isolation of C. trachomatis, it is critical that the practitioner scrape the endocervix or urethra to ensure the collection of host columnar epithelial cells that harbor the organisms. C. trachomatis is extremely labile, and clinical specimens sent to the laboratory for culture must be placed in a chlamydial transport medium to maintain the viability of the chlamydia. Culture of specimens for C. trachomatis is typically performed on McCoy cells in a shell vial system. The specimen is inoculated onto the cell line, the vial is incubated for 48-72 hours, and the cells are harvested and stained with a fluoresceinated antibody that has specificity for C. trachomatis. Cultures for N. gonor-

Table 12.4 Typical Genital Tract Organisms Targeted by Molecular-Based Detection Methods25,60

Organism

Specimen Sources

Traditional Diagnostic Methods

Gene Target

Treponema pallidum

Mycoplasma genitalium

Mycoplasma hominis Ureaplasma urealyticum Haemophilus ducreyi

Neisseria gonorrhoeae

Chlamydia trachomatis

Genital ulcers

Blood

Brain tissue

Cerebrospinal fluid

Amniotic fluid

Placenta

Umbilical cord

Fetal tissue

Serum

Urine

Urethral

Vaginal

Cervical

Genital tract

Amniotic fluid

Genital tract

Amniotic fluid

Urine

Urethral

Cervical

Thin preparation vials

Urine

Urethral

Cervical

Thin preparation vials Conjunctiva

Serological (indirect and direct) Direct antigen detection (dark field, DFA)

Culture

Culture

Culture

Gram's stain

Culture

Serological

Culture

Culture

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