Previous Article | Next Article ![]()
Journal of Clinical Microbiology, April 2005, p. 1782-1788, Vol. 43, No. 4
0095-1137/05/$08.00+0 doi:10.1128/JCM.43.4.1782-1788.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Departments of Biochemistry,1 Laboratory Medicine,3 Internal Medicine,4 College of Medicine, Department of Microbiology, College of Natural Science,2 Interdisciplinary Program of Bioinformatics, Graduate School, Pusan National University,6 Department of Laboratory Medicine, College of Medicine, Kosin University, Busan, Korea5
Received 22 March 2004/ Returned for modification 3 June 2004/ Accepted 8 October 2004
|
|
|---|
|
|
|---|
In order to overcome these disadvantages, the development of molecular assays has accelerated diagnosis (19). Molecular assays require highly technical tools and target genes. Technical improvements in PCR, sequencing, and oligonucleotide array have increased the sensitivity, specificity, and speed of assays (4, 8, 11, 13, 24). Among these improvements, the oligonucleotide array has recently become a powerful tool for microbial genotyping, drug resistance associated with mutations, and single-nucleotide polymorphisms. This technology also permits the simultaneous monitoring and analysis of a large number of target genes depending on sequence diversity (5). Several target genes, such as 16S rRNA, rpoB, hsp65, internal transcribed spacer (ITS), and gyrB, have been used as targets for the genotyping of microorganisms (10, 17, 19, 21, 23). The important feature of target genes is that these genes are present in all bacteria and contain both conserved and polymorphic regions (11, 18). This sequence diversity of target genes provides a possibility in the speedy and accurate design of molecular assays.
We have previously studied the PCR assay by using the ITS region for identifying mycobacteria to the species level (16). Based on the previous PCR assay, we tried to develop molecular testing that could identify and discriminate medically important mycobacteria in a single reaction. The purpose of this study was to develop an oligonucleotide array based on the ITS sequence for the genotyping of medically important mycobacteria containing M. tuberculosis and 19 NTM. We evaluated the oligonucleotide array with reference strains, clinical isolates, and clinical specimens.
|
|
|---|
Preparation of genomic DNA and PCR. Clinical specimens, taken from sputum, had to be liquefied, decontaminated, and concentrated using the NaOH method. Then, the DNA was extracted using an InstaGene matrix kit (Bio-Rad Laboratories Inc., Hercules, Calif.) according to the manufacturer's protocol. PCR was performed as previously described with thermal cycler system PCT-100 (MJ Research, Waltham, Mass.) (16). The biotin-labeled primers ITS-F (forward, 5'-TGGATCCGACGAAGTCGTAACAAGG-3') and PAN-04R (reverse, 5'-ATGCTCBCAABCACTATCCA-3') were used for PCR amplification.
Designs of genotype-specific probes and quality control (QC) probes. (i) Genotype-specific probes. The ITS sequences of mycobacteria were obtained from GenBank. According to the multiple alignment analysis data obtained by using CLUSTALW, genus- and species-specific probes were designed from the conserved and polymorphic regions of the ITS sequences of mycobacteria, respectively. The uniqueness of the sequences of the probes designed from the mycobacterial ITS region was analyzed with the BLAST search. The probes were designed to meet the following parameters. The oligonucleotides were between 15 and 22 nucleotides long, and the position of the potential mismatch in similar sequences was close to the center of the probe. The 5' end of each probe was modified by adding poly(T) and an aminolink group to enable covalent immobilizing on the aldehyde-coated glass surface.
(ii) QC probe. We prepared QC probes to check the spot uniformity of each array. The 6-carboxytetramethylrhodamine (TAMRA)-labeled QC probe [20-mer poly(T)] was also modified by an aminolink group at the 5' end. We spotted mixed probes of genotype-specific and QC probes. Before hybridization, we could confirm the proper spotting of genotype-specific probes by scanning at a 532-nm wavelength.
Fabrication of oligonucleotide arrays. Probes were printed in glass slides as shown in Fig. 1W. The oligonucleotide arrays were spotted on silylated glass slides (Cell Associated, Inc., Pearland, Texas) by using a PixSys nQUAD 4500 Microarrayer (Cartesian Technologies, Inc., Irvine, CA). The spotting solution contained a mixture of genotype-specific and QC probes (9:1 ratio) in solution. The spotted slides were dried for 3 h at 50°C and treated for 5 min with a freshly prepared 0.25% NaBH4 solution. The slides were washed once for 5 min with 0.2% sodium dodecyl sulfate and distilled water to remove unbound probes.
![]() View larger version (44K): [in a new window] |
FIG. 1. Hybridization results with mycobacterial species and other bacteria by oligonucleotide array. (A through T) Each species of mycobacterium was expected to show a unique pattern of reactivity to each set of probes. (U) Hybridization result is shown for other pathogenic bacteria. (V) Image by QC probe before hybridization is shown. (W) Scheme of probes is laid out in oligonucleotide array. The probes were patterned in duplicate. Probe names are as explained in Results. PAN, PAN-03 probe.
|
Sequence analysis of mycobacterial ITS. For the sequencing of the ITS of the mycobacteria, ITS-F (forward, 5'-TGGATCCGACGAAGTCGTAACAAGG-3') and ITS-R (reverse, 5'-TGGATCCTGCCAAGGCATCCACCAT-3') primers were designed to amplify the ITS region to approximately 350 to 450 bp in length. The amplicons were purified using a QIAquick PCR purification kit (QIAGEN, Inc., Valencia, Calif.) according to the manufacturer's protocol. The sequence of PCR products was determined using the ABI Prism BigDye Terminator Cycle Sequencing Ready Reaction kit (Applied Biosystems, Inc., Foster City, Calif.) in the BaseStation-1 DNA fragment analyzer (MJ Research).
|
|
|---|
|
View this table: [in a new window] |
TABLE 1. Genus- and species-specific probes used for the genotyping of mycobacteria
|
We selected the optimal genus-specific probe and species-specific probes through hybridization with the clinical isolate of M. leprae as well as with reference strains of 31 mycobacterial species and of 15 other pathogenic bacteria. We produced amplicons with biotin-labeled primers in order to hybridize the oligonucleotide array. The amplicon size ranged from 270 to 400 bp from species to species. The results of independent experiments on mycobacterial species and other pathogenic bacteria for genotyping are presented in Fig. 1. The genus-specific probe (PAN-03) showed a unique hybridization pattern with all mycobacteria (Fig. 1A through T) and a mismatch with 15 other bacteria containing C. diphtheriae, which is taxonomically close to mycobacteria, and M. pneumoniae, which is the most common bacterium causing respiratory tract infections (Fig. 1U). The genus Mycobacterium can be identified with a single genus-specific probe (PAN-03). The most optimal species-specific probes, among 2 to 13 individual probes for discriminating mycobacteria, are shown in Table 1. Each mycobacterial species was expected to show a unique hybridization pattern with the species-specific probes (Fig. 1). For example, M. tuberculosis was expected to hybridize with the PAN-03 and M. tuberculosis complex (MTB)-specific probes (Fig. 1A), and other mycobacteria were hybridized with the PAN-03 probe but not with the MTB probe. The PAN-03 probe and each species-specific probe (MTB, MAC, FOR, CHE, ABC, KAN, GOR, SCO, SZU, VAC, XEN, TER, FLA, SME, MAL, SIM, M-U, GAS, and LEP) were hybridized specifically with M. tuberculosis, M. avium-intracellulare complex, M. fortuitum, M. chelonae, M. abscessus, M. kansasii, M. gordonae, M. scrofulaceum, M. szulgai, M. vaccae, M. xenopi, M. terrae, M. flavescens, M. smegmatis, M. malmoense, M. simiae, M. marinum and M. ulcerans, M. gastri, and M. leprae, respectively. But only the PAN-03 probe was hybridized with M. acapulsensis, M. agri, M. asiaticum, M. gallinarum, M. gilvum, M. lentiflavum, M. phlei, M. peregrinum, M. procinum, M. shimoidei, and M. triviale, which were not species-specific probes contained in the oligonucleotide array (Fig. 1T). The oligonucleotide array allowed for the differentiation of M. chelonae from M. abscessus and M. kansasii from M. gastri but could not differentiate M. marinum from M. ulcerans. The mycobacterial species except MAC were each identified with a respective single probe. For MAC, in which there are many subspecies, a combination probe of MAC-01 (degenerate species-specific probe) and MAC-05 was designed. The species-specific probes could discriminate mycobacteria to the species level under uniform hybridization conditions.
Oligonucleotide array assay with clinical isolates and specimens of mycobacteria. We tested 149 clinical isolates and 155 clinical specimens with the oligonucleotide array. We performed two approaches used to identify the clinical samples. The clinical isolates were identified by using typing results known prior to diagnosis. The 155 clinical specimens were performed blind, and the results were compared at a later date. Some discrepant results occurred when the results by conventional identification were compared with those by oligonucleotide array (Table 2). But the results by oligonucleotide array and sequencing were identical in all cases for each species. In cases where there was a mismatch between the phenotypic and genotypic identifications, we repeated the test three times to confirm our oligonucleotide array and sequencing results. Of the nine M. chelonae isolates, which were identified by conventional methods, eight were identified as M. chelonae and one was identified as M. abscessus by oligonucleotide array. Of the nine M. gordonae isolates, six were identified as M. gordonae, two were identified as M. tuberculosis, and one was identified as M. flavescens by oligonucleotide array. The PCR and direct sequencing results matched oligonucleotide array results (Table 2). Among the clinical specimens, two were shown to hybridize with the PAN-03 probe and were identified as Mycobacterium spp. by oligonucleotide array. By sequencing, they were identified as M. celatum and M. lentiflavum. The oligonucleotide array allowed for the identification of a number of mycobacterial species in one reaction. It is advantageous for diagnostic purposes to identify a number of bacteria simultaneously in a clinical sample.
|
View this table: [in a new window] |
TABLE 2. Interpretations obtained with clinical isolates and clinical specimens of mycobacteriaa
|
![]() View larger version (42K): [in a new window] |
FIG. 2. Sensitivity of oligonucleotide array for the detection of mycobacteria. We performed experiments of detection limit three times. Among three separate experiments, we used the representative single-experiment result. (A) Image and signal intensities were obtained through hybridization with the amplified DNA of M. tuberculosis. The PAN (PAN-03) and MTB probes were the number 1 and number 30 probes and the number 2 probe, respectively, at the result of signal intensity. The detection limit was determined by amplification and hybridization of serial dilutions of DNA (500-pg/µl to 500-fg/µl dilutions of the M. tuberculosis DNA initial concentrations) and based on triplicate samples. Signal intensity (S.I) of hybridization was analyzed by using an intelligent analysis program (CombiView). (B) Before hybridization with oligonucleotide array, ITS of M. tuberculosis (5-ng/µl to 500-fg/µl dilutions of the DNA initial concentrations) was amplified with biotin-labeled primers and separated in a 2% agarose gel. Lanes M, 100-bp DNA ladder size markers.
|
|
|
|---|
In this study, we developed the oligonucleotide array to differentiate 20 medically important pathogenic mycobacteria. There were several hurdles in the development of the oligonucleotide array for genotyping of mycobacteria, such as probe design, the adjusting of the hybridization condition of various kinds of probes, and the conforming of spotting of probes, among others. QC is an important aspect of oligonucleotide array fabrication. Among several important QC systems, the monitoring of spot uniformity is critical to manufacturing the oligonucleotide array. Many studies have reported on the QC systems of array (1, 2, 5, 22, 26). These QC systems allow for the confirmation of spot uniformity after hybridization. But we introduced a QC probe which could confirm spot uniformity before hybridization. In our study, the QC probe was manufactured as follows. A TAMRA-labeled QC probe [20-mer poly(T)] was modified by an aminolink group in the same way as a genotype-specific probe. The spotting solution contained a mixture of a genotype-specific probe and a QC probe (9:1 ratio) in solution. Before the hybridization experiment, a fluorescent image of QC probes was obtained at 532 nm (TAMRA) (Fig. 1V). That is, we could confirm the spot uniformity of the genotype-specific probes before hybridization. This approach may be a valuable tool for the monitoring of spot uniformity. And it was hard to design species-specific probes because there are interspecies variations in several species. MAC shows high interspecies variations. In order to overcome these problems, we tried to design species-specific probes in species-conserved regions. The interspecies variation regions within target sequences were avoided in designing species-specific probes for hybridization by the oligonucleotide array.
The limits of the phenotype-based identification method have led to the development of methods based on the microbial genotype or DNA sequences which minimize problems with typeability and reproducibility and, in some cases, enable the establishment of large databases of characterized organisms (7, 15). Tortoli et al. (20) reported that M. gordonae is misidentified as M. szulgai by conventional identification, including biochemical investigation. Among strains with overlapping phenotypic patterns, the conventional procedures are unable to distinguish organisms belonging to a certain taxon from the other species. There was a similar result by conventional identification in our study. As shown in Table 2, nine M. gordonae isolates which were identified by conventional methods were identified as six M. gordonae, two M. tuberculosis, and one M. flavescens by the oligonucleotide array. Since M. tuberculosis is considered clinically significant, whereas M. gordonae is not, to distinguish among them is essential. ITS-based oligonucleotide array could discriminate between M. gordonae and M. szulgai, M. gordonae and M. tuberculosis, and M. chelonae and M. abscessus, which are hard to differentiate by a conventional method. Two clinical specimens were shown to hybridize with only the PAN probe. They were interpreted as an unusual or unknown mycobacterium by the oligonucleotide array. After the sequencing of ITS, we could identify them as M. celatum and M. lentiflavum (Table 2). Therefore, even if mycobacteria are not species-specific probes contained in this oligonucleotide array, we can say that it is a mycobacterial species by hybridization with the PAN probe, so unusual or unknown strains of mycobacteria may be detected from clinical specimens. Consequently, some studies have tried to develop an oligonucleotide array containing a huge number of probes for the detection of many kinds of microorganisms. Photolithography chips displaying over 250,000 oligonucleotides can now be produced to perform sequence analysis on amplified small-subunit rRNA genes (23). A high-density microarray has also been described in the identification of 54 different mycobacterial species by using 82 unique 16S rRNA sequences and all known mutations associated with rifampin and isoniazid resistance in M. tuberculosis (12). On the other hand, a relatively small number of probes were included in our oligonucleotide array because of cost-effectiveness. But a genus-specific probe (PAN-03) makes it possible to detect the presence of unusual or unknown mycobacteria. This is a reasonable approach to routine diagnosis with low cost, we believe.
In conclusion, we developed the oligonucleotide array based on the ITS sequence for the identification and discrimination of medically important mycobacterial species in a single hybridization. The advantages of this oligonucleotide array are the identification of 20 mycobacterial species, except for MAC, with a single probe in a single hybridization; the discrimination between closely related species, such as M. chelonae and M. abscessus; and detection of the presence of unusual or unknown mycobacteria in a rapid, easy-to-perform manner and at a low cost. Our results indicate that oligonucleotide array is very useful for the rapid identification and accurate discrimination of mycobacteria from clinical isolates and specimens in an ordinary clinical laboratory.
We thank Gue-Tae Chae for providing us with the clinical isolate of M. leprae.
|
|
|---|
This article has been cited by other articles:
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Copyright © 2009 by the American Society for Microbiology. For an alternate route to Journals.ASM.org, visit: http://intl-journals.asm.org | More Info»