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Journal of Clinical Microbiology, May 2003, p. 2213-2218, Vol. 41, No. 5
0095-1137/03/$08.00+0 DOI: 10.1128/JCM.41.5.2213-2218.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Department of Biomedical Laboratory Science, College of Health Sciences, Yonsei University, Wonju 220-710,1 Department of Microbiology and Brain Korea 21 Project for Medical Sciences, Yonsei University College of Medicine, Seoul 120-752,2 Department of Microbiology, Korean Institute of Tuberculosis, Seoul 137-140,3 The International Vaccine Institute, Seoul 151-600, Republic of Korea4
Received 5 August 2002/ Returned for modification 23 September 2002/ Accepted 23 February 2003
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Although biochemical tests are available for Mycobacterium species identification, it has proven to be difficult to use these tests because of time-consuming and often incorrect identification. In order to overcome such difficulties, high-performance liquid chromatography has been widely used for species identification based on mycolic acid analysis (1, 14, 25). In addition, with recent developments in molecular techniques and the availability of genome sequencing data, several molecular tests have been developed and are used in clinical mycobacterial laboratories. rRNA sequences, notably that of 16S rRNA, have been most widely used for mycobacterium species identification (18), and commercial kits based on such sequences are available (AccuProbe; Gen-Probe Inc., LiPA, Innogenetics N. V., Zwijnaarde, Szinjdrecht, Belgium). In addition, the hsp65 gene (8, 15, 16, 23), the intergenic region between 16S and 23S rRNA (19), and the rpoB gene (9, 10, 13) are among the targets for molecular technique-based species identification. Sequencing (11, 15, 18, 20-22), DNA hybridization (6, 12), PCR-restriction fragment length polymorphism analysis (RFLP) (8, 9, 13, 16, 23), and microarray technology have also been employed to differentiate Mycobacterium species (5, 24).
Among the target genes, we were particularly interested in the rpoB gene. PCR sequence analysis of a region of the rpoB gene was suggested as a possible means of differentiating 44 species (9, 10, 13). We have also reported a new RFLP method (13) based on a different region of the rpoB gene, which is located between the first variable region (V1) and the second conserved region (C2), as determined using the genetic information of the Escherichia coli rpoB gene. The 360-bp region of the rpoB gene (bases 902 to 1261 and codons 302 to 420 of the rpoB gene of M. tuberculosis; GenBank accession number P47766) was found to be useful in the differentiation of more than 50 Mycobacterium species by a simple RFLP using two restriction enzymes. This clearly indicates that this 360-bp region of the rpoB gene contains highly informative sequences. In the present study, we analyzed sequences of this rpoB region of 35 mycobacterial strains representing 26 different mycobacterial species and prepared DNA probes that can be used in simple DNA hybridization tests for the identification of Mycobacterium species.
A total of 48 mycobacterial reference strains representing 39 Mycobacterium species were used for the PCR amplification of the 360-bp region of the rpoB gene in the present study (Table 1). Among them, 39 mycobacterial strains were obtained from the Korean Institute of Tuberculosis (KIT), Seoul, Korea, and three species were obtained from the Korean Collection for Type Cultures (KCTC) at the Korean Research Institute of Bioscience and Biotechnology (KRIBB). M. abscessus, which was recently separated from M. chelonae as an independent new species, was obtained from Department of Clinical Pathology at Yonsei University Medical Center (YUMC). Finally, five subtypes of M. kansasii were generously provided by V. Vincent at the Laboratoire de Référence des Mycobactéries, Institut Pasteur, Paris, France. Clinical isolates that were subjected to dot blot hybridization to evaluate the specificity of each mycobacterial species-specific probes were obtained from the KIT. All clinical isolates used in this study were identified on the basis of conventional tests that included microbiological characterization and biochemical tests and an rpoB-based RFLP method (13) to precisely identify the clinical isolates.
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TABLE 1. Bacterial strains used in this study
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For sequencing, PCR products were purified using the GeneClean III kit (Bio 101, Vista, Calif.) and cloned into a PCR-TOPO vector in the TOPO TA cloning kit (Invitrogen Co., Carlsbad, Calif.). The TOPO vectors containing PCR products were used for transformation of TOP10 competent cells (Invitrogen Co.). Plasmids containing inserts were purified from broth cultures with a Qiagen (Valencia, Calif.) plasmid kit and sequenced with the AutoRead sequencing kit and ALF DNA sequencer (Pharmacia Biotech, Uppsala, Sweden). Sequences were aligned using the Multialign program developed by F. Corpet (2).
In order to characterize the genetic nature of the 360-bp region of the rpoB gene, a total of 35 reference strains representing 26 different mycobacterial species were sequenced. Some species such as M. gordonae, M. kansasii, M. celatum, and M. fortuitum are known to have several subspecies, and thus these subspecies were also included in the sequence analysis. Among the 360-bp region sequenced in this study, sequences of 216 bp that have not been reported elsewhere are shown in Fig. 1, which shows that there exists a highly polymorphic region (black letters) flanked by highly conserved regions (red letters). Interestingly, the highly polymorphic region seemed to be suitable for the differentiation of mycobacterial species. For example, species differentiation between M. kansasii and M. gastri was possible since the sequences of M. kansasii are different from those of M. gastri, whose differentiation is not possible by 16S rRNA sequence analysis (18). Moreover, these polymorphic sequences were different even between highly closely related species, such as M. abscessus and M. chelonae or M. fortuitum and M. peregrinum, whose exact species identification has been extremely difficult by conventional culture-based microbiological and biochemical tests. In addition, the sequences of this polymorphic region in subspecies of M. kansasii, M. fortuitum, and M. gordonae were also differentiable, suggesting that this region of the rpoB gene may be used as a molecular signature for the differentiation of mycobacteria to the species or even to the subspecies level. However, there was no sequence difference in this region among species of the M. tuberculosis complex, including M. tuberculosis, M. bovis, M. microti, and M. africanum.
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FIG. 1. Multialignment sequences identified in the rpoB region of 35 strains of 26 mycobacterial species of clinical importance. The software used for the alignment of multiple sequences was obtained from reference 2. Sequences with high consensus are shown as red letters, low consensus as blue letters, and neutral consensus as black letters. Among the 360-bp sequences identified in this study, only the 216-bp region of the upstream sequences is shown, since the downstream sequences have been reported elsewhere. The sequences shown here have been submitted to GenBank.
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TABLE 2. Mycobacterial species-specific oligonucleotide probes designed and confirmed for their specificity by dot blot hybridization in this study
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Figure 2 showed dot blot hybridization results using the M. tuberculosis complex probe and M. gastri probe as examples. Each probe hybridized only to the corresponding mycobacterial species, indicating the specificity of molecular probes to each mycobacterial species. The rest of the probes in Table 2 also showed species-specific hybridization to the corresponding species (data not shown).
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FIG. 2. The results of the dot blot hybridizations using each mycobacterial species-specific oligomer probe derived from the novel region of the rpoB genes of species. Dot blot hybridizations were conducted using probes specific for M. tuberculosis complex and M. gastri. The PCR-amplified products from 48 mycobacterial species were blotted on the membrane, and this was followed by hybridization with probes. The identification numbers in the membranes are matched with numbers and species names in Table 1.
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FIG. 3. Dot blot hybridizations using the M. avium specific oligomer probe. The membrane contained PCR products amplified from 36 clinical isolates of mycobacteria, including six isolates of M. avium species. The clinical isolates were identified by conventional culture and biochemical tests at the KIT.
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The rpoB gene encodes the ß subunit of RNA polymerase, which produces RNA molecules in cells. Thus, rpoB is one of the very critical housekeeping genes that are closely related to cellular vitality and thus becomes the target for rifampin, the major bactericidal drug for M. tuberculosis and M. leprae. It is, therefore, reasonable to assume that the genetic structure of the rpoB gene is highly conserved within the same species. However, unlike 16S rRNA or any other rRNA whose primary structure is functionally critical, the rpoB gene seems to tolerate a more diverse sequence alteration without causing any changes in protein function. In particular, the DNA region that is not involved in the active site of the protein seems to be more polymorphic and does not cause major functional defects. Based on these relationships, it is easily understood that there exist highly conserved DNA regions and relatively variable DNA regions in the rpoB gene. The tolerable sequence variation in the rpoB gene becomes a useful clue for species identification of mycobacteria as reported previously (9, 10, 13). However, sequence analysis of the 360-bp region of the rpoB that we reported herein clearly revealed more extensive variation than expected, leading to develop mycobacterial species-specific probe molecules.
The oligonucleotide probes designed on the basis of this polymorphic region were useful in Mycobacterium species identification by dot blot hybridization assay. No cross-reactive hybridization was found between the 16 mycobacterial species-specific probes (Table 2) and 48 strains of 39 Mycobacterium species (Fig. 2). For example, the M. gastri-specific probe did not hybridize with five subtypes of M. kansasii, although the two species could not be differentiated by 16 rRNA sequence analysis (18). One of our concerns was sequence variation in the 360-bp region among clinical isolates of each Mycobacterium species. In this study, however, there seems to be no variation in the nucleotides of the region among M. avium clinical isolates, because the probe hybridized with all M. avium clinical isolates, as shown in Fig. 3. This was also supported by our previous study in which no variation was found in RFLP enzyme restriction sites among 40 clinical isolates of M. tuberculosis, 40 clinical isolates of M. avium, 50 clinical isolates of M. intracellulare, and 25 clinical isolates of M. gordonae, etc. (13). Although other probes still need to be confirmed for their sensitivity and specificity using multiple clinical isolates of each Mycobacterium species, the probes will be useful in developing a reverse blot hybridization assay by which many isolates can be analyzed for their species identification at the same time. In addition, since the 360-bp region is located near the rpoB mutation sites, which are associated with resistance to rifampin, one can develop an assay in the future which can simultaneously provide information about mycobacterial species identity and rifampin resistance.
Nucleotide sequence accession numbers. The nucleotide sequences listed in Table 2 have been submitted to the EMBL database and have been given the following accession numbers: AY271315 for M. microti, AY271316 for M. terrae, AY271317 for M. scrofulaceum, AY271318 for M. marinum, AY271319 for M. szulgai, AY271320 for M. gastri, AY271321 for M. malmoense, AY271322 for M. avium, AY271323 for M. bovis, AY271324 for M. peregrinum, AY271325 for M. fortuitumI, AY271326 for M. celatum type II, AY271327 for M. flavescens, AY271328 for M. intracellulare, AY271329 for M. abscessus, AY271330 for M. africanum, AY271331 for M. haemophilum, AY271332 for M. xenopi, AY271333 for M. kansasii type I, AY271334 for M. kasasii type II, AY271335 for M. kansaii type IV, AY271336 for M. kansasii type IV, AY271336 for M kansaii AY271337 M. celatum type I, AY271338 for M. genavense, AY271339 for M. simiae, AY271340 for M. fortuitum type II, AY271341 for M. gordonae type IV, AY271342 for M. gordonae type I, AY271343 for M. gordonae type II, AY271344 for M. gordonae type III, and AY271345 for M. smegmatis.
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