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Journal of Clinical Microbiology, November 2006, p. 3855-3862, Vol. 44, No. 11
0095-1137/06/$08.00+0 doi:10.1128/JCM.01214-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Department of Microbiology and Immunology, Cancer Research Institute and Liver Research Institute, College of Medicine, Seoul National University, Seoul 110-799,1 Mogam Biotechnology Research Institute Diagnostic Lab, Yongin 449-903,2 The Korean Institute of Tuberculosis, The Korean National Tuberculosis Association, Seoul 137-140, South Korea3
Received 13 June 2006/ Returned for modification 6 August 2006/ Accepted 15 August 2006
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Instead of a culture-based identification scheme, which takes 4 to 6 weeks or longer for the identification of slowly growing mycobacteria, PCR and PCR-linked methods have been widely used to diagnose mycobacteria (6, 7, 14, 18). The insertion element, IS6110, which is uniquely found in multiple copies in M. tuberculosis complex (19), is most widely used for its detection and identification. Because of the increasing incidence of NTM infection, it is possible that methods which detect only M. tuberculosis fail to detect NTM. Thus, any method that can detect and identify M. tuberculosis and NTM strains simultaneously would be useful. For this purpose, multiplex PCR employing two or three different gene targets is frequently used. They could specifically detect and identify different species in the genus Mycobacterium (4, 13, 15, 22) or distinguish members of the M. tuberculosis complex (4, 8) in the routine diagnostic laboratory. Usually the 16S rRNA gene (22), groEL (8), and alpha-antigen gene (4) have been used as genus Mycobacterium-specific genes. The IS6110 insertion sequence (8), mpb70 gene (21), and mtp40 gene (4) have been used as M. tuberculosis complex-specific genes. However, some of these are not specific for M. tuberculosis. IS6110 was reported to cause false-negative (24) and false-positive results (10), and the mpt40 gene is not present in all M. tuberculosis strains (21). Duplex PCR targeting of a single gene, the RNA polymerase gene (11), has been developed for the differential identification of M. tuberculosis complex and NTM groups. However, this method has been reported to have problems associated with the short sequence length of the target gene. Therefore, a novel PCR method for the differential identification of M. tuberculosis complex and NTM groups and for the further species differentiation of NTM isolates is required.
Previously, we reported that sequence analysis of 604-bp hsp65 DNA is useful for differentiating mycobacterial species, and we introduced several signature nucleotides specific for M. tuberculosis and NTM strains (12). In the present study, we developed a novel duplex PCR method using these signature nucleotides. The devised method can differentiate M. tuberculosis and NTM strains by amplifying hsp65 DNAs of different sizes, i.e., of 195 bp and 515 bp, respectively, in a single PCR. Moreover, duplex PCR-restriction analysis and direct sequencing protocols for the further differentiation of NTM strains were also developed based on hsp65 DNA sequences. To demonstrate the usefulness of these protocols for mycobacterial identification, we applied them to 54 reference strains and 170 clinical mycobacteria isolates.
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TABLE 1. List of reference strains used in this study and their duplex PCR amplification products
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TABLE 2. Methods used for the identification of clinical isolates
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DNA extraction. Chromosomal DNA was extracted by the bead beater-phenol extraction method (12). To disrupt Mycobacterium cell walls, a bacterial mixture containing phenol and glass beads was oscillated on a mini-bead beater. The aqueous phase was then transferred to a clean tube, and the DNA pellet was precipitated by adding isopropyl alcohol and then solubilized with 60 µl TE buffer (10 mM Tris-HCl, 1 mM EDTA, pH 8.0). Two microliters of purified DNA was used as a PCR template.
Duplex PCR. The devised duplex PCR used two hsp65 DNA fragments, 195 bp and 515 bp. These were specific for M. tuberculosis and NTM strains, respectively. Primers enabling the production of specific amplicons for each group were carefully designed using the signature nucleotides previously reported (12), especially the three consecutive signature nucleotides at codon 240 (Fig. 1). The primers were designed for specific nucleotides of M. tuberculosis members or NTM strains (each group) to be located at the 3' hydroxyl end. In a 20-µl PCR mixture tube (AccuPower PCR PreMix; Bioneer, Dae Jeon, Korea) containing 2 U of Taq polymerase, 10 mM Tris-HCl (pH 8.3), 1.5 mM MgCl2, and primers (10 pmol of 2TBF-2TBR and 20 pmol of MOTT3F-MOTT3R) were added, and the volume was adjusted to 20 µl. PCR was performed using initial denaturation at 95°C for 5 min, 30 amplification cycles (30 s at 95°C, 60 s at 72°C), and a final elongation at 72°C for 5 min (model 9600 thermocycler; Perkin-Elmer Cetus). Denaturation was extended to 15 min for the clinical isolates. The PCR products obtained were analyzed by agarose gel electrophoresis (2.5%). DNA bands were visualized by ethidium bromide staining and photographed.
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FIG. 1. Primer location and sequences for duplex PCR system. Primers enabling the production of specific amplicons for each group were carefully designed using the signature nucleotides, especially the three consecutive signature nucleotides at codon 240 (nucleotide positions from 718 to 720) (NTM-specific reverse primer MOTT3R and M. tuberculosis-specific forward primer 2TBF). The 2TBF-2TBR primer set producing M. tuberculosis-specific 195-bp amplicons and MOTT3F-MOTT3R primer set producing NTM-specific 515-bp amplicons were carefully designed for specific signature nucleotides to be located at the 3' hydroxyl group of each primer. The 422 bp among the 515-bp NTM amplicons were used as targets of direct sequencing analysis for the NTM differentiation. Bold letters in boxes indicate signature nucleotides specific to M. tuberculosis (TB) or NTM strains.
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Direct sequencing. Purified PCR amplicons (515-bp) from 105 NTM clinical isolates were directly sequenced. During direct sequencing analysis, we used 515-bp PCR products as templates and MOTT3R, a reverse PCR primer, as a direct sequencing primer. Although approximately 470-bp sequences were obtained by direct sequencing, for reasons of simplicity, only 422-bp sequences (nucleotides 233 to 654 in the hsp65 gene of M. tuberculosis) were used as a target for direct sequencing analysis, as reported previously (12), for species differentiation (Fig. 1). An Applied Biosystems model 373A automatic sequencer and a BigDye terminator cycle sequencing kit (Perkin-Elmer Applied Biosystems) were used for the sequencing. For sequencing reactions, 60 ng of PCR-amplified DNA, 5 pmol of reverse primer (MOTT3R), and 4 µl of BigDye terminator v2.0 100 RR mix (Perkin-Elmer Applied Biosystems) were mixed. Contents were adjusted to a final volume of 10 µl by adding distilled water, and the reaction was run for 30 cycles of 10 s at 96°C, 5 s at 60°C, and 4 min at 60°C. Determined sequences (422 bp) were aligned with the sequences of 56 mycobacterial reference strains (GenBank accession no. AY299133 to AF299182 and AY373453 to AY373458) previously reported (12) by using the multiple-alignment algorithm in the MegAlign package (Windows version 3.12e; DNASTAR, Madison, Wis.) and were used for the species differentiation of NTM strains. A phylogenetic tree was constructed using the MEGA version 2.1 (13a), and a bootstrap analysis (1,000 repeats) was performed to evaluate the topology of the phylogenetic tree using Tsukamurella paurometabola KCTC 9821T as an out-group (AY373483).
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To evaluate affinities for target DNA and the sensitivities of each primer set, PCRs using each primer set were applied to the serially diluted DNAs of Mycobacterium avium (ATCC 25291) and M. tuberculosis (ATCC 27294). In PCRs using 2TBF-2TBR, the 195-bp amplicons were observed from 1 pg of M. tuberculosis DNA but not from even 100 ng of M. avium DNA. In PCRs using MOTT3F-MOTT3R, 515-bp amplicons were observed from 100 pg of M. avium DNA but not from 100 ng of M. tuberculosis DNA (data not shown).
Sensitivity of DPCR. The sensitivity of the developed DPCR method was evaluated in terms of the differential detection of M. avium (ATCC 25291) and M. tuberculosis (ATCC 27294) DNA in the serially diluted DNA samples. Amplification products were observed from 100 fg of M. tuberculosis DNA and 10 pg of M. avium DNA (Fig. 2).
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FIG. 2. Sensitivity of the duplex PCR determined by using the serially diluted DNAs of M. tuberculosis and M. avium. PCR products are M. tuberculosis and M. avium. Lanes: M, marker DNA (50-bp ladder); 1, 10 ng; 2, 1 ng; 3, 100 pg; 4, 10 pg; 5, 1 pg; 6, 100 fg; 7, negative control.
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FIG. 3. Differential identification of the M. tuberculosis complex and of NTM by duplex PCR. Duplex PCR produced a 195-bp DNA from 5 strains of M. tuberculosis complex (lanes 2 to 6) and 515-bp DNA from the NTM strains. Lanes: M, 100-bp ladder; 1, M. avium (ATCC 25291); 2, M. tuberculosis (ATCC 27294); 3, M. bovis; 4, M. bovis BCG; 5, M. africanum; 6, M. microti; 7, M. kansasii type I; 8, M. szulgai; 9, M. fortuitum (ATCC 6841); 10, M. chelonae.
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TABLE 3. Differentiation of M. tuberculosis complex and NTM clinical isolates by duplex PCR and conventional biochemical testing
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FIG. 4. Algorithm for species differentiation by duplex PRA with AvaII and HaeIII. This algorithm was based on the hsp65 sequences of 54 mycobacterial reference strains previously reported. After completing duplex PCR which can differentiate NTM strains and M. tuberculosis complex strains only in a single PCR, subsequent application of 2 restriction enzymes, AvaII and HaeIII, to 515-bp duplex PCR amplicons of NTM strains can differentiate most NTM strains to the species level.
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FIG. 5. Identification of NTM reference strains (A) and MAC clinical isolates (B) by AvaII digestion of 515-bp duplex PCR amplicons. (A) Lanes: M, 50-bp ladder; 1, M. avium (ATCC 25291); 2, M. avium KIT41182; 3, M. intracellulare; 4, M. scrofulaceum; 5, M. kansasii; 6, M. szulgai; 7, M. fortuitum ATCC 6841; 8, M. peregrinum; 9, M. abscessus; 10, M. chelonae. (B) Lanes: M, 50-bp ladder; 1, M. avium (ATCC 25291); 2, M. avium KIT41182; 3, M. intracellulare; 4, MAC clinical isolate 1; 5, MAC clinical isolate 2; 6, MAC clinical isolate 3; 7, MAC clinical isolate 4; 8, MAC clinical isolate 5; 9, MAC clinical isolate 6; 10, MAC clinical isolate 7; 11, MAC clinical isolate 8; 12, MAC clinical isolate 9; 13, MAC clinical isolate 10.
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FIG. 6. Identification of NTM reference strains by HaeIII digestion of 515-bp duplex PCR amplicons. Lanes: M, 50-bp ladder; 1, M. avium (ATCC 25291); 2, M. intracellulare; 3, M. scrofulaceum; 4, M. kansasii type I; 5, M. kansasii type II; 6, M. szulgai; 7, M. gordonae; 8, M. marinum; 9, M. fortuitum ATCC 6841; 10, M. fortuitum ATCC 49403; 11, M. peregrinum; 12, M. abscessus; 13, M. chelonae.
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TABLE 4. Identification of NTM clinical isolates by duplex PRA and direct sequencing analysis
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FIG. 7. Identification of NTM clinical isolates by referring to the hsp65 phylogenetic tree (GenBank accession no. AY299133 to AF299182 and AY373453 to AY373458) (12). Differentiation of MAC to the species level, M. avium (NTM-7) and M. intracellulare (NTM-37), was possible. Differentiation of M. kansasii into subspecies I (NTM-27) and II (NTM-94) and differentiation between closely related rapid growing mycobacteria (between M. chelonae [NTM-24] and M. abscessus [NTM-7] or between M. fortuitum [NTM-53] and M. peregrinum [NTM-65]) was also possible. The tree was constructed by the neighbor-joining method. The percentages at the nodes represent the levels of the bootstrap supported by the 1,000 resampled data sets. Bootstrap values of less than 50% are not shown. The bar indicates the sequence difference of 0.01%. The solid circle () indicates the phylogenetic location of clinical isolates of NTM. SGM, slowly growing mycobacteria; RGM, rapidly growing mycobacteria.
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Since in hsp65 duplex PCR, only a single gene, hsp65, is targeted irrespective of the M. tuberculosis complex or NTM species, the following were taken into account for successful amplification: (i) that the sizes of the amplicons of the two groups should be different enough to be discriminated in an agarose gel (195 versus 515 bp), (ii) that the optimal PCR annealing temperature of the respective primer sets should be nearly equal (60°C), and (iii) that the respective primer sets should be specific for target templates. In a previous study, we observed three consecutive signature nucleotides at codon 240 of HSP65 which are specific for M. tuberculosis complex strains (GGA) and NTM strains (CAG), suggesting that they could be effectively used for the development of M. tuberculosis or NTM detection methods. To construct TB-specific or NTM-specific primers, these consecutive signature nucleotides were used for one direction primer of each two-direction-primer pair (2TBF or MOTT3R) (Fig. 1). When PCR using the respective primer set was applied to the serially diluted DNA of M. tuberculosis or M. avium, it did not produce amplicons even at a nontarget DNA concentration of 1 µg (data not shown). As shown in a previous report (12), the present study also confirms that signature nucleotides of the hsp65 gene, specific for the M. tuberculosis complex and for the NTM strains, used for the development of the primer, are invariably constant. Thus, no false-positive or -negative results were obtained during the analysis of the 170 clinical isolates.
Since duplex PCR yields only one product irrespective of species for NTM, hsp65 DNAs of NTM (515-bp) can be further analyzed by PRA or by direct sequencing for species identification. Therefore, when duplex PRA and direct sequencing protocols were applied to the 105 NTM clinical isolates, all isolates were clearly differentiated at the species level. Furthermore, in M. kansasii, separation at the subspecies level and separation between M. abscessus and M. chelonae, which is not possible using rpoB duplex PCR-based protocols, were possible.
The other advantage of our hsp65 duplex PCR system is that a procedure required to separate MAC strains is straightforward, and these are the most frequently encountered and the most important NTM strains in the clinical setting. AvaII PRA alone can separate MACs from other NTM strains and for species differentiation between MACs. Therefore, in the general clinical setting, only DPCR-linked AvaII PRA, which can completely identify the three most clinically important strains, i.e., M. tuberculosis, M. avium, and M. intracellulare, is sufficient without further HaeIII PRA or a duplex PCR-based direct sequencing procedure.
In conclusion, duplex PCR targeting the hsp65 gene provides a rapid and reliable means of obtaining differential identification of M. tuberculosis complex and NTM in culture using only a single PCR. In addition to its advantages of simplicity and sensitivity, the method provides a clue for the differentiation of these two most important mycobacterial groups, which have different modes of infection and require different treatments. Furthermore, PRA and the sequencing of the duplex PCR products will add to the usefulness of the technique by allowing NTM identifications to be performed in clinical diagnostic laboratories.
Published ahead of print on 23 August 2006. ![]()
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