The Mycobacterium tuberculosis complex (MTC) comprises closely related species responsible for strictly human and zoonotic tuberculosis. Accurate species determination is useful for the identification of outbreaks and epidemiological links. Mycobacterium africanum and Mycobacterium canettii are typically restricted to Africa and M. bovis is a re-emerging pathogen. Identification of these species is difficult and expensive.
The Exact Tandem Repeat D (ETR-D; alias Mycobacterial Interspersed Repetitive Unit 4) was sequenced in MTC species type strains and 110 clinical isolates, in parallel to reference polyphasic identification based on phenotype profiling and sequencing of pncA, oxyR, hsp65, gyrB genes and the major polymorphism tandem repeat. Inclusion of M. tuberculosis isolates in the expanding, antibiotic-resistant Beijing clone was determined by Rv0927c gene sequencing. The ETR-D (780-bp) sequence unambiguously identified MTC species type strain except M. pinnipedii and M. microti thanks to six single nucleotide polymorphisms, variable numbers (1–7 copies) of the tandem repeat and two deletions/insertions. The ETR-D sequencing agreed with phenotypic identification in 107/110 clinical isolates and with reference polyphasic molecular identification in all isolates, comprising 98 M. tuberculosis, 5 M. bovis BCG type, 5 M. canettii, and 2 M. africanum. For M. tuberculosis isolates, the ETR-D sequence was not significantly associated with the Beijing clone.
ETR-D sequencing allowed accurate, single-step identification of the MTC at the species level. It circumvented the current expensive, time-consuming polyphasic approach. It could be used to depict epidemiology of zoonotic and human tuberculosis, especially in African countries where several MTC species are emerging.
The Mycobacterium tuberculosis complex (MTC) comprises several closely related species responsible for strictly human and zoonotic tuberculosis. Some of the species are restricted to Africa and were responsible for the high prevalence of tuberculosis. However, their identification at species level is difficult and expansive. Accurate species identification of all members is warranted in order to distinguish between strict human and zoonotic tuberculosis, to trace source exposure during epidemiological studies, and for the appropriate treatment of patients. In this paper, the Exact Tandem Repeat D (ETR-D) intergenic region was investigated in order to distinguish MTC species. The ETR-D sequencing unambiguously identified MTC species type strain except M. pinnipedii and M. microti, and the results agreed with phenotypic and molecular identification. This finding offers a new tool for the rapid and accurate identification of MTC species in a single sequencing reaction, replacing the current time-consuming polyphasic approach. Its use could assist public health interventions and aid in the control of zoonotic transmission in African countries, and could be of particular interest with the current emergence of multidrug-resistant and extended-resistance isolates.
Citation: Djelouadji Z, Raoult D, Daffé M, Drancourt M (2008) A Single-Step Sequencing Method for the Identification of Mycobacterium tuberculosis Complex Species. PLoS Negl Trop Dis 2(6): e253. doi:10.1371/journal.pntd.0000253
Editor: Pamela L. C. Small, University of Tennessee, United States of America
Received: October 11, 2007; Accepted: May 20, 2008; Published: June 18, 2008
Copyright: © 2008 Djelouadji et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This work was supported by Unité des Rickettsies - CNRS UMR 6236 UMR_D 198 and l'Oeuvre Antituberculeuse des Bouches-du-Rhône, Marseille, France. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors declare that they are the inventors of a protective patent deposited by the Mediterranée University, Marseilles, France on this matter.
The Mycobacterium tuberculosis complex (MTC) comprises several closely related species responsible for strictly human and zoonotic tuberculosis (Figure 1). In addition to M. tuberculosis, which represents the leading cause of human tuberculosis worldwide and is now emerging as extensively drug-resistant tuberculosis strains , other MTC species have been found in patients, typically in African countries (Figure 2). Mycobacterium bovis is a re-emerging, zoonotic agent of bovine tuberculosis  whose prevalence probably depends on variations in direct exposure to cattle and consumption of unpasteurised dairy products . The prevalence of Mycobacterium africanum type I (West Africa) and type II (East Africa)  has decreased in several African countries over the last decades ,. Mycobacterium canettii, a rare MTC species, has been isolated recently in patients exposed in Africa . Mycobacterium microti, a vole and small rodent pathogen  that is closely related to the so-called Dassie-bacillus and infects small mammals in South Africa and the Middle East ,, has been isolated in humans . Mycobacterium caprae is a rare cause of tuberculosis in cattle , and zoonotic tuberculosis in humans  while Mycobacterium pinnipedii has been isolated from seal lions and fur seals . A recent description of the re-emergence of M. bovis in cattle, along with the direct interhuman transmission of this zoonotic organism  in a six-case cluster that included one death in United Kingdom , illustrates the potential of emerging and re-emerging zoonotic tuberculosis due to MTC species other than M. tuberculosis and the necessity for accurate species identification.
Figure 1. Distribution of rare Mycobacterium tuberculosis complex species in humans (filled circles) and animals (open circles).
Green circles, M. canettii; blue circles, M. caprae; pink circles, M. microti; orange circles, M. pinnipedii.doi:10.1371/journal.pntd.0000253.g001
Figure 2. Distribution of Mycobacterium tuberculosis complex species in human tuberculosis cases in Africa.
Based on a review of papers published in 1999–2007 , –. For each country, prevalence rates and reference are indicated in cartoons. Mtub, M. tuberculosis; M afr, M. africanum; Mbov, M. bovis; Mcan, M. canettii; MTC, Mycobacterium tuberculosis complex.doi:10.1371/journal.pntd.0000253.g002
Accurate species identification of all MTC members is warranted in order to distinguish between strict human and zoonotic tuberculosis and to trace source exposure during epidemiological studies. Indeed, phenotypic methods of identification relying on colony morphology, oxygen preference, niacin accumulation, nitrate reductase activity, growth kinetics and resistance to thiophene-2-carboxylic acid hydrazide (TCH) and PZA  are hampered by slow growth of MTC members and subjective interpretation of colony morphology and cross-resistance to drugs . They do not always allow unambiguous species identification in every case. Recent studies of MTC species responsible for animal and human tuberculosis in tropical countries have relied on molecular methods including mycobacterial interspersed repetitive-unit-variable-number tandem-repeat (MIRU-VNTR) typing, IS6110-RFLP and spoligotyping –. Molecular differentiation of MTC members has been complicated by low sequence variability at the nucleotide level, illustrated by a 85–100% DNA/DNA relatedness and a 99–100% 16S rDNA sequence similarity ,. Nucleic acid-based assays such as acridinium ester-labelled DNA probes (AccuProbe; Gene Probe Inc, San Diego, CA) have proven to be reliable tools for assigning an isolate to the MTC ,, but they do not allow for identification at the species level. Molecular identification based on deleted regions (RD), RD1, RD9 and RD10 , are limited by the necessity of interpreting negative results in the case of the absence of a specific deletion. The detection of single nucleotide polymorphisms (SNP) in the pncA gene , the oxyR locus , the mtp40 gene , and the restriction fragment length polymorphism of the hupB gene  differentiated M. tuberculosis from M. bovis but not from other MTC species. The major polymorphism of tandem repeat (MPTR) sequencing differentiated M. tuberculosis (Sequevar long), M. bovis and M. microti (Sequevar Med-G), M. bovis BCG (Sequevar Med-C) and M. africanum (Sequevar short), but other MTC species were not studied . The gyrB gene proved to be an effective target ,, as an identification scheme has been proposed based on Pyrosequencing analysis of four single nucleotide polymorphism (SNPs) in gyrB , and a DNA strip based on gyrB is commercially available (HAIN Genotype MTBC DNAstrip test, Hain Lifescience, Nehren, Germany) . Both approaches, however, fail to differentiate M. tuberculosis from M. africanum type II and M. canettii; and M. africanum type I from M. pinnipedii. IS6110-RFLP, VNTR typing and Spoligotyping , emerged as reference methods to study the diversity of MTC species in resource-limited countries, despite the fact that these methods may not recognize rarely encountered species and may not appreciate the entire genetic diversity of strains, as they are not based upon the sequencing of molecular targets .
When investigating intergenic spacers in the genotyping of M. tuberculosis, we found that one spacer, previously identified as the Exact Tandem Repeat D (ETR-D)  and aliased Mycobacterial Interspersed Repeat Unit 04 (MIRU04) , exhibited a variable sequence among M. tuberculosis isolates. Analysis of this spacer had been previously shown to distinguish between M. bovis and the M. bovis BCG type . We therefore further investigated whether sequencing the ETR-D could identify all of the MTC at the species level. In this study, we demonstrate that ETR-D sequencing offers a new tool for the rapid and accurate identification of MTC species in a single reaction.
M. tuberculosis CIP103471, M. bovis CIP105050, M. africanum CIP105147T (type I), M. bovis BCG vaccine strain type 105060, M. microti CIP104256T, M. canettii CIP140060001T, M. pinnipedii ATCC BAA-688, and M. caprae CIP105776T reference strains were purchased from the Collection Institut Pasteur (CIP, Paris, France) and American Type Culture Collection (ATCC, Rockville, USA). The following non-tuberculosis mycobacteria were tested in order to assess the specificity of ETR-D spacer sequencing: Mycobacterium avium IWGMT49 T, Mycobacterium intracellulare CIP104243 T, Mycobacterium chimaera CIP107892 T, Mycobacterium colombiense CIP108962 T, Mycobacterium haemophilum CIP105049 T, Mycobacterium ulcerans CIP105425 T, Mycobacterium xenopi CIP104035 T, Mycobacterium abscessus CIP104536 T, Mycobacterium chelonae CIP104535 T, Mycobacterium fortuitum ATCC49404 and Mycobacterium mucogenicum CIP 105223 T. Quality of DNA was controlled by parallel partial rpoB PCR amplification as previously described . One hundred and ten MTC clinical isolates (Table 1) recovered from Microbiology Laboratory in Marseille (n = 102), from Institut Pasteur in Djibouti (n = 3) and from Institut de Pharmacologie et Biologie Structurale, Toulouse (n = 5) were also analyzed. All isolates were identified as members of the MTC by phenotypic characterization and a gene probe assay according to the manufacturer (AccuProbe; Gene Probe Inc, San Diego, Calif). This study was approved by the ethics committee of the Institut Féfératif de Recherche 48, Marseilles, France.
Table 1. Origin of Mycobacterium tuberculosis complex reference strains and clinical isolates analyzed in this studydoi:10.1371/journal.pntd.0000253.t001
Phenotypic characterisation included colony morphology, a urease test controlled after 3 and 18 hour incubation, and oxygen consumption measured after inoculation of a 0.2 ml actively growing mycobacterial suspension into 40 ml of Middlebrook 7H10 into the Bactec 9000MB system (Becton and Dickinson, Le Pont de la Claix, France) after a 3-week incubation. Drug susceptibility tests for thiophene-2-carboxylic acid hydrazide (TCH) and PZA were performed as previously described .
Reference tests for molecular identification
The identification of reference strains and clinical isolates identified as M. bovis BCG type, M. canettii and M. africanum by ETR-D sequencing (see below) was confirmed by parallel reference molecular tests. Every isolate coated on beads was inactivated as previously described  and the DNA was extracted using a Qiagen kit (Qiagen, Courtaboeuf, France). DNA was used as a template for PCR amplification of pncA, oxyR, hsp65, gyrB genes and sequence analysis of MPTR was performed as previously described , , –, In addition, we sequenced the Rv0927c-pstS3 intergenic region in all clinical isolates identified as M. tuberculosis in order to identify the Beijing genotype . Amplified products were visualized by agarose gel electrophoresis and direct sequencing was performed as described above. Sequences were edited using the Auto assembler program (Applied Biosystems, Courtaboeuf, France) and aligned using CLUSTAL W (http://pbil.ibcp.fr). Original sequences were deposited into GenBank (http://www.ncbi.nlm.nih.gov/sites/entrez/).
ETR-D spacer sequencing
Amplification and sequencing of the ETR-D spacer located between the putative histidine kinase Senx3 upstream and the sensory transduction protein Regx3 downstream were done using direct primers: 5′-GTTGATCGAGGCCTATCACG-3′ and 5′-GAATAGGGCTTGGTCACGTA-3′. The PCR mixture contained 33 µl H2O, 5 µl 10× buffer (Qiagen), 2 µl 25× MgCl2, 5 µl 10× dDNTP, 1 µl forward primer, 1 µl reverse primer, 0.25 µl hotstart Taq (Qiagen) and 2 µl target DNA. Appropriate negative controls consisting of PCR mix without target DNA were also included. PCRs were performed using the following program: 15 min enzyme activation at 95°C, followed by 34 cycles consisting of 95°C for 30 s, 58°C for 30 s, 72°C for 1 min, followed by a 5 min elongation step at 72°C. After agarose gel electrophoresis, PCR products were purified and subjected to sequencing in both directions by using the BigDye Terminator 1.1 Cycle Sequencing kit (Applied Biosystems). Sequencing electrophoresis was performed on a 3130 genetic analyzer (Applied Biosystems). The sequences were edited using the Auto assembler program (Applied Biosystems) and aligned using CLUSTAL W (http://pbil.ibcp.fr). Original ETR-D sequences were deposited into Genbank (http://www.ncbi.nlm.nih.gov/sites/entrez/).
Phenotypic identification (Table 2)
As for reference strains, M. tuberculosis exhibited eugonic growth that was inhibited by the presence of PZA but not by TCH and showed aerophilic growth on Middlebrook agar positive urease at 18 hours. M. bovis and M. bovis BCG type strains exhibited microaerophilic dysgonic growth, did not grow in the presence of TCH, but were resistant to PZA and exhibited a positive urease activity at 3 hours for M. bovis BCG type and at 18 hours for M. bovis. M. africanum type I differed from M. bovis by its susceptibility to PZA. M. canettii exhibited eugonic growth in the presence of PZA and TCH, and showed a positive urease activity at 3 hours and aerophilic growth on Middlebrook. M. microti, M. capare and M. pinnipedii exhibited eugonic growth that was inhibited by TCH and PZA, and a positive urease at 18 hours. As for clinical isolates, 101/110 of isolates were phenotypically identified as M. tuberculosis, 5 as M. canettii, 3 as M. bovis BCG type and one as M. africanum.
Table 2. Biochemical and antibiotic susceptibility profiles observed for M. tuberculosis complex reference strains and clinical isolates in this studydoi:10.1371/journal.pntd.0000253.t002
Reference molecular identification (Table 3)
In all PCR experiments, negative controls remained negative. All reference strains and clinical isolates yielded an amplicon of the expected size when amplified for pncA, oxyR, hsp65, gyrB genes, Rv0927c-pstS3 intergenic region and MPTR. By comparison with M. tuberculosis, the 410-bp oxyR gene sequence exhibited a previously known A285G polymorphism in M. bovis and M. bovis BCG type  and a newly identified T136G polymorphism in M. canettii. The 561-bp pncA gene sequence exhibited a previously known G253C polymorphism in M. bovis and M. bovis BCG type  and a G222A polymorphism in M. canettii . The 441-bp hsp65 gene exhibited a previously known T235C polymorphism in M. canettii  and a newly identified G376C polymorphism in M. africanum type I. The 1.020-bp gyrB gene sequence exhibited an identical sequence in M. tuberculosis, M. canettii and M. caprae, a previously known A756G polymorphism in M. bovis and M. bovis BCG type, a T675C polymorphism in M. microti, and an identical sequence was identified in common with M. africanum type I and M. pinnipedii . Sequence analysis of MPTR (300-bp) exhibited a unique sequence for M. tuberculosis (Sequevar Long), M. africanum type I strain (Sequevar Short), M. bovis and M. microti (Sequevar MED-G) and M. bovis BCG type (Sequevar MED-C) ; MPTR sequencing newly identified C99T, G164C and A267G polymorphisms in M. canettii; the M. caprae strain exhibited Sequevar Long in common with M. pinnipedii and M. tuberculosis reference strains. Original sequences found in this study were deposited in GenBank under the following accession numbers (GenBank: EF656461, EF 656463, EF 656464).
Table 3. Single nucleotide polymorphisms in five housekeeping genes and MPTR sequence analysis in eight MTC reference strainsdoi:10.1371/journal.pntd.0000253.t003
Sequence analysis of clinical isolates using the five previous targets yielded four different profiles. One profile comprised 98 isolates identified as M. tuberculosis, including three isolates identified as W-Beijing strains using a G127A polymorphism in Rv0927c-pstS3 intergenic region, a second profile comprised five isolates identified as M. bovis BCG type; a third profile included five isolates identified as M. canettii and a fourth profile included two isolates identified as M. africanum type I.
ETR-D sequence analysis
For all ETR-D experiments, negative controls remained negative. All the non-tuberculosis mycobacteria yielded a negative ETR-D PCR amplification whereas they yielded an amplicon of the expected size through rpoB PCR amplification. The size of PCR products obtained with MTC reference strains varied from 497-bp for M. canettii, 545-bp for M. bovis, 564-bp for M. caprae, 598-bp for M. bovis BCG type, 651-bp for M. africanum type I, 805-bp for M. tuberculosis and 959-bp for M. microti and M. pinnipedii. Each of the eight reference strains exhibited a unique ETR-D sequence, exhibiting one to seven copies of a tandem repeat, six mutations and two deletions/insertions.
M. tuberculosis exhibited three different alleles combining two or five copies of a 77-bp repeat unit and one T/G SNP at the fifth base of the tandem repeat, in addition to one 53-bp repeat unit. The M. tuberculosis reference strain exhibited five copies of a 77-bp repeat unit followed by one 53-bp repeat unit copy; M. microti and M. pinnipedii exhibited seven copies of a 77-bp repeat unit and one 53-bp repeat unit, M. africanum type I exhibited three copies of a 77-bp repeat unit and one 53-bp repeat unit, in addition to one T/C polymorphism at position 75. M. bovis exhibited four copies of a 77-bp repeat unit and one 53-bp repeat unit in addition to an A/G SNP at position 773; M. bovis BCG type exhibited three copies of a 77-bp repeat unit in addition to an A/G SNP at position 773. M. caprae exhibited three copies of a 77-bp repeat unit, a 34-bp deletion and one 53-bp repeat unit. M. canettii exhibited one copy of a 77-bp repeat unit and three SNPs following the tandem repeat in addition to a 53-bp repeat unit. ETR-D sequences of MTC type strains were deposited in GenBank under the following accession numbers (GenBank: EU180228-EU180234). ETR-D sequencing identified 98/110 MTC clinical isolates as M. tuberculosis including 45 isolates presenting allele 1, 26 isolates presenting allele 2, and 27 isolates presenting allele 3; the ETR-D allele did not correlate with Beijing genotype (P = 0.2). 5/110 isolates were identified as M. bovis BCG type, 5/110 isolates as M. canettii, and 2/110 isolates as M. africanum type I. All unique ETR-D sequences were deposited into our freely available database at http://ifr48.timone.univ-mrs.fr/MST_Mtuberculosis/mst.
Comparison between ETR-D identification and reference phenotypic and molecular identifications of clinical isolates
ETR-D identification was in agreement with phenotypic identification in 107/110 (97.27%) of clinical isolates. Three isolates phenotypically identified as M. tuberculosis were identified by ETR-D sequencing and reference molecular methods as M. bovis BCG type in two cases and M. africanum type I in one case. Reference molecular identification agreed with ETR-D identification in 100% (110/110) of clinical isolates.
Previous methods for MTC species identification either combined the amplification of several genomic regions in order to identify all species , or analyzed one gene polymorphism to distinguish between only two species. ETR-D spacer sequencing herein developed proved to be specific for the MTC and allowed the differentiation of the 7/8 MTC species in a single reaction. Indeed all the non-tuberculosis mycobacteria yielded a negative ETR-D PCR amplification as previously described .
The fact that M. africanum type II was not included in the present study may not modify this conclusion. In fact, the taxonomic status of M. africanum type II has been disputed , but it is now regarded as a phenotypic variant of M. tuberculosis (genotype Uganda) ,. ETR-D sequencing agreed in all cases with reference molecular identification. In this study, new mutations were identified because some genes were sequenced for the first time in some MTC species including the oxyR gene and MPTR in M. canettii and the hsp65 gene in M. africanum type I (Table 3). ETR-D sequencing revealed that 3/110 clinical isolates identified as M. tuberculosis by phenotypic tests comprised two M. bovis BCG type isolate and one M. africanum type I isolate. The 497-959-bp size of ETR-D allows one-step sequencing using a modern capillary sequencer and software and may be easily sequenced using Pyrosequencing and additional internal primers. Cost was decreased in comparison with the current polyphasic approach and any microbiologist could compare the ETR-D sequence with those that we deposited in the versatile, freely accessible databank at http://ifr48.timone.univ-mrs.fr/MST_MTuberculosis/mst. This identification technique, based on PCR amplification, could be directly applied to clinical specimens exhibiting acid-fast bacilli.
ETR-D sequence identification relied not only on the variation in the number of tandem repeats illustrated by various PCR product sizes, as previously described  for M. tuberculosis, M. africanum, M. bovis group , but also on specific SNPs, which are stable events  accounting for 55.5% of genetic events observed in this study and on insertion/deletion events (accounting for 22.2% of genetic events). However, the ETR-D sequence was not correlated with the Beijing genotype as defined by Rv0927c-pstS3 intergenic region sequencing. This indicates that, although 3 ETR-D genotypes were found among M. tuberculosis isolates in this study, ETR-D sequencing alone cannot be used for genotyping. It is not surprising that the same, limited genomic region does not have the potential to identify at the species and strain levels. ETR-D sequencing provides, for the first time, a unique sequencing test capable of distinguishing all MTC species in a single step.
Accurate identification of MTC isolates at the species level is of particular interest in Africa where species other than M. tuberculosis were characterized in human tuberculosis and M. bovis remains a huge problem for cattle  (Figure 2). Their identification may direct specific epidemiological investigation. In Africa, the prevalence of M. bovis in human tuberculosis was correlated with the prevalence in the local cattle population . Consumption of unpasteurised milk and of poorly heat-treated meat, and close contact with infected animals represent the main sources of infection for humans . However, human to human transmission of M. bovis was recently reported in a 6-case cluster including one death due to M. bovis meningitis in United-Kingdom . In addition, the emergence of MDR M. bovis has been documented, raising infection control in health care settings ,. M. bovis BCG type derived from the closely related virulent M. bovis after 230 serial passages had led to a considerably increased rate of disseminated BCG disease in HIV-infected infants reported in South Africa , although diagnoses were based on a few biochemical tests including the urease test and RD1deletion . ETR-D sequencing allows unambiguous distinguishing of BCG type strains from M. bovis strains using a minute quantity of starting material. M. africanum identification indicated a tuberculosis microepidemic in a defined area when repeated isolation was observed . Sporadic isolation of M. africanum strains has been reported in Europe and the United States, including outbreaks of multidrug-resistant (MDR) strains ,. In recent studies, variations in the reported prevalence of M. africanum among various African countries may also reflect difficulties in accurate identification of this species (Figure 2). M. microti, M. pinnipedii, M. caprae and M. canettii remain difficult to identify because of the extremely slow growth of these organisms, the difficulties with their identification under traditional bacteriological methods  and the fact that these recently described species have not been incorporated into current molecular identification schemes.
ETR-D spacer sequencing offers a new tool for the rapid and accurate identification of all MTC species in a single sequencing reaction without the need for expensive, time-consuming and potentially harmful polyphasic approaches. Its use could assist public health interventions and aid in the control of zoonotic transmission in African countries. Accurate identification of MTC isolates from Africa and tropical Asia would be of particular interest from the perspective of the current emergence of multidrug resistant and extended resistance isolates in these countries .
The authors acknowledge the technical expertise of Christian Fontaine and Bernard Campagna. In addition, the authors recognize Dr. Christophe Guilhot, Dr. Sylvain Godreuil, and Dr. Cristina Gutierrez for providing the M. canettii clinical isolates.
Conceived and designed the experiments: Z. Djelouadji M. Drancourt. Performed the experiments: Z. Djelouadji. Analyzed the data: Z. Djelouadji D. Raoult M. Daffé M. Drancourt. Contributed reagents/materials/analysis tools: D. Raoult. Wrote the paper: Z. Djelouadji D. Raoult M. Drancourt.
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