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Journal of Clinical Microbiology, May 2005, p. 2148-2154, Vol. 43, No. 5
0095-1137/05/$08.00+0 doi:10.1128/JCM.43.5.2148-2154.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Targeted Hybridization of IS6110 Fingerprints Identifies the W-Beijing Mycobacterium tuberculosis Strains among Clinical Isolates
Natalia Kurepina,1
Ekaterina Likhoshvay,1
Elena Shashkina,1
Barun Mathema,1
Kristin Kremer,2
Dick van Soolingen,2
Pablo Bifani,3 and
Barry N. Kreiswirth1*
TB Center, Public Health Research Institute, Newark, New Jersey,1
Mycobacteria Reference Unit, Laboratory of Infectious Diseases and Perinatal Screening, National Institute of Public Health and the Environment (RIVM), The Netherlands,2
INSERM U629, Institut Pasteur de Lille, Lille, France3
Received 1 November 2004/
Returned for modification 7 December 2004/
Accepted 1 January 2005

ABSTRACT
Targeted IS
6110-based RFLP genotyping can be applied to rapidly
identify specific groups of biomedically/epidemiologically relevant
Mycobacterium tuberculosis clinical isolates. One such group
is the W-Beijing strain family (also known as Beijing/W), implicated
in significant nosocomial and community outbreaks worldwide.
Using previously defined criteria, we developed a simple and
accurate method to identify members of the W-Beijing family,
based on rehybridization of Southern blot membranes used previously
in routine IS
6110 DNA fingerprint analysis. The hybridization
probe constructed ("W-Beijing polyprobe") contains the PCR-amplified
fragments specific for three
M. tuberculosis chromosomal loci
used for the identification of W-Beijing strains. The targets
include the
dnaA-dnaN and NTF regions and the direct repeat
locus. A total of 526 selected clinical isolates (representative
of 253 different IS
6110-defined strain types) were analyzed
using the W-Beijing polyprobe. A total of 148 isolates from
this collection were found to be members of the W-Beijing phylogenetic
lineage, comprising 106 strains from the W-Beijing family (46
clusters) and 42 related isolates. Rehybridization results were
confirmed by computer-assisted analysis. The sensitivity and
specificity of this method were estimated at 98.7% and 99.7%,
respectively. This study demonstrates that the W-Beijing polyprobe
can accurately and reliably discriminate members of the W-Beijing
phylogenetic lineage and the W-Beijing family of
M. tuberculosis strains.

INTRODUCTION
Over the last decade of
Mycobacterium tuberculosis genotyping,
one group of clinical isolates, the W-Beijing strain family,
has received considerable attention. These strains have been
shown to be highly prevalent throughout Asia and the countries
of the former Soviet Union, and they have been associated with
nosocomial and community outbreaks, global transmission, and
drug resistance (
3,
10,
28). The W-Beijing strains have been
also associated with tuberculosis (TB) treatment failures and
relapse cases (
16). In vivo murine infection studies have suggested
that W-Beijing strains fail to efficiently elicit a Th1-dependent
protective immune response and have shown that they are more
virulent (
17,
18). As a result of the current epidemiology and
laboratory studies associated with W-Beijing strains, public
health research laboratories and TB control efforts would benefit
from the ability to accurately identify these strains in any
given population.
Independent genetic markers specific for W-Beijing strains have been described previously (3, 8, 15). These strains can be characterized on the basis of a unique IS6110 insertion in the origin of replication (oriC) and a specific spacer oligotype (spoligotype; NY_S00034; type 1, SpolDB3) lacking spacers 1 through 34 in the direct repeat (DR) chromosomal locus. Strains bearing both these genetic characteristics have recently been classified into a large phylogenetic lineage (the W-Beijing lineage) defined by synonymous single-nucleotide polymorphism (sSNP) analysis as members of cluster II (12). W-Beijing strains with multiple (15 to 26) copies of IS6110 that share similar restriction fragment length polymorphism (RFLP) genotyping patterns (>65%), and that have at least one specific IS6110 insertion within the NTF locus, define the W-Beijing family of strains (3). A second IS6110 insertion in the NTF locus defines the multidrug-resistant (MDR) W strains, recent descendants of the W-Beijing strain family that caused multiple institutional outbreaks in New York City (NYC) during the early 1990s and more than 500 active cases (5, 22, 24).
We have exploited the uniqueness of these genetic markers in establishing a single-step method that is based on the standardized IS6110 Southern blot hybridization protocol. We report here the construction and utility of the "W-Beijing polyprobe," which includes three genetic markers, for rapid recognition of W-Beijing strains among a collection of M. tuberculosis clinical isolates by rehybridizing membranes used for routine IS6110-based RFLP genotyping.

MATERIALS AND METHODS
Clinical isolates.
Since 1992 the TB Center at the Public Health Research Institute
(PHRI) has been systematically subculturing, stocking, and genotyping
clinical isolates of
M. tuberculosis. The PHRI TB Center collection
contains over 18,000
M. tuberculosis clinical isolates that
have been catalogued on the basis of their IS
6110 RFLP patterns,
which have been digitized, analyzed, and archived using a Sun
Sparcs5 workstation and a BioImage Whole Band Analyzer (version
3.4; Genomic Solutions, Ann Arbor, Mich.). Clinical isolates
originate primarily from New York City and New Jersey patients,
but the collection also includes isolates from diverse global
populations including Brazil, Chile, the Czech Republic, Egypt,
Germany, Haiti, Honduras, India, Israel, Kenya, Mexico, Romania,
Russia, Singapore, South Africa, Thailand, and Venezuela. From
this collection, approximately 2,850 isolates have previously
been assigned to the W-Beijing family according to the computer-assisted
analysis and predefined criteria described elsewhere (
3). Representative
isolates from this collection were selected for the purpose
of this study.
Selection of membranes.
Thirty-two Southern blot nylon (Hybond-N+) membranes (526 isolates) were selected for rehybridization with the W-Beijing polyprobe among a collection of more than 1,000 membranes generated during routine IS6110 RFLP genotyping. Twenty-two blots with at least one W-Beijing IS6110 fingerprint pattern previously determined by computer-assisted analysis were selected, and 10 additional blots were randomly selected.
PCR amplification of desired fragments.
Genomic loci used for PCR amplification have been described previously (14, 15, 24). Chromosomal DNA of the laboratory strain H37Rv, ATCC 35837, was used as a template for PCR amplification (http://genolist.pasteur.fr/TubercuList/) (6). Amplicons were designed to limit the number of hybridization bands. The primers used for PCR, their respective locations in the genome of M. tuberculosis H37Rv, and the sizes of the resulting amplicons are shown in Table 1 and Fig. 1. DNA amplification was conducted using thermocycler GenAmp PCR System 2700 (Applied Biosystems, Inc., Foster City, CA). A 100 µl reaction mixture with a final MgCl2 concentration of 4 mM was incubated at 94°C for 2 min, followed by 35 cycles of 94°C for 20 s, 62°C for 10 s, and 72°C for 30 s, and a final 2 min at 72°C, with Taq DNA polymerase (Roche Diagnostics GmbH). PCR products can be purified if needed using the QIAquick PCR Purification kit (QIAGEN, Valencia, CA, catalog no. 28106).The proportion of the PCR-amplified fragments of the three W-Beijing polyprobe amplicons to be used for rehybridization of a single membrane is 1:2:2.5 [
5 µl (DR) + 10 µl (region A) + 12 µl (region B)]. Enhanced chemiluminescence labeling was performed as the manufacturer recommended.
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TABLE 1. Primers used for PCR DNA amplification, their locations in the M. tuberculosis H37Rv genome, sizes of the PCR amplicons, and expected patterns after rehybridization of routinely IS6110-probed membranes with the W-Beijing polyprobe
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Rehybridization procedure.
M. tuberculosis chromosomal DNA was isolated, PvuII digested,
electrophoresed, and transferred to a nylon membrane (Hybond-N+)
as described elsewhere (
26). Membranes previously hybridized
with the 312-bp IS
6110 3'-specific probe for RFLP genotyping
were subjected to rehybridization with the W-Beijing polyprobe,
prepared as a combination of PCR amplicons of three chromosomal
loci. Single-locus-specific probes (Fig.
1) were also used for
hybridization of 1,560 clinical
M. tuberculosis isolates. Membranes
were rehybridized with the W-Beijing polyprobe without additional
washing, following daylight exposure for at least 20 h prior
to secondary hybridization.
Composite blot of outlier isolates.
In order to facilitate the recognition of exceptions among the W-Beijing isolates which do not display all the predicted hybridization patterns, a composite blot containing DNA from two controls (CDC1551 and H37Rv) and 15 W-Beijing clinical isolates with unusual hybridization patterns was assembled and rehybridized as a reference (Fig. 2A and B).

RESULTS
Rehybridization of Hybond-N+ membranes.
The selected loci, primer sequences, and PCR amplicon sizes
and the predicted hybridization patterns are shown in Table
1. The ultimate selection of the PCR amplicons proposed in this
study is the result of the analysis of more than 1,560 strains
hybridized independently with each single probe specific for
the three designated chromosomal loci
dnaA-dnaN, DR, and NTF,
as shown in Fig.
1A, B, and C, respectively. These hybridization
results identified probe sequences that cross-hybridized or
failed to generate a robust signal and facilitated the classification
of the strains (Table
1). Rehybridization with the W-Beijing
polyprobe can be performed according to standard protocol on
membranes previously used for IS
6110 RFLP genotyping and does
not require an additional blot washing step (
26). W-Beijing
polyprobe rehybridization of membranes stored at room temperature
yielded clear banding patterns regardless of the age of the
membrane (up to 4 to 8 years). The rehybridization procedure
might be limited to 4 h; hybridization buffer with resuspended
enhanced chemiluminescence-labeled W-Beijing polyprobe can be
reused for rehybridization of membranes at least twice if stored
at 20°C.
Analysis of rehybridization results.
An algorithm for interpretation of hybridization patterns is proposed (Fig. 3). Using the algorithm, each individual strain can be assigned to the W-Beijing phylogenetic lineage or the W-Beijing family, or classified as a non-W-Beijing strain.
Region A (oriC; dnaA-dnaN intergenic region).
The A1 insertion of IS
6110 in the origin of replication (
dnaA-dnaN intergenic region;
oriC) results in the generation of two hybridizing
bands (3.36 and 8.55 kb, instead of a single 10.85-kb band in
wild-type
oriC) when the full wild-type
dnaA-dnaN fragment is
used as a probe. This is due to the insertion of an additional
PvuII restriction site brought in by transposition of an IS
6110 copy, as previously described (
15). To simplify the hybridization
patterns, a new pair of primers specific for the amplification
of a 450-bp fragment was used (Table
1; Fig.
1). This probe
demonstrated the presence of the A1 insertion by revealing a
single 3.36-kb hybridization band (Fig.
2B, lanes 3 to 12 and
15 to 17). Comparison of two blots (Fig.
2A and B), generated
by independently hybridizing and rehybridizing the same membrane
with IS
6110- and region A-specific probes, reveals matching
3.36-kb bands on both blots.
M. tuberculosis strains lacking
an IS
6110 copy in
oriC display a characteristic 10.85-kb band
(Fig.
2B, lanes 1 and 2), which does not match any band in the
corresponding IS
6110 image. A previous study has shown that
the
dnaA-dnaN intergenic region is a "hot spot" for IS
6110 transposition
events; insertions in 11 different positions within this region
have been identified in genetically distinct lineages of
M. tuberculosis (
15). With the exception of A1, as described here,
IS
6110 insertions within the
dnaA-dnaN intergenic region will
result in hybridization patterns with one or two bands of different
corresponding sizes.
DR region.
W-Beijing isolates have a large deletion within the chromosomal DR locus (spacers 1 to 34 deleted). This results in a single 3.65-kb hybridization band on blots with PvuII-digested DNA from W-Beijing strains (Fig. 2B, lanes 3, 7 to 11, and 15 to 17), when they are rehybridized with the DR-specific probe (14). This band does not match that of any IS6110-specific pattern on an IS6110 Southern hybridization blot. In contrast, non-W-Beijing strains show patterns with one, two, or three bands of variable sizes that correspond to the presence of 1 or 2 copies of IS6110 in the DR (Fig. 2B, lanes 1 and 2). In these non W-Beijing isolates, some DR-specific bands may match IS6110-specific bands as long as such strains contain an IS6110 insertion within the DR locus (25).
Region B (chromosomal locus NTF).
All members of the W-Beijing family have at least one insertion within the NTF chromosomal locus, resulting in a 1.6-kb hybridizing band (Fig. 2B, lanes 9, 13 to 16). Additionally, some members of the W-Beijing family may have a second insertion within this locus. A specific second insertion in the same orientation 556 bp downstream has been identified in the New York City W-MDR strain, resulting in a unique 1.1-kb hybridization band which can be used to identify this specific strain (Fig. 2B, lane 17) (15, 24) but not other, unrelated W-Beijing-MDR isolates. However, most clinical isolates analyzed to date do not have an IS6110 insertion within the NTF locus and show a 1.4-kb hybridization band that corresponds to the wild-type chromosome (Fig. 2B, lanes 1 and 2); H37Rv, CDC1551, and Mycobacterium bovis share this characteristic genotype. Likewise, some strains from the W-Beijing phylogenetic lineage ("ancestral" or "parental" strains) which probably diverged early during the course of evolution, prior to the IS6110 transposition event in the NTF locus (P. Bifani, unpublished data), also have a wild-type genotype (Fig. 2B, lanes 3 to 8 and 10 to 12).
Evaluation of the polyprobe.
Table 2 summarizes the rehybridization results using the W-Beijing polyprobe on membranes containing PvuII-digested DNA from M. tuberculosis clinical isolates. An algorithm for the interpretation of the results is shown in Fig. 3.
To evaluate this method, 526 clinical isolates (32 membranes)
were rehybridized with the W-Beijing polyprobe and the banding
patterns were analyzed blindly. Patterns were subdivided into
three groups using the proposed algorithm. The first group consisted
of 104 isolates with hybridization banding patterns of 3.65,
3.36, and 1.6 kb (corresponding to DR-,
dnaA-dnaN-, and NTF-associated
fragments) and 2 isolates with 3.65-, 3.36-, and 1.1-kb bands.
These 106 strains were recognized as members of the W-Beijing
family, as confirmed by previously performed computer-assisted
comparative analysis of highly similar IS
6110 RFLP banding patterns
(>65% pattern similarity). Comparison with RFLP cluster analysis
confirmed that they represented 46 independent clusters that
differ from each other by at least 1 IS
6110 copy or by a shift
in the banding pattern. A second group of 42 isolates with hybridization
bands of 3.65, 3.36, and 1.4 kb corresponds to the members of
the W-Beijing phylogenetic lineage (ancestral strains). These
isolates were previously classified at the PHRI TB Center as
members of N, HD, CI, KY, LB, and other arbitrarily designated
families. Ancestral strains cannot be recognized by IS
6110 profiling
given that they share less than 65% IS
6110 RFLP similarity with
W-Beijing family isolates. These two groups together represent
members of the W-Beijing phylogenetic lineage (
n = 148). The
third group consisted of 378 isolates which displayed one or
two bands of variable sizes in addition to 10.85- and 1.4-kb
hybridization bands. As expected, these 377 isolates were distinguished
by the absence of IS
6110 insertions in
dnaA-dnaN and NTF regions
and at least 1 copy in the DR region and therefore did not belong
to the W-Beijing phylogenetic lineage or family. With two false-negative
identifications and one false-positive identification, the sensitivity
and specificity of the W-Beijing polyprobe were determined (
n = 526) to be 98.7% (147/149) and 99.7% (377/378) (Table
2),
respectively. False-negative cases were represented by two W-family
strains that had acquired a second IS
6110 copy in the
oriC locus
(Fig.
2, lanes 13 and 14); one non-W-Beijing-related strain
with an IS
6110 insertion in a position other than A1 gave false-positive
results.

DISCUSSION
An MDR
M. tuberculosis strain associated with several nosocomial
outbreaks and high mortality rates was identified in New York
City in the early 1990s (
5,
9,
21). This strain, arbitrarily
designated strain W, has been the focus of several investigations.
While strain W was identified in New York City, a concurrent
study conducted in China reported on the predominance of a group
of
M. tuberculosis isolates characterized by a high copy number
of IS
6110 elements (15 to 24 per genome) and related IS
6110 RFLP patterns which were named the Beijing strains (
28). It
is now recognized that W, W-related strains, and the Beijing
strains are genetically related; they are now jointly referred
to as the W-Beijing or Beijing/W strain family (
3,
10). The
genetic linkage between the W-Beijing strains has been determined
by a combination of independent molecular markers, including
IS
6110-based RFLP, variable number of tandem repeats, deletion
analysis, and spoligotyping (
2-
4,
15). Furthermore, recent work
by Gutacker et al. showed that the W-Beijing strains are members
of sSNP cluster II and part of a phylogenetic lineage (
12).
In Fig.
2 we show the schematic phylogenetic relationship of
M. tuberculosis strains based on the genetic markers described
in this report.
Recent reports have indicated that disease manifestations may be dictated in part by the interplay between strains of M. tuberculosis and the host immune system. One report showed that a particular strain, 210 (a member of the W-Beijing family), was able to replicate in cultured human macrophages at a four- to eightfold-higher rate than other, unrelated strains (29). Another study observed up to a 10-fold-higher bacillary load in the lungs and poorer survival of mice infected with a W-Beijing strain compared to other strains tested (18). These investigators note that these phenomena correlated with weak host immune response induced by W-Beijing strain infection. That is, W-Beijing strains were shown to elicit a poor cytokine-dependent Th1 protective host immune response, thereby delaying intracellular M. tuberculosis elimination by macrophages, which might in part explain the success of these strains. There have been numerous reports implicating the W-Beijing strains in outbreaks, extensive transmission, and high prevalence (3, 9, 23, 28). These attributes highlight the importance of this family of strains in TB control programs and necessitate their rapid identification within TB populations.
The goal of this study was to develop a fast, accurate, and low-cost method for M. tuberculosis strain identification, capable of distinguishing W-Beijing strains in any collection of clinical isolates where IS6110 Southern blot hybridization genotyping has been previously performed. This method relies on the combination of three selected probes for rehybridization of existing membranes. The genetic probes target variable genomic loci unique to W-Beijing strains (Table 1); the proposed markers are well studied and result in effective and easy-to-interpret rehybridization pattern profiles.
The most definitive marker chosen includes the dnaA-dnaN intergenic region of the chromosomal origin of replication, the oriC locus. All W-Beijing strains have an IS6110 insertion in this region in position A1 and a 3.36-kb hybridization band, accordingly; although no W-Beijing strains lacking the A1 insertion have yet been identified in our collection, some rare isolates which contain a second insertion within this locus have been described (15). These strains lack the 3.36-kb band (in routine IS6110 RFLP patterns), and hybridization with the W-Beijing polyprobe demonstrates two additional bands of smaller size, in addition to the 3.65-kb (DR) and 1.6-kb (NTF) bands (Fig. 2B, lanes 13 and 14). It is important to recognize these exceptions, though they are rare, in order to avoid false-negative strain identification.
The second selected marker relies on a well-studied locus of the M. tuberculosis chromosome which includes a series of direct repeats, referred to as the DR region. The variability in the presence/absence of specific spacers in the DR forms the basis for spoligotyping (11). A search of the PHRI TB Center Spoligotype Database shows that most W-Beijing isolates have spoligotype NY_S00034 (octal code 000000000003771, type 1 in Spoldb3), lacking spacers 1 to 34, with positive hybridization to spacers 35 to 43; however, a few exceptions have been identified (Fig. 2, lanes 4 to 6 and 8) (2). As a result of IS6110-associated rearrangement, strains from the W-Beijing lineage have a characteristic deletion which removes part of the DR (spacers 1 through 34) and flanking chromosomal sequence (1, 27). These strains have a single 3.65-kb hybridization band that corresponds to the DR-specific fragment (Fig. 1).
The chromosomal NTF locus has been previously used as a target in multiplex PCR for the recognition of NYC W MDR strains (Fig. 2, lane 17) (24). In addition, the presence of at least 1 IS6110 copy in the NTF locus provides a useful genetic marker for the identification of closely related clinical isolates which share a high similarity of RFLP profiles and were classified as members of the W family. The NTF-specific fragment used in the W-Beijing polyprobe can differentiate NYC W MDR strains (1.1-kb hybridization band) from W-Beijing family strains (1.6-kb band) and (in combination with region-A and region-B markers) the W-Beijing lineage strains from unrelated isolates.
Taken alone, each of the three chromosomal markers described above provides limited information for the identification of W-Beijing strains. However, when the markers are combined, the W-Beijing polyprobe yields high sensitivity and specificity for proper identification. An algorithm was proposed to interpret and identify the W-Beijing polyprobe banding patterns of strains that are members of the W-Beijing family and strains that belong to the W-Beijing phylogenetic lineage and to differentiate them from other, unrelated M. tuberculosis clinical isolates (Fig. 3).
Spoligotyping, considered the "gold standard" method for W-Beijing strain identification, combines into one group isolates with high similarity of RFLP images (>65% banding similarity) and phylogenetically related strains with much lower similarity of hybridization patterns. The latter group of strains does not share all the characteristics of W-Beijing family strains (RFLP pattern similarity, <65%), and its epidemiological significance is not clearly known, although several TB outbreaks have been associated with the various representatives of the W-Beijing phylogenetic lineage (7, 20).
Recently, a protocol to identify W-Beijing strains was proposed based on computer-assisted analysis of IS6110-based RFLP profile similarity of clinical isolates with a panel of 19 W-Beijing reference strains (13). In that study, clinical isolates that shared at least 80% IS6110 banding similarity with any reference strain were determined to belong to the W-Beijing lineage. The W-Beijing polyprobe, described here, extends the recognition of isolates in at least two ways. First, the polyprobe increases the discriminatory power to identify W-Beijing strains. All 19 reference strains were positively recognized by the W-Beijing polyprobe and were further delineated as ancestral members of the W-Beijing phylogenetic lineage or as belonging to the W-Beijing family (data not shown). Second, the polyprobe, which relies not on IS6110 patterns (and is not limited by the diversity of the reference panel) but rather on defined genetic characteristics, is more inclusive.
It must be noted that although the method described in this study (W-Beijing polyprobe) gives clear results that can be easily interpreted, each hybridization subgroup has exceptions which should be recognized in order to avoid inaccurate strain identification. As an example, some unrelated M. tuberculosis strains may also have IS6110 insertions within the dnaA-dnaN region; however, they differ in the exact insertion site, and the orientation of IS6110 differs from the characteristic A1 insertion (19).
The W-Beijing polyprobe can be used to identify rare M. tuberculosis isolates lacking the IS6110 element (resulting in a non-W-Beijing hybridization profile). Additionally, the W-Beijing polyprobe is specific for members of the M. tuberculosis complex (M. africanum, M. bovis, M. canetti, M. microti, and M. tuberculosis) and does not hybridize with other mycobacterial species (e.g., M. avium, M. fortuitum, M. xenopi, M. intracellulare, M. marinum, M. phlei).
In conclusion, we have developed a rapid, effective, and low-cost method for W-Beijing strain identification. This approach highlights the possibility of using various genetic markers specific for different phylogenetic groups of M. tuberculosis strains. Currently, chromosomal markers such as SNP substitutions, deletions, or site-specific IS6110 transpositions are being used to facilitate rapid identification of other major M. tuberculosis strain families (e.g., Haarlem or LAM strain families [8, 14]).

ACKNOWLEDGMENTS
We gratefully acknowledge W. Eisner and A. Ravikovitch for assistance
in preparing the manuscript.

FOOTNOTES
* Corresponding author. Mailing address: PHRI TB Center, 225 Warren St., Newark, NJ 07103. Phone: (973) 854-3240. Fax: (973) 854-3241. E-mail:
barry{at}phri.org.


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Journal of Clinical Microbiology, May 2005, p. 2148-2154, Vol. 43, No. 5
0095-1137/05/$08.00+0 doi:10.1128/JCM.43.5.2148-2154.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
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