J Med Microbiol 57 (2008), 304-309; DOI: 10.1099/jmm.0.47498-0
© 2008 Society for General Microbiology
ISSN 1473-5644
A comparative study of three different PCR assays for detection of Mycoplasma genitalium in urogenital specimens from men and women
Andreas Edberg1,
Margaretha Jurstrand2,
Eva Johansson3,
Elisabeth Wikander1,
Anna Höög1,
Thomas Ahlqvist1,
Lars Falk4,5,
Jørgen Skov Jensen6 and
Hans Fredlund2
1 Department of Clinical Microbiology, Central Hospital, SE-651 85 Karlstad, Sweden
2 Department of Clinical Microbiology, University Hospital, SE-701 85 Örebro, Sweden
3 Department of Infectious Diseases, Central Hospital, SE-651 85 Karlstad, Sweden
4 Department of Dermatology and Venereology, Linköping University Hospital, County of Östergötland, SE-581 85 Linköping, Sweden
5 R&D Department of Local Health Care, County of Östergötland, SE-581 85 Linköping, Sweden
6 Mycoplasma Laboratory, Statens Serum Institute, DK-2300 Copenhagen S, Denmark
Correspondence
Andreas Edberg
andreas.edberg{at}liv.se
Received 5 July 2007
Accepted 22 November 2007
The aim of this study was to compare conventional 16S rRNA gene PCR, real-time 16S rRNA gene PCR and real-time Mycoplasma genitalium adhesin protein (MgPa) gene PCR as detection methods for M. genitalium infection. The study also determined the prevalence of M. genitalium in male and female patients attending a sexually transmitted infections clinic in a rural area in the west of Sweden. First void urine (FVU) and/or urethral swabs were collected from 381 men, and FVU and/or cervical swabs and/or urethral swabs were collected from 298 women. A total of 213 specimens were used in the PCR comparative study: 98 consecutively sampled specimens from patients enrolled in the prevalence study, 36 consecutively sampled specimens from patients with symptoms of urethritis and 79 specimens from patients positive for M. genitalium by real-time MgPa gene PCR in the prevalence study. A true-positive M. genitalium DNA specimen was defined as either a specimen positive in any two PCR assays or a specimen whose PCR product was verified by DNA sequencing. The prevalence of M. genitalium infection in men and women was 27/381 (7.1 %) and 23/298 (7.7 %), respectively. In the PCR comparative study, M. genitalium DNA was detected in 61/76 (80.3 %) of true-positive specimens by conventional 16S rRNA gene PCR, in 52/76 (68.4 %) by real-time 16S rRNA gene PCR and in 74/76 (97.4 %) by real-time MgPa gene PCR. Real-time MgPa gene PCR thus had higher sensitivity compared with conventional 16S rRNA gene PCR and had considerably increased sensitivity compared with real-time 16S rRNA gene PCR for detection of M. genitalium DNA. Real-time MgPa gene PCR is well suited for the clinical diagnosis of M. genitalium.
Abbreviations: FVU, first void urine; MgPa, M. genitalium adhesin protein; NCNGU, non-chlamydia, non-gonococcal urethritis; NGU, non-gonococcal urethritis; STI, sexually transmitted infections.
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INTRODUCTION
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Mycoplasma genitalium was first isolated in 1980 (Tully et al., 1981). There is strong evidence of M. genitalium having a causative role in non-chlamydia, non-gonococcal urethritis (NCNGU) in men and cervicitis in women (Deguchi & Maeda, 2002; Jensen, 2004; Taylor-Robinson, 2002; Uusküla & Kohl, 2002). M. genitalium infection has also been associated with pelvic inflammatory disease but the exact role has not yet been determined (Cohen et al., 2002). In genital tract infection, the clinical implications of M. genitalium infection may not differ significantly from those of the well-established pathogens Chlamydia trachomatis and Neisseria gonorrhoeae (Falk et al., 2004, 2005). Repeated attempts have been made to recover this extremely fastidious organism from clinical samples by culture techniques but isolates have been rare and difficult to obtain. Serology for the diagnosis of M. genitalium infection has not been widely used because of cross-reactivity with other mycoplasmas (Lind et al., 1984; Taylor-Robinson et al., 1983). With the development of PCR methods in the early 1990s, detection of M. genitalium infection became more feasible. In 1991, conventional PCR for M. genitalium targeting the M. genitalium adhesin protein (MgPa) gene was introduced (Jensen et al., 1991). This method was subsequently used in M. genitalium studies (Gambini et al., 2000; Martinelli et al., 1999). Jensen et al. (2004b) presented a further development of the MgPa gene PCR for application in real-time PCR. Other methods are based on the 16S rRNA gene of M. genitalium and have also been used with both conventional and real-time PCR to detect M. genitalium infection (Björnelius et al., 2000; Jensen et al., 2003; Jurstrand et al., 2005; Yoshida et al., 2002). In a review by Jensen (2004), there was an urge for more studies of M. genitalium infection in women as fewer studies have been published on women compared with men. There is also a lack of studies addressing the prevalence in both men and women from the same catchment area. The aim of this study was to compare conventional 16S rRNA gene PCR, real-time 16S rRNA gene PCR and real-time MgPa gene PCR as detection methods for M. genitalium and to determine the prevalence of M. genitalium in patients, both male and female, attending a sexually transmitted infections (STI) clinic in a rural area in Sweden.
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METHODS
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Patients and clinical specimens.
From April to October 2003, specimens were obtained from 381 men (18–82 years of age, median 27 years) and 298 women (17–55 years of age, median 25 years) attending the STI clinic at the Central Hospital Karlstad, Sweden. All new attendees who were at risk of being infected with an STI due to unprotected sex with a new partner or having a sexual partner who was PCR-positive for M. genitalium were enrolled in the study after providing informed consent. Fewer than ten patients declined to be enrolled. Following a standard protocol, the patients were asked for historical symptoms and on-going symptoms of urethritis/cervicitis (dysuria, urethral and cervical discharge, intermenstrual or post-coital bleeding and lower abdominal pain). A specimen for a smear was obtained with either a plastic loop or a small swab from the distal urethra of men. The smears were stained with methylene blue. Patients with 
5 polymorphonuclear leukocytes per high-power (x1000) microscopic field were considered to have urethritis. Following clinical examination, all men were asked to collect first void urine (FVU) for detection of M. genitalium. In patients with a urine bladder incubation time of 
1 h, a urethral swab specimen was collected instead using a rayon-tipped wire shaft Copan 160 C and placed in 1.5 ml 2-SP medium containing 0.2 M sucrose in 0.02 M phosphate buffer (pH 7.0), 10 % inactivated fetal calf serum, garamycin (20 µg ml–1), vancomycin (100 µg ml–1) and Fungizone (5 µg ml–1). From women, an endocervical sample was obtained with a plastic loop from the posterial and lateral vaginal fornices for wet smear examination in 0.9 % NaCl. Patients with more polymorphonuclear leukocytes than epithelial cells by microscopy were considered to have cervicitis. After the wet smear was taken, an endocervical swab for detection of M. genitalium was collected using a rayon-tipped wire shaft Copan 160 C and placed in 1.5 ml 2-SP medium. Following clinical examination, all women were asked to collect FVU for detection of M. genitalium. FVU was gathered in a 10 ml screw-capped polypropylene tube (Sarstedt). In patients with a urine bladder incubation time of 1 h, a urethral swab specimen was collected instead of FVU using a rayon-tipped wire shaft Copan 160 C and placed in 1.5 ml 2-SP medium.
Diagnostic tests.
In the PCR comparative study, we used 213 specimens: 98 consecutively sampled specimens from patients enrolled in the prevalence study, 36 consecutively sampled specimens from patients with symptoms of urethritis and 79 selected specimens from patients positive for M. genitalium in the prevalence study by real-time MgPa gene PCR.
DNA extraction
For real-time PCR.
A volume of 1800 µl from urine specimens was centrifuged at 20 000 g for 15 min. Aliquots of swab specimens (100 µl) in 2-SP medium were mixed with 1 ml 0.85 % NaCl prior to centrifugation as for the urine specimens. The pellet was resuspended in 300 µl 5 % (w/v) Chelex 100 slurry (Bio-Rad) in distilled water, vortexed for 60 s and incubated at 99 °C for 10 min. Finally, the specimens were centrifuged at 12 000 g for 5 min and the supernatant was analysed by real-time 16S rRNA gene PCR and real-time MgPa gene PCR.
For conventional PCR.
DNA extraction for the conventional 16S rRNA gene PCR was performed as described previously by Jensen et al. (2004a). Briefly, 100 µl swab specimen in 2-SP medium was mixed with 300 µl 20 % (w/v) Chelex 100 slurry in TE buffer [10 mM Tris/HCl (pH 8.0), 1 mM EDTA], vortexed for 60 s and incubated at 95 °C for 10 min. After centrifugation at 20 000 g for 5 min, the supernatant was analysed by conventional 16S rRNA gene PCR. Urine specimens were concentrated by centrifugation: 1800 µl was centrifuged at 20 000 g for 15 min. Chelex 100 slurry (300 µl) was added to the pellet and the mixture was treated as described above for the swab specimens.
Real-time MgPa gene PCR.
The PCR was carried out in a 25 µl SmartCycler reaction tube containing 1x reaction buffer, 3.5 mM MgCl2, 200 µM dNTP mix, 0.625 U Hot Gold Star Taq polymerase (Eurogentec), 1.0 µM each primer using the previously described (Jensen et al., 2004b) forward primer MgPa-355F and reverse primer MgPa-432R (Cybergene) and 0.1 µM MgPa-380 Taqman MGB probe (Applied BioSystems) (Table 1
). Subsequently, 5 µl template DNA was added to the mixture. Amplification was performed in a SmartCycler (Cepheid) under the following conditions: Hot Gold Star Taq polymerase activation at 95 °C for 10 min, followed by a touch-down protocol of 1 cycle of denaturation at 95 °C for 15 s, annealing at 64 °C for 30 s and extension at 72 °C for 30 s; 1 cycle of denaturation at 95 °C for 15 s, annealing at 62 °C for 30 s and extension at 72 °C for 30 s; and 48 cycles of denaturation at 95 °C for 15 s, annealing at 60 °C for 30 s and extension at 72 °C for 30 s.
Real-time 16S rRNA gene PCR.
The PCR was carried out in a 20 µl LightCycler glass capillary. Primers and probes in this PCR assay were designed previously as a further development of the conventional PCR (Jensen et al., 2003). The 18 µl reaction mix contained FastStart DNA Master Hybridization Probe mix, containing FastStart Taq polymerase, reaction buffer, dNTP mix, 10 mM MgCl2 (Roche Diagnostics), 0.6 µM forward primer MG16-45F and 0.4 µM reverse primer MG16-447R (Scandinavian Gene Synthesis). Two hybridization probes, Mg16S-137 and Mg16S-169 (TIB-Molbiol), were used at a final concentration of 0.2 µM each (Table 1
). Additional MgCl2 was added to a final concentration of 5 mM. An aliquot (2 µl) of template DNA was added to the mixture. Amplification was performed on a LightCycler PCR system (Roche Molecular Biochemicals) under the following conditions: FastStart Taq polymerase activation at 95 °C for 10 min, followed by 50 cycles of denaturation at 95 °C for 10 s, annealing at 60 °C for 10 s and extension at 72 °C for 16 s (Jurstrand et al., 2005).
Conventional 16S rRNA gene PCR.
The conventional 16S rRNA gene PCR was performed as described previously by Jensen et al. (2003). In brief, the PCR was carried out with a final reaction volume of 100 µl containing 1x PCR buffer (Super Taq buffer; HT Biotechnology) with 2.5 mM MgCl2; 0.4 mM each primer (Table 1); 62.5 µM each dATP, dGTP and dCTP; 125 µM dUTP; 10 µl of the appropriate dilution of internal process control; and 2 U Taq polymerase (Platinum Taq; Life Technologies). A Perkin Elmer 9600 thermal cycler was used with 0.2 ml MicroAmp tubes. After denaturation at 94 °C for 2 min, 40 cycles were performed. The first ten cycles were used in a touch-down procedure and consisted of denaturation at 94 °C for 15 s, annealing at 72–62 °C for 30 s with a 1 °C decrement per cycle and extension at 72 °C for 15 s. The following 30 cycles consisted of denaturation at 92 °C for 15 s, annealing at 62 °C for 30 s and extension at 72 °C for 30 s. Amplicons were visualized after electrophoresis on 2 % agarose gels containing 1 µg ethidium bromide ml–1 and examined by UV transillumination.
DNA sequencing.
The PCR products were purified by a standard ethanol precipitation method. In brief, 50 µl PCR product was mixed with 5 µl 3 M sodium acetate (pH 4.6) and 100 µl 95 % ethanol in a 1.5 ml microcentrifuge tube. The tubes were left at –20 °C for 30–40 min to precipitate the PCR products. Following the freeze precipitation, the tubes were centrifuged at 20 000 g for 20 min. The supernatant was carefully aspirated and the pellets were rinsed with 300 µl 70 % ethanol. The tubes were then centrifuged at 20 000 g for 5 min. Again, the supernatant was carefully aspirated and discarded. The pellets were left to dry at room temperature and then resuspended in 50 µl sterile water. The purified PCR products were then DNA sequenced using an ABI BigDye Terminator Cycle Sequencing Ready Reaction kit (Applied Biosystems) using the same primers used for the MgPa gene real-time PCR assay. Sequencing reactions were purified using a DyeEx 2.0 Spin kit (Qiagen) before separation using an ABI PRISM 310 Genetic Analyzer (Applied Biosystems). The nucleotide sequences were analysed using CromaPro version 1.33 software and compared with sequence databases at the National Center for Biotechnology Information, using the basic local alignment search tool (BLAST) at http://www.ncbi.nlm.nih.gov/BLAST/.
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RESULTS
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Prevalence
Patients.
M. genitalium was detected in 27/381 men (7.1 %) and in 23/298 women (7.7 %) by real-time MgPa gene PCR.
Specimens from men.
M. genitalium was found in 27/361 (7.5 %) single urine specimens tested. Five men provided a urethral swab specimen together with a urine specimen, whereas 15 men submitted only a urethral swab specimen. None of these specimens was positive for M. genitalium. Clinical information from the 27 M. genitalium PCR-positive men showed a 59 % concordance between a positive PCR result and symptoms of urethritis and/or a positive urethral smear indicating urethritis.
Specimens from women.
A cervical swab specimen together with a urine specimen was provided by 255 women and 19 (7.5 %) were positive for M. genitalium in either specimen (ten urine- and endocervical-positive; eight urine-positive and endocervical-negative; one urine-negative and endocervical-positive). A FVU specimen only was submitted from four women, none of which was positive for M. genitalium. A cervical swab specimen only was obtained from 30 women from whom M. genitalium was found in two (6.7 %). Nine women provided a urethral swab specimen together with a cervical swab specimen. Of these, two were positive in either specimen (one cervical-positive and urethral-negative; one cervical-negative and urethral-positive) for M. genitalium (Table 2). Clinical information from the 23 M. genitalium PCR-positive women showed a 30 % concordance between a positive PCR result and symptoms of cervicitis and/or a positive wet smear indicating cervicitis.
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Table 2. Distribution according to specimen type of 23 M. genitalium PCR-positive specimen sets from 298 women included in the prevalence study
ND, Not determined.
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PCR comparative study
A total of 213 specimens were used in the PCR comparative study. The real-time MgPa gene PCR assay established a 92 and 89.7 % agreement in comparison with analysis results obtained with the conventional 16S rRNA gene PCR and real-time 16S rRNA gene PCR assays, respectively. A 93 % agreement was demonstrated in analysis results in comparison between real-time 16S rRNA gene PCR and conventional 16S rRNA gene PCR assays. Seventy-six specimens were considered to be true-positive specimens defined as either a specimen positive in any two PCR assays or a specimen whose PCR product was verified by DNA sequencing. Forty-nine specimens were positive in all three assays (Fig. 1). Ten specimens were positive only by conventional 16S rRNA gene PCR and real-time MgPa gene PCR. Three specimens were positive only by real-time 16S rRNA gene PCR and real-time MgPa gene PCR. Twelve specimens were positive for M. genitalium only by real-time MgPa gene PCR. Four of these specimens came from women with another true-positive sample. All 12 specimens were retested with the real-time MgPa gene PCR and the PCR products were verified by DNA sequencing (data not shown). Two specimens were positive for M. genitalium only by conventional 16S rRNA gene PCR. These two specimens also came from women with another true-positive sample. Both specimens were able to be retested by real-time MgPa gene PCR and the PCR products were verified by DNA sequencing.
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Fig. 1. Distribution of 213 specimens (76 positive and 137 negative) in a comparison of real-time 16S rRNA gene PCR, conventional 16S rRNA gene PCR and real-time MgPa gene PCR assays for detection of M. genitalium. Positive specimens are indicated within the circles.
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DISCUSSION
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The main purpose of this study was to compare different PCR assays for the detection of M. genitalium in urogenital specimens from men and women. We also wanted to determine the prevalence of M. genitalium in patients, both male and female, attending an STI clinic in the study catchment area. Many clinical studies published to date have used primers targeting the M. genitalium adhesin gene according to Jensen (2006). Moreover, the majority of studies to date on M. genitalium have used conventional PCR as the detection method. Yoshida et al. (2002) published the first real-time PCR assay for detection of M. genitalium. In the current study, 213 specimens were used to compare two different real-time PCR assays targeting the MgPa gene and the 16S rRNA gene of M. genitalium. We also compared results with conventional 16S rRNA gene PCR. The real-time MgPa gene PCR assay, using the MgPa-355F/432R primers published by Jensen et al. (2004b), detected 12 specimens in the PCR comparative study that were not able to be detected by the two other assays tested. These specimens were retested and the PCR products were verified by DNA sequencing as containing M. genitalium DNA. Four of the retested specimens came from women with another true-positive sample, which verified the result. However, the possibility of contamination rendering the remaining eight specimens positive could not be excluded. Two specimens were found to be positive for M. genitalium only by conventional 16S rRNA gene PCR. These two specimens also came from women with another true-positive sample. Both specimens were able to be retested with real-time MgPa gene PCR and the PCR products were verified by DNA sequencing. No retested specimens could be subjected to a new DNA extraction due to insufficient specimen material left after three previous DNA extractions. Previous studies have also demonstrated lower sensitivity for real-time 16S rRNA gene PCR compared with conventional 16S rRNA gene PCR. Jurstrand et al. (2005) showed 72.2 % sensitivity and 99.7 % specificity for real-time PCR compared with conventional PCR for detection of M. genitalium in urogenital specimens from men. Recently, Svenstrup et al. (2005) evaluated real-time MgPa gene PCR, real-time gap gene PCR and conventional 16S rRNA gene PCR as detection methods for M. genitalium in 246 urethral swab specimens from men. Real-time MgPa gene PCR proved to be the most sensitive method, detecting M. genitalium DNA in three specimens that were negative by real-time gap gene PCR (range 0.03–2.65 copies µl–1). A low load of M. genitalium DNA in clinical specimens underlines the need for highly sensitive assays. Jensen et al. (2004a) demonstrated that 28 % of urethral swab specimens and 14 % of FVU specimens contained less than ten genome equivalents of M. genitalium DNA. In our PCR comparative study, the real-time 16S rRNA gene PCR was performed on a LightCycler using only 2 µl template DNA, whilst the real-time MgPa gene PCR was performed on a SmartCycler using 5 µl template DNA. This could explain the lower sensitivity for the real-time 16S rRNA gene assay. However, the conventional 16S rRNA gene PCR had the advantage of being performed with a total volume of 100 µl, allowing 10 µl template DNA to be analysed. The conventional assay increased the sensitivity of the 16S rRNA gene assay somewhat, but clearly using the adhesin gene as the target provided a more sensitive PCR method for detection of M. genitalium. All clinical specimens in this study were subjected to a crude Chelex extraction of DNA as the sample preparation method. In a recent study at the Department of Clinical Microbiology, Central Hospital, Karlstad, Sweden, we demonstrated a 6 % partial inhibition compared with <1 % partial inhibition in specimens prepared by Chelex extraction compared with an automated extraction using a MagAttract DNA Mini M48 kit (Qiagen) on a GenoM48 Biorobot (F. Aronsson, personal communication). In our PCR comparative study, only the conventional 16S rRNA gene PCR assay used an internal control for inhibition in the set-up. Inhibitors and the probability of low DNA load in specimens emphasize the need for improved protocols for specimen preparation to increase the sensitivity in assays for clinical purpose. There is an urge for more studies of M. genitalium infection in women as claimed by Jensen (2004), as more studies have been published on men. There are only a few studies addressing the prevalence in both men and women from the same catchment area. A study of M. genitalium in non-gonococcal urethritis (NGU) in Swedish male STI patients showed a high occurrence of 26 and 36 % in patients with NGU and with NCNGU, respectively, compared with 10 % in control patients without urethritis (Björnelius et al., 2000). A prevalence study on M. genitalium in relation to the number of life-time sexual partners in patients visiting STI clinics in western Sweden showed considerably lower numbers of infected patients. Seven per cent of the examined men, 14 % of men with urethritis, 3.5 % of the examined women and 1 % of the control patients without urethritis were infected with M. genitalium (Johannisson et al., 2000). Other Swedish studies have shown a prevalence in concordance with our results (Anagrius et al., 2005; Falk et al., 2004, 2005). Mellenius et al. (2005) presented slightly lower numbers of prevalence compared with our study: 4.1 % in men and 3.8 % in women. The clinical information from men and women in our study showed a 59 % concordance for men and a 30 % concordance for women between a positive PCR result for M. genitalium and symptoms of urethritis/cervicitis and/or a positive urethral/wet smear indicating urethritis/cervicitis. This is a lower proportion than previous reports for men but is congruent with other reports for women (Falk et al., 2004, 2005). The lower percentage for men could be attributed to the fact that a plastic loop or a swab was used instead of a blunt curette in sampling. Several recent studies on M. genitalium have utilized real-time PCR as the detection method. The real-time MgPa gene PCR can be used in a quantitative setting, determining the M. genitalium load in clinical specimens, which may prove to be useful in clinical studies, giving information about the number of organisms. To our knowledge, this is the first comparison between two real-time PCR methods for detection of M. genitalium using specimens from both men and women. In conclusion, real-time MgPa gene PCR demonstrated higher sensitivity compared with conventional 16S rRNA gene PCR and considerably increased sensitivity compared with real-time 16S rRNA gene PCR for detection of M. genitalium DNA. It has a number of advantages over conventional PCR as it is a closed format, decreasing the contamination risk, it is less labour-intensive and the use of probes increases specificity. With the implementation of an internal processing control, this method will be well suited for clinical diagnostics of M. genitalium in urogenital specimens from men and women.
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ACKNOWLEDGEMENTS
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We are grateful for the excellent work of the staff at the outpatient STI clinic, Karlstad Central Hospital, during the study. We are also grateful for excellent technical assistance with the DNA sequencing by Paula Mölling, Department of Clinical Microbiology, Örebro University Hospital. This work was partly supported by grants from the Örebro University Hospital Foundation and Värmland County Hospital Foundation.
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TOP
INTRODUCTION
METHODS
