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Journal of Clinical Microbiology, October 2001, p. 3747-3749, Vol. 39, No. 10
Department of Internal Medicine, Dankook
University College of Medicine, Chonan,1 and
Department of Pediatrics, Seoul National University College of
Medicine, Seoul,4 Korea, and Edith
Nourse Rogers Memorial Veterans Hospital,
Bedford,2 and Lahey Clinic,
Burlington,3 Massachusetts
Received 12 February 2001/Returned for modification 1 July
2001/Accepted 23 July 2001
CTX-M-14 Because resistance is more
than locally relevant with increasingly mobile populations and since
unique resistance mechanisms may evolve
anywhere antibiotics are used, we have investigated several unusually
resistant clinical isolates from Korea.
0095-1137/01/$04.00+0 DOI: 10.1128/JCM.39.10.3747-3749.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Identification of CTX-M-14 Extended-Spectrum
-Lactamase in Clinical Isolates of Shigella sonnei,
Escherichia coli, and Klebsiella
pneumoniae in Korea
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ABSTRACT
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Abstract
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References
-lactamase was identified in a stool isolate of
Shigella sonnei and in blood isolates of
Escherichia coli (one isolate) and Klebsiella
pneumoniae (two isolates) from different parts of Korea. The
amino acid sequence differed by one amino acid from CTX-M-9 (Ala-231
Val) and was identical to that of
-lactamases recently found in
China and Japan.
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TEXT
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Abstract
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FIG. 1.
The nucleotide and deduced amino acid sequences of the
CTX-M-14
-lactamase identified in four strains found in Korea. The
upstream and downstream sequences are shown in italics. *,
termination codon.
One strain of Escherichia coli and two strains of
Klebsiella pneumoniae that had high levels of resistance to
cefotaxime were isolated from the blood of patients in Seoul National
University Children's Hospital in 1995 and 1996. One strain of
Shigella sonnei isolated from a pediatric patient on Cheju
Island in 2000 also had a high level of cefotaxime resistance. By disk
susceptibility testing, the strains were resistant to amoxicillin,
cephalothin, and cefotaxime but were susceptible to ceftazidime,
cefoxitin, and amoxicillin-clavulanic acid. Isoelectric focusing showed
that the four strains produced a
-lactamase with an isoelectric
point (pI) of 8.0. PCR with SHV-specific primers (3) was
negative for all strains. Cefotaxime resistance was transferred by
conjugation (9) from K. pneumoniae
strain 95151 along with a plasmid of 160 kb to E. coli J53 Azir (met pro
azide resistant) to produce E. coli J53
Azir/pMG267. The
-lactamase gene was
cloned from plasmid pMG267 with EcoRI as an 8-kb insert into
vector plasmid pBC SK (Stratagene, La Jolla, Calif.) carrying
chloramphenicol resistance to produce plasmid pMG268. For sequencing, a
Tn7-based transposon carrying a kanamycin resistance gene
was inserted into purified pMG268 using the GPS-1 Genome Priming
System-1 kit (New England BioLabs, Beverly, Mass.), and the resulting
derivative was introduced into E. coli DH10B
(Gibco BRL, Rockville, Md.) by electroporation. After selection with 50 µg of kanamycin per ml and 30 µg of chloramphenicol per ml,
colonies were screened for loss of resistance to 100 µg of ampicillin
per ml. In ampicillin-susceptible colonies, the transposon was assumed
to have been inserted into the
-lactamase gene. With primers
(primerN and primerS) that matched nucleotides at the extremities of
the inserted transposon, cycle sequencing (Perkin-Elmer Cetus, Norwalk,
Conn.) was initiated and continued by primer walking until both DNA
strands were analyzed.
The open reading frame of the bla gene was 876 bp, and
it encoded a protein with 291 amino acid residues (Fig. 1). The deduced amino acid sequence had one amino acid difference from that of the
CTX-M-9
-lactamase discovered in Spain (10), a change
from Ala to Val at position 231. A BLAST search (2)
indicated that the 99 nucleotides upstream from
blaCTX-M-14 differed by only one base from
a sequence upstream from the gene for Toho-2
-lactamase, another
member of the CTX-M family (4). The DNA from nucleotides 882 to 1055 downstream from the bla gene was 99% identical
to the sequence of transposable element IS903 in
kanamycin-resistant transposon Tn2680
(7).
To sequence the
-lactamase genes of the other strains, they were
amplified using primers designed for a CTX-M-type gene. The primers
used for amplification were CTX-1 (5'-CGCTTTATGCGCAGACGA) and CTX-2 (5'-GATTCTCGCCGCTGAAGC). The PCR
amplification mixture was denatured at 95°C for 30 s and
annealed at 58°C for 1 min, and the chain was extended at 72°C for
1 min in a thermal cycler (Perkin-Elmer Cetus). This cycle was repeated
35 times. The nucleotide sequencing analyses were performed with the
BigDye Terminator Cycle Sequencing Ready Reaction kit and ABI 377 automated sequencer (Applied Biosystems, Inc., Foster City, Calif.).
Primers CTX-1, CTX-2, and CTX-3 (5'-TCAAAGGCAATACGACCG) were
used for sequencing. The nucleotide sequences of the genes from the
other Korean strains (K. pneumoniae 96062, E. coli 960402, and S. sonnei 153) were identical to those of K. pneumoniae 95151.
To compare the resistance plasmids of the four strains, plasmid
analyses were performed. Cefotaxime resistance was transferred by
conjugation at frequencies of 10
3 to
10
4 per donor from each strain. Plasmids
from strains 96062 (pMG269) and 960402 (pMG270) mediated
resistance to
-lactams, streptomycin, and sulfonamide, whereas
plasmid pMG267 carried resistance to amikacin, chloramphenicol,
gentamicin, streptomycin, sulfonamide, tetracycline, and
trimethoprim as well as to
-lactams. S. sonnei 153, however, transferred cefotaxime resistance alone.
Plasmids from the transconjugants were extracted and separated by
electrophoresis (6). Transconjugants from strains 960402, 96062, and 95151 each had one plasmid with a size of about 160 kb.
S. sonnei 153 contained nine plasmids of
different sizes, one of which was a plasmid of 77 kb (pMG271) that was
most likely responsible for cefotaxime resistance (Fig.
2).
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The MICs of the
-lactams were determined using Etest strips (AB
Biodisk, Dalvägen, Sweden). Similar values were found for all
strains (Table 1). The strains all had
high levels of resistance to cefotaxime but much less resistance to
ceftazidime or aztreonam. The MICs of amoxicillin decreased more than
eightfold for each strain when the drug was combined with clavulanic
acid.
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A search of GenBank detected two as yet unpublished
-lactamase gene
sequences identical to that found in the Korean isolates: one in an
E. coli isolate from the People's Republic of
China (accession number AF252622) and the other in an E. coli isolate from Japan (accession number AF311345). The
prevalence of this enzyme in these countries is not known. In our
study, we identified strains of different genera producing CTX-M-14
-lactamase in different parts of the country over a 5-year period,
suggesting dissemination throughout Korea. This
-lactamase is also
the first CTX-M-type extended-spectrum
-lactamase (ESBL) to be
identified in Korea.
In 2000, an epidemic of S. sonnei gastroenteritis
occurred on Cheju Island. During the outbreak, several strains of
S. sonnei resistant to expanded-spectrum
cephalosporins were isolated, including strain 153, which was studied
here. The widespread use of ceftriaxone during the epidemic may be the
reason for the emergence of Shigella species that produced
the CTX-M-type ESBL. Until now there have been few reports of
ESBL-producing Shigella species. In India, an
SHV-11-producing Shigella dysenteriae strain was isolated
(1), and in Japan, a Shigella flexneri strain
which produced a metallo-
-lactamase has been identified
(8). This is the third report of ESBL-producing Shigella species strains and the first report of such a
strain producing an enzyme of the CTX-M type.
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ACKNOWLEDGMENTS |
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This work was supported in part by a Merit Review award from the VA/DoD Collaborative Research Program on Mechanisms of Emerging Pathogens.
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FOOTNOTES |
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* Corresponding author. Mailing address: Lahey Clinic, 41 Mall Rd., Burlington, MA 01805. Phone: (781) 744-8508. Fax: (781) 744-1264. E-mail: george.a.jacoby{at}lahey.org.
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