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Journal of Clinical Microbiology, November 2005, p. 5435-5439, Vol. 43, No. 11
0095-1137/05/$08.00+0 doi:10.1128/JCM.43.11.5435-5439.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Toshinori Hayashi,2
Motohisa Tomita,3
Katsuhiro Suzuki,3
Tatsuro Hasegawa,4
Takanori Tagami,4
Atsuyuki Kurashima,4 and
Satoshi Ichiyama1
Department of Clinical Laboratory Medicine, Kyoto University Graduate School of Medicine, 54 Shogoin-Kawaharacho, Sakyo-ku, Kyoto 6068507, Japan,1 Scientific Instruments Division, Tosoh Corporation, 2743-1 Hayakawa, Ayase, Kanagawa 2521123, Japan,2 Clinical Research Center, Kinki-chuo Chest Medical Center, 1180 Nagasone-cho, Sakai, Osaka 5918555, Japan,3 National Tokyo Hospital, 3-1-1 Takeoka, Kiyoshe-shi, Tokyo 2048585, Japan4
Received 6 June 2005/ Returned for modification 28 June 2005/ Accepted 10 August 2005
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Recently, we have reported on a novel method designated the transcription-reverse transcription concerted (TRC) method (9). This method, a schematic of which is shown in Fig. 1, is based on isothermal RNA amplification at 43°C with transcriptase and reverse transcriptase in the presence of the intercalation activating fluorescence (INAF) probe (10). Measurement of the fluorescence intensity of the reaction mixture with a dedicated multicolor detector enables completely homogeneous real-time monitoring of the amplification of specific RNA, while it requires only 30 min for simultaneous amplification and detection. We have used the TRC method to establish a system for the detection of specific mRNA transcripts: tdh and trh of Vibrio parahaemolyticus (13), mecA of methicillin-resistant Staphylococcus aureus (11), and pab of M. tuberculosis (9, 19).
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FIG. 1. Schematic description of the elementary steps of the TRC method. The progress of the reaction is monitored by measuring the fluorescence intensity of the reaction mixture. dsRNA, double-stranded RNA.
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The amplified gene sequence was cloned into the EcoRI site of pUC19. The plasmid DNAs were then digested with EcoRI to yield linear DNAs containing the SP6 promoter, followed by in vitro transcription in a reaction mixture composed of 40 mM Tris-HCl (pH 7.5), 6 mM MgCl2, 2 mM spermidine, 10 mM dithiothreitol, 0.5 mM nucleoside triphosphates, 0.1 mg/ml of bovine serum albumin, 1 U/ml of RNase inhibitor, 2.5 U/ml of SP6 RNA polymerase (TaKaRa Bio, Otsu, Shiga, Japan), and 0.025 mg/ml of template DNA. The resultant RNAs were purified by gel filtration with Chromaspin-100 columns (BD Biosciences, Palo Alto, CA). The concentration of the purified RNA was determined spectrophotometrically at an optical density at 260 nm and adjusted to 102 to 107 copies/5 ml with TE (Tris-EDTA) buffer containing 0.25 U/ml of RNase inhibitor and 5 mM dithiothreitol. The RNAs were then stored at 20°C until use.
Primers, probe, and internal control for the detection of M. tuberculosis complex by TRC method. Synthetic oligonucleotides used for the TRC reaction included a pair of amplification primers (designated the promoter primer [5'-AAT TCT AAT ACG ACT CAC TAT AGG GAG ACG GAA AGG TCT CTT CGG AGA TAC-3'] and the antisense primer [5'-ACA AGA CAT GCA TCC CGT-3']), a scissors probe (5'-TTT CCG TTC GAC TTG CAT GTG TTA-3') to initiate the TRC reaction, and an INAF probe (5'-CGA AGT GCA GGG C*AG ATC, where the asterisk indicates the base position linked by oxazole yellow) to detect the RNA amplicons. For the specific detection of MTC, primers were designed to amplify base positions 313 to 443 of M. tuberculosis 16S rRNA (GenBank accession no. Z83862). As shown in Fig. 2, oxazole yellow-linked and ethidium bromide-linked INAF probes were synthesized for the homogeneous and simultaneous detection of the target 16S rRNA and the internal amplification control in the same tube by means of two-color fluorescence monitoring of the reaction mixture at 520 nm and 610 nm, respectively. For the experiments to be able to evaluate the species specificity of the assay, the 16S rRNAs of 16 mycobacterial species other than MTC were prepared in the same manner described above for MTC 16S rRNA. These species comprised M. avium, M. intracellulare, M. kansasii, M. simiae, M. scrofulaceum, M. gordonae, M. szulgai, M. gastri, M. xenopi, M. nonchromogenicum, M. terrae, M. triviale, M. fortuitum, M. chelonae, M. abscessus, and M. peregrinum.
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FIG. 2. Primers and probes used in the TRC method designed for amplification and detection of MTC 16S rRNA (A) and internal control (B). Ex, excitation wavelength; Em, emission wavelength.
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30 min were considered to have a positive signal, and those with positive signals at 520 nm were judged to be positive. Samples without a positive signal at 520 nm and with a positive signal at 610 nm were considered negative, and those with negative signals at both 520 nm and 610 nm were considered indeterminate. Sample processing for detection of MTC in experimental sputum samples by TRC method. Mycobacterium culture-negative, clinical sputum specimens were obtained from patients diagnosed as not having mycobacterial infection at Kyoto University Hospital, Kyoto, Japan. The samples were mixed and homogenized by vigorous vortexing and were decontaminated by treatment with N-acetyl-L-cysteine (NALC)-NaOH. Various amounts of M. bovis BCG (100 to 104 cells per 100 µl), M. avium (102 to 106 cells per 100 µl), and M. kansasii (102 to 106 cells per 100 µl) were added to these culture-negative sputum specimens. Pretreatment consisted of mixing of 100 µl of the decontaminated samples with 500 µl of 67 mM phosphate buffer (pH 8.6) containing glass beads. After centrifugation at 3,000 x g for 5 min at room temperature, the supernatant was removed, followed by the addition of 300 µl of extraction buffer containing 10 mM Tris-HCl (pH 8.0), 1 mM EDTA, 0.25 U/µl RNase inhibitor, and 1 mM dithiothreitol. After sonication for 5 min, followed by centrifugation at 3,000 x g for 5 min, 5 µl of the supernatant was used as the template.
Sample processing for detection of MTC in clinical sputum samples by culture, TRC method, and COBAS AMPLICOR PCR. Clinical samples were obtained from patients diagnosed with or suspected of having a mycobacterial infection at the National Central Hospital for Chest Disease, Osaka, Japan, and the National Tokyo Hospital, Tokyo, Japan. All samples were decontaminated by treating them by a commercially available NALC-NaOH-based method, CC-E Nichibi (Japan BCG Laboratory, Tokyo, Japan) or BBL Micoprep (Becton Dickinson, Franklin Lakes, NJ). After centrifugation at 3,000 x g for 15 min at room temperature, the sediment was resuspended in 1.0 ml of phosphate buffer (pH 7.0). A smear of the decontaminated suspension was stained with the Ziehl-Neelsen stain, and a 500-µl aliquot of the suspension was cultivated in the BACTEC MGIT 960 liquid culture system (Becton Dickinson) for 6 weeks, in accordance with the manufacturer's recommendations. The remaining decontaminated suspension of the samples was immediately used or was stored at 20°C until use. The TRC method was applied to 100 µl of the suspension; the Roche COBAS AMPLICOR PCR was applied to 200 µl of the suspension, which had been treated with the Amplicor Mycobacterium Specimen Pretreatment Set II (Roche Diagnostics). The isolated mycobacteria were identified by an immunochromatographic assay with the anti-MPB64 antibody (Capillia; Becton Dickinson), a chemiluminescent DNA hybridization probe assay (AccuProbe; Gen-Probe Inc.), or a DNA hybridization assay (DDH Mycobacteria; Kyokuto Pharmaceutical Co., Tokyo, Japan).
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102 M. avium or M. kansasii organisms per 100 µl were judged to be negative by the TRC method (Table 1). The species specificity of the TRC method was examined by testing the 16S rRNA solution with 16 mycobacterial species other than MTC as the templates. The TRC method did not show positivity for any of the 16S rRNAs (106 copies, equivalent to an rRNA content of 103 cells, per 100 µl) of the species examined (data not shown).
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FIG. 3. (A) Fluorescence monitoring of the TRC reaction of the MTC 16S rRNA calibrator. The initial copy numbers of the calibrator are indicated as follows: diamonds, 105; circles, 104; triangles, 103; squares, 102; crosses, 0. (B) Initial copy numbers of the calibrator on a logarithmic scale plotted against the time needed to reach the cutoff value of 1.2. Circles indicate the detection time at 520 nm, and diamonds indicate that at 610 nm. The average of the values obtained with triplicates of each sample is plotted.
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TABLE 1. Detection of Mycobacterium tuberculosis complex-specific 16S rRNA sequence by TRC method in sputum containing BCG (100 to 104 cells/100 µl), Mycobacterium avium, and M. kansasii (102 to 106 cells/100 µl)
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TABLE 2. Detection of Mycobacterium tuberculosis complex in 201 clinical sputum samples from 173 patients with diagnosed or suspected mycobacterial infection by TRC method, the COBAS AMPLICOR PCR, MGITa culture for MTC, and smear test
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TABLE 3. Agreement between of results of TRC method and COBAS AMPLICOR PCR for detection of Mycobacterium tuberculosis complex in 201 respiratory samples from 173 patients
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In the case of MTC smear-negative and culture-positive samples, the TRC method showed substantially lower rates of positivity than it did for smear- and culture-positive samples, as did the COBAS AMPLICOR PCR. This less favorable result was, however, comparable to the results of previous studies that evaluated nucleic acid amplification assays in similar settings (18). As for the specificity, the TRC method did not show positivity for 106 M. avium or 106 M. kansasii organisms per 100 µl of sputum, nor did it show positivity for 106 initial copies of 16S rRNA of 16 non-MTC mycobacterial species. The 129 MTC culture-negative samples comprised 40 samples which were culture positive for nontuberculous mycobacterial isolates as well as 89 mycobacterial culture-negative samples. These results clearly demonstrate the excellent specificity of the TRC method for the detection of MTC.
As to sensitivity, the difference between the two methods was not significant, although the comparatively small sample size provided this study with insufficient power to detect small differences in sensitivity. The slightly smaller number of MTC culture-positive samples detected by the TRC method than by the COBAS AMPLICOR PCR may have been due to a reduction in the numbers of viable cells during treatment, since this study included samples from patients undergoing antituberculosis chemotherapy. The levels of mycobacterial rRNA may therefore have decreased faster than the levels of DNA in proportion to the decrease in viable cells. This finding may, rather, indicate some advantage of the TRC method over DNA-based amplification methods in terms of greater correctness in the diagnosis of active disease. Actually, Moore et al. has reported that elimination of MTC rRNA from sputum samples may indicate successful antituberculosis therapy (12).
We found unaccountable false-negative results, four by the TRC method and two by the COBAS AMPLICOR PCR, among the 43 smear- and culture-positive samples. The reason could not be that no viable cells remained in the samples, since MTC was indeed identified by smear as well as culture of these samples. The promptly appearing positive signal of the internal control with the use of either method excluded the possibility that inhibitors were present in the samples. The slightly greater number of false-negative results obtained by the TRC method could be explained by a reduction in the number of viable cells during treatment, although the exact reason is difficult to identify. The fact that these false-negative results were found with the use of both methods together, however, makes it less likely that there are any critical problems specific to the TRC method in terms of false-negative results.
The samples examined in this study showed a prevalence of MTC-positive samples considerably higher than the prevalence in most clinical laboratories in developed nations. This is not surprising, since Japan is characterized by a substantially higher prevalence of tuberculosis compared with that in other developed countries: 24.8 new cases per million population in 2003 (11). In addition, public health policies in Japan have been promoting the concentration of tuberculosis patients in core referral centers specializing in tuberculosis, which include the Kinki-Chuo Chest Medical Center and National Tokyo Hospital.
Of special importance are the several advantages that the TRC method has over the other nucleic acid amplification tests. First, an internal control is amplified to avoid false-negative results; second, the entire procedure is conducted in a completely homogeneous and isothermal format, thus eliminating contamination by postamplification analysis; and third, the results from the time that the samples are decontaminated are available within an hour because amplification and detection require only 30 min. The TRC method also proved to be clinically useful for the rapid identification of MTC in respiratory samples, at least for those smear-positive ones, as is the case for COBAS AMPLICOR PCR.
Pulmonary tuberculosis and nontuberculous mycobacterial infection are sometimes difficult to differentiate based on clinical findings. The nucleic acid amplification test, which could detect MTC only, may add little information in such cases if the result was negative. Development of the TRC method for the detection of M. avium complex and M. kansasii, which is now in progress in the laboratory at Tosoh Corporation, could further enhance the clinical usefulness of this method for the management of patients with suspected mycobacterial infection especially when the smear result is positive.
In summary, the sensitivity of the TRC method was found to be comparable to that of the COBAS AMPLICOR PCR, and its specificity was excellent. Moreover, less than an hour was required to obtain results after the specimens had been treated with NALC-NaOH. This novel method can thus be expected to be suitable for routine use for the rapid diagnosis of tuberculosis.
Present address: Department of Food Biology, Kyoto University Graduate School of Agricultural Science, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto 6068502, Japan. ![]()
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