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Antimicrobial Agents and Chemotherapy, August 2006, p. 2820-2823, Vol. 50, No. 8
0066-4804/06/$08.00+0 doi:10.1128/AAC.00324-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Evolution of Drug Resistance in Different Sublineages of Mycobacterium tuberculosis Beijing Genotype
Igor Mokrousov,1*
Wei Wei Jiao,2
Gui Zhi Sun,3
Jia Wen Liu,3
Violeta Valcheva,4
Mo Li,2
Olga Narvskaya,1 and
A Dong Shen2*
Laboratory of Molecular Microbiology, St. Petersburg Pasteur Institute, 197101 St. Petersburg, Russia,1
Public Central Laboratory, Beijing Pediatric Institute, Beijing Children's Hospital Affiliated to Capital University of Medical Sciences, 56 Nan Li Shi Road, Xi Cheng District, Beijing 100045, China,2
Clinical Laboratory, Beijing Chest Hospital, Wenquan Town, Haidian District, Beijing 100095, China,3
Department of Pathogenic Bacteria, The Stephan Angeloff Institute of Microbiology, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria4
Received 16 March 2006/
Returned for modification 15 May 2006/
Accepted 2 June 2006

ABSTRACT
We compared the population structure and drug resistance patterns
of the
Mycobacterium tuberculosis strains currently circulating
in the Beijing area of China. One hundred thirteen of 123 strains
belonged to the Beijing family genotypes defined by spoligotyping.
The Beijing genotype strains were further subdivided into old
and modern sublineages on the basis of NTF locus analysis. A
stronger association with resistance to the more recently introduced
antituberculosis drugs has been observed for old versus modern
strains of the Beijing genotype, suggesting that its different
sublineages may differ in their mechanisms of adaptation to
drug selective pressure.

TEXT
In some areas of the world, the increased rate of multidrug-resistant
(MDR) tuberculosis (TB) appears to be linked to a disequilibrium
in the local population structure of
Mycobacterium tuberculosis,
manifested as a predominance of particular genetic lineages
and sublineages. For example, strains of the Beijing genotype
are endemically prevalent in eastern Asia, South Africa, and
northern Eurasia (reviewed in references
1 and
9), and new unexpected
routes of their transmission are being uncovered (
7). This genotype
was first identified in
M. tuberculosis strains isolated in
the Beijing area of China, for which it was named (
23). Currently,
these strains attract great attention worldwide because they
demonstrate important pathogenic features (
15,
24) and association
with drug resistance (
4,
9).
China is one of the MDR TB hot spots, along with Russia and India (6). The increased rate of drug-resistant and MDR strains of M. tuberculosis remains a serious problem of TB control in China (14, 20). In the present study, we investigated the current population structure of M. tuberculosis in the Beijing area of China and compared it with drug resistance patterns in order to gain new insights into the evolution of drug-resistant TB.
One hundred twenty-three M. tuberculosis isolates were recovered from 123 adult pulmonary TB patients admitted to the Beijing Chest Hospital in 2002 to 2005. No epidemiological connection of these patients could be detected by standard investigation. For each patient, only the first available isolate was included in this study. Löwenstein-Jensen medium was used for cultivation of isolates. Testing of susceptibility to rifampin (RIF), isoniazid (INH), streptomycin (STR), ethambutol (EMB), and pyrazinamide (PZA) was done by the method of absolute concentration as previously described (3).
DNA from cultured cells was extracted as described by van Embden et al. (22). Strain differentiation was performed by spoligotyping (11). A PCR approach was used to determine a possible IS6110 insertion(s) in the NTF region of the M. tuberculosis Beijing genotype strains (18). Three NTF variants of the Beijing strains are distinguished on the basis of the presence or absence of the IS6110 sequence, thus providing a rough subdivision within this genotype. The W branch prevalent in the United States harbors two head-to-tail IS6110 insertions separated by a 556-bp noncoding spacer (13). Most Beijing strains worldwide harbor only one IS6110 insertion (1, 13); we previously defined them as the NTF::IS6110 "modern" branch (16). Finally, the Beijing strains without an IS6110 insertion in the NTF region (12, 13) were previously defined as atypical (12, 17), ancient/primordial (16), or ancestral (17, 19); here, we assigned these strains to an "old" sublineage of the Beijing genotype.
Odds ratios and P values, were calculated with EpiCalc software (8).
Results and discussion.
A total of 123 M. tuberculosis strains were included in this study. They comprised 40 pansusceptible and 83 resistant isolates randomly selected among those isolated between 2002 and 2005 in Beijing, China. A majority of the resistant strains were multidrug resistant; the distribution of the drug resistance profiles of all of the strains is shown in Table 1.
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TABLE 1. Drug resistance profiles of all of the M. tuberculosis strains studied and of modern versus old Beijing genotype strains
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All strains were subjected to spoligotyping in order to assess
their genetic relatedness. This genotyping method subdivided
the 123 strains into 14 types (Table
2). The largest cluster
(profile T1, Table
2) had a nine-signal spoligoprofile (signals
35 to 43) known to be a characteristic signature of the Beijing
genotype (
1,
9,
12). Additionally, eight strains (types T2 to
T7) had abridged Beijing-like profiles with some of the nine
signals absent. Thus, 113 (91.9%) of the 123 strains in this
study belonged the Beijing family genotypes. The remaining spoligoprofiles
(T8 to T14) were compared against the global spoligotype database
SpolDB 4.0 (
2) and the spoligotype signatures of the different
M. tuberculosis families (
2,
5). Accordingly, the respective
codes (types and families) were assigned where available (Table
2).
A further rough subdivision within the Beijing genotype was
performed by analysis of the NTF region. It revealed that 27
strains had an intact NTF region (old Beijing genotype sublineage)
while 86 strains had one IS
6110 direct insertion on the right
side of the NTF region (modern Beijing genotype sublineage).
In our sample, old Beijing strains constituted 24% of the isolates.
Previously, this rate was reported to be 5% in Russia (
17),
14% in Hong Kong, and 25% in Vietnam (
12). Mokrousov et al.
(
16) hypothesized that primary dispersal of the Beijing genotype
strains took place in China and had been driven by Neolithic
Proto-Sino-Tibetan farmers, whereas introduction of these strains
into northern Eurasia was historically recent and might be associated
with expansion of the Mongol empire in the 13th to the 15th
centuries. In this view, it seems that the ratio of the old
(ancient [16]) versus modern Beijing sublineages is a specific
feature reflecting how "old" or "young," as a whole, a Beijing
strain population circulating in a given geographic area is,
although it may be influenced by TB control measures such as
therapy and vaccination.
The Beijing genotype strains constituted the majority of our collection (91.9%), and we further compared the distribution of drug resistance patterns within the Beijing genotype sample for old versus modern Beijing genotype sublineages. Interestingly, the spectrum of resistance patterns was much wider in the modern versus old Beijing group although this could mainly be due to the higher number of isolates in the modern group (Table 1). Our comparison revealed that resistance to RIF and PZA, as well as MDR and HRZ patterns, was statistically significantly associated with old Beijing strains (Tables 1 and 3). In the case of INH, this association was not significant while STR resistance was very weakly associated with modern Beijing strains. Similar observations on STR and INH resistance and multidrug resistance in modern (typical) and old (atypical) Beijing genotype strains have been made by Kremer (12) for strains from Hong Kong and Vietnam. Anti-TB chemotherapy was only introduced some 60 years ago (10), and neither old nor modern Beijing strains were subjected to the selective pressure of the drugs before then. A comparison of the order of implementation of the particular drugs used for anti-TB therapy with P values in Table 3 revealed an enigmatic and somewhat quantitative association pattern. It shows that resistance to the historically more recently introduced drugs (RIF and PZA) is associated with old Beijing strains and furthermore is the most prominent for the most recently implemented drug, PZA. Contrarily, this difference between old and modern Beijing sublineages is smoothed and even reversed for INH and STR, respectively, which were introduced earlier (Table 3).
It may be that different mechanisms including second-order selection
acting on DNA repair systems (
19,
21) lie behind mycobacterial
adaptation and, in particular, behind the development of drug
resistance in different sublineages of the Beijing genotype.
The observed variation of the drug resistance patterns in the
M. tuberculosis Beijing strain populations in different countries
may be explained by different ratios of modern to old Beijing
genotype subpopulations in the local populations of
M. tuberculosis.
Perhaps the prevalence of the modern Beijing strains in the
currently circulating global population of this genotype results
not from drug-driven selection but from a real situation prior
to the advent of anti-TB therapy 60 years ago, an evolutionarily
negligible period of time. It seems reasonable to assume that
old (ancestral [17, 19], ancient [16], or atypical [12]) strains
were also less numerous than modern "progeny" (typical [12])
strains in that population of the Beijing genotype. Alternatively,
the current worldwide dissemination of the modern Beijing genotype
strains may be due to their hypervirulent features rather than
increased mutability leading to the rapid acquisition of drug
resistance, although further studies of this effect are undoubtedly
required.

ACKNOWLEDGMENTS
I.M. and W.W.J. contributed equally to this study.

FOOTNOTES
* Corresponding author. Mailing address for Igor Mokrousov: Laboratory of Molecular Microbiology, St. Petersburg Pasteur Institute, 14 Mira Street, 197101 St. Petersburg, Russia. Phone: 7812 2332149. Fax: 7812 2329217. E-mail:
igormokrousov{at}yahoo.com. Mailing address for A Dong Shen: Public Central Laboratory, Beijing Pediatric Institute, Beijing Children's Hospital Affiliated to Capital University of Medical Sciences, 56 Nan Li Shi Road, Xi Cheng District, Beijing 100045, China. Phone: 8610 68028401. Fax: 8610 68011503. E-mail:
shenad{at}sohu.com.


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Antimicrobial Agents and Chemotherapy, August 2006, p. 2820-2823, Vol. 50, No. 8
0066-4804/06/$08.00+0 doi:10.1128/AAC.00324-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
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