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Journal of Clinical Microbiology, January 2005, p. 174-178, Vol. 43, No. 1
0095-1137/05/$08.00+0 doi:10.1128/JCM.43.1.174-178.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Department of Microbiology and Cancer Research Institute, Institute of Endemic Diseases, SNUMRC, Seoul National University College of Medicine, and Clinical Research Institute, Seoul National University Hospital,1 Department of Orthopedic Surgery, Soonchunhyang University Hospital,2 Department of Orthopedic Surgery, Seoul National University College of Medicine, Seoul, Korea3
Received 16 March 2004/ Returned for modification 8 July 2004/ Accepted 26 August 2004
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Thus, the estimated global prevalence of latent joint and bone TB is approximately 19 million to 38 million cases (17). Moreover, since osteoarticular tuberculosis (OAT) can cause functional disability, it should be accurately diagnosed and treated early.
As the identification of mycobacterial species from clinical samples usually requires culture (26), the diagnosis of OAT depends upon microbiologic testing (i.e., smear or culture) and the histologic examination of tissue samples. Although culture is the "gold standard," it may take 6 to 8 weeks before a positive culture is detected (22), unless the radiometric BACTEC 460 method or the nonradiometric BACTEC 960/Mycobacteria Growth Indicator Tube method (4) is used.
In recent years, several nucleic acid-based techniques have been developed for the rapid detection of Mycobacterium tuberculosis in clinical samples. By testing sputa and bronchoalveolar lavage specimens, mycobacteria can be detected and identified by PCR or PCR-linked methods (1, 18). However, unlike pulmonary TB samples, such as sputa and bronchoalveolar lavage specimens, from which tubercle bacilli are concentrated for culture and further testing, joint biopsy samples usually contain only a small number of bacteria. This causes difficulties with culture and staining for acid-fast bacilli (8, 17, 19) that necessitate the use of molecular biology-based methods.
In the present study, rpoB PCR-plasmid TA cloning-sequencing for Mycobacterium species was applied directly to clinical specimens from patients suspected of having OAT without culture. rpoB encodes the ß subunit of RNA polymerase (3), and recently, partial rpoB DNA sequences containing the Rifr region, which is related to rifampin resistance, have been used to identify Mycobacterium species (5, 10, 11) and non-Mycobacterium species (12, 13, 14, 15, 16).
M. tuberculosis is readily differentiated by its rpoB sequence. In addition, important information on rifampin susceptibility can be provided by rpoB sequence analysis, which is impossible by IS6110 PCR or 16S rRNA gene PCR. Moreover, because the amplified rpoB DNA of M. tuberculosis has one HindII restriction site, it can easily be differentiated from other mycobacteria (10). We used these characteristics of rpoB DNA to rapidly and accurately detect and identify M. tuberculosis in specimens from patients with OAT, and because PCR amplicons are usually weak due to the minimal number of mycobacteria in biopsy samples, TA cloning-sequencing analysis was used.
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DNA preparation and rpoB PCR. DNA was extracted by using the previously described bead beater-phenol extraction method (11). Two or three fragmented specimens were suspended in 200 µl of distilled water in a screw-cap microcentrifuge tube filled with 200 µl (packed volume) of glass beads (diameter, 0.1 mm; Biospec Products; Bartlesville, Okla.) and 200 µl of phenol-chloroform-isopropyl alcohol (50:49:1). To disrupt the tissues and the bacteria, the tube was oscillated on a Mini-Bead Beater (Biospec Products) for 1 min and then centrifuged (12,000 xg, 5 min).
After the aqueous phase had been transferred to another clean tube, 10 µl of 3 M sodium acetate and 250 µl of ice-cold ethanol were added, and the mixture was kept at 20°C for 10 min. The DNA pellet obtained was then washed with 70% ethanol, dissolved in 60 µl of TE buffer (10 mM Tris-HCl, 1 mM EDTA [pH 8.0]), and used as a template for PCR. rpoB PCR was carried out as described previously (11). Plasmid TA-cloned M. tuberculosis DNA and sterile distilled water were used as positive and negative (nontemplate) controls, respectively. The PCR products obtained were electrophoresed in a 1.5% agarose gel and purified by use of a QIAEX II gel extraction kit (Qiagen, Hilden, Germany). Separately, rpoB nested PCR (9), 16S rRNA gene PCR (7), and IS6110 PCR (23) were performed with those samples with negative rpoB PCR results. The potential effects of PCR inhibitors for the rpoB PCR-negative samples were also tested. Helicobacter pylori DNA (0.001 µg) and cagA-specific PCR primers (29) were added to the reaction mixtures containing each PCR-negative sample. Then, the cagA PCR products were compared with those obtained with a control, which contained only the H. pylori DNA.
TA cloning. The purified PCR product (5 to 10 ng) was cloned by use of a TA cloning kit (Invitrogen, Carlsbad, Calif.), according to the manual provided by the supplier. Three to 10 colonies of transformed Escherichia coli were picked in each reaction, cultured, and used to prepare plasmid DNA by use of a High Pure Plasmid Isolation kit (Roche, Mannheim, Germany).
The EcoRI restriction site in the plasmid and the HindII restriction site in the amplified M. tuberculosis rpoB DNA were used to confirm the presence of the rpoB DNA insert.
Double digestion with EcoRI and HindII (TAKARA, Shiga, Japan) generated a large plasmid fragment and either an uncleaved insert fragment (368 bp) or two cleaved insert fragments (248 and 120 bp). Constructs possessing the M. tuberculosis rpoB DNA insert produced cleaved DNA fragments. Specimens yielding more than one positive clone were regarded as positive for M. tuberculosis infection.
Nucleotide sequencing. The nucleotide sequences of the cloned rpoB DNAs were directly determined from the purified plasmid by using M13 forward and reverse primers, which were supplied in the TA cloning kit, by using a 373A automatic sequencer and a BigDye Terminator Cycle Sequencing kit (PE Applied Biosystems, Warrington, United Kingdom). For the sequencing reaction, 60 ng of PCR-amplified DNA, 3.2 pmol of either the forward or the reverse primer, and 4 µl of BigDye Terminator RR mix (part no. 4303153; PE Applied Biosystems) were mixed and adjusted to a final volume of 20 µl by adding distilled water. The reaction was run with 5% (vol/vol) dimethyl sulfoxide for 30 cycles of 15 s at 95°C, 10 s at 50°C, and 4 min at 60°C. Both strands were sequenced as a cross-check. The sequences determined (306 bp) were aligned and compared to sequences in GenBank by using the multiple-alignment algorithms in the MegAlign package (Windows version 3.12e; DNASTAR, Madison, Wis.). The procedures are summarized in the flow sheet (Fig. 1).
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FIG. 1. Flow sheet of the rpoB PCR-TA cloning-sequencing protocol used to detect and identify M. tuberculosis in joint biopsy samples. If the rpoB PCR result was negative, nested PCR (a) (9), 16S rRNA gene PCR (b) (26), or IS6110 PCR was used for confirmation. c, false positive indicates that nonspecific human DNAs or other unknown sequences were detected; d, the sequences determined were compared to those in GenBank, which confirmed the presence of mutation sites (10). MF-MR, PCR primers.
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FIG. 2. Presumptive identification of M. tuberculosis by double digestion (EcoRI and HindII) of cloned constructs. Lanes M, marker DNA (100-bp ladder). (a) Of the 10 plasmid constructs, only the constructs in lanes 4, 5, 7, 8, and 10 harbored the M. tuberculosis rpoB DNA insert. Lanes 1 to 10, plasmids purified from 10 colonies of transformed E. coli. (b) The plasmid constructs in all lanes harbored the M. tuberculosis rpoB DNA insert.
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TTG), which results in a high level of resistance to rifampin, was found in 3 of the 23 samples. On the other hand, 20 samples showed negative results. No amplicons were found by rpoB nested PCR, 16S rRNA gene PCR, or IS6110 PCR. These 20 DNA extracts were not observed to have an inhibitory effect on the PCR. |
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TABLE 1. Rates of positivity by rpoB PCR-TA cloning and sequencing from clinical specimens
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However, 20 samples consistently produced negative results. Because the sample DNA extracts were not shown to have inhibitory effects on the PCR and other causes of the negative results were not identified by the clinical laboratory testing results, it was possible that the tubercle bacilli disappeared after they triggered an inflammatory change or that intact whole mycobacterial DNA did not exist locally in joint tissue. Thus, it might not be possible to detect M. tuberculosis by PCR in such samples, as was postulated previously (25, 27).
The unique HindII restriction site on the rpoB sequence of M. tuberculosis is useful for construct screening. Not knowing whether a given insert was M. tuberculosis DNA, we randomly selected transformed colonies for sequencing. If the detection of M. tuberculosis in a tissue sample is all that is required, double digestion of the construct is sufficient. However, the rpoB DNA insert must be sequenced to determine the presence of mutations related to rifampin resistance. Basically, any construct showing an uncleaved insert by double digestion should also be sequenced to rule out NTM. The HindII restriction site is not present in NTM rpoB DNA, and it has been reported that NTM are detectable in joint tissues (24). However, it is noteworthy that the undigested DNA inserts of our tested samples, which might be suspected of being NTM, were human DNAs or other unknown sequences by a sequence database search with the BLAST algorithm. These may be contaminants that have been introduced during PCR product purification.
IS6110 and 16S rRNA gene PCRs have been widely used to detect mycobacteria (2, 7). They have also been used to detect mycobacteria in joint biopsy samples (6, 26). The reported rates of positivity were 40 and 5.1% by IS6110 and 16S rRNA gene PCR, respectively (6, 26).
In our study, however, the rate of positivity was higher (53.5%). It is not clear whether this difference is due to the sensitivities of the methods used. The endemicity of M. tuberculosis infection in the area tested and the criteria used for patient selection may have affected the rate of positivity in important ways. In general, the high sensitivity of the IS6110 PCR due to the presence multiple copies of IS6110 in the M. tuberculosis complex (23) and the versatility of 16S rRNA gene PCR, which can detect many bacteria, including mycobacteria, by use of universal primers, might be considered useful. However, considering that the identification of pathogenic bacteria should be complemented by information regarding antimicrobial susceptibility, both PCR methods are inadequate, because they provide no information on susceptibility to antimicrobial agents. However, because rifampin resistance is related to rpoB mutations, rpoB PCR presents a useful method of determining rifampin resistance by analyzing amplicon sequences (9, 21). The most common mutations (65 to 85%) alter codon 526 or 531, which results in high levels of resistance to rifampin.
Reports on multidrug-resistant M. tuberculosis infections are increasing (28), and most of the rifampin-resistant strains are also resistant to isoniazid. Therefore, rifampin resistance as a surrogate marker of multidrug resistance (20) supports the importance and usefulness of the rpoB PCR. However, compared to other routine PCR methods, the rpoB PCR-TA cloning method may be a laborious protocol, as it requires a sequencing unit and expert involvement in a clinical laboratory. Although it can be applied to sputa or other clinical samples, we suggest that the indications for the use of this method are limited to samples from patients with OAT, which are not common and which usually contain small amounts of bacilli.
In conclusion, although several difficulties are associated with the detection of M. tuberculosis in tissue specimens, rpoB PCR-TA cloning-sequencing offers a useful method for the detection and simultaneous identification of M. tuberculosis in specimens from patients with OAT.
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