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Antimicrobial Agents and Chemotherapy, April 2004, p. 1249-1255, Vol. 48, No. 4
0066-4804/04/$08.00+0 DOI: 10.1128/AAC.48.4.1249-1255.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Dissemination of CTX-M-Type ß-Lactamases among Clinical Isolates of Enterobacteriaceae in Paris, France
C. Eckert,1 V. Gautier,1 M. Saladin-Allard,2 N. Hidri,3 C. Verdet,4 Z. Ould-Hocine,4 G. Barnaud,4 F. Delisle,4 A. Rossier,4 T. Lambert,5 A. Philippon,6 and G. Arlet1,4*
Laboratoire de Bactériologie, UPRES EA 2392, UFR Saint-Antoine, Université Paris VI,1
Service de Bactériologie-Hygiène, Hôpital Tenon, AP-HP,4
Service de Bactériologie, Hôpital Saint-Michel,5
Service de Bactériologie, CHU Cochin, AP-HP, Paris,6
Service de Biologie, Hôpital Robert Ballanger, Aulnay-sous-bois,2
Service de Microbiologie, Hôpital Louis Mourier, AP-HP, Colombes, France3
Received 19 March 2003/
Returned for modification 9 August 2003/
Accepted 30 December 2003

ABSTRACT
We analyzed 19 clinical isolates of the family
Enterobacteriaceae (16
Escherichia coli isolates and 3
Klebsiella pneumoniae isolates)
collected from four different hospitals in Paris, France, from
2000 to 2002. These strains had a particular extended-spectrum
cephalosporin resistance profile characterized by a higher level
of resistance to cefotaxime and aztreonam than to ceftazidime.
The
blaCTX-M genes encoding these ß-lactamases were
involved in this resistance, with a predominance of
blaCTX-M-15.
Ten of the 19 isolates produced both TEM-1- and CTX-M-type enzymes.
One strain (
E. coli TN13) expressed CMY-2, TEM-1, and CTX-M-14.
blaCTX-M genes were found on large plasmids. In 15 cases the
same insertion sequence, IS
Ecp1, was located upstream of the
5' end of the
blaCTX-M gene. In one case we identified an insertion
sequence designated IS
26. Examination of the other three
blaCTX-M genes by cloning, sequencing, and PCR analysis revealed the
presence of a complex
sul1-type integron that includes open
reading frame ORF513, which carries the
bla gene and the surrounding
DNA. Five isolates had the same plasmid DNA fingerprint, suggesting
clonal dissemination of CTX-M-15-producing strains in the Paris
area.

INTRODUCTION
Plasmid-mediated extended-spectrum ß-lactamases (ESBLs)
are becoming increasingly frequent among clinical isolates of
the family
Enterobacteriaceae throughout the world. The emergence
of new enzyme groups that have a typical ESBL resistance phenotype
but that are non-TEM and non-SHV derivatives have recently been
reported (
11). CTX-M-type ß-lactamases constitute
a novel group of enzymes encoded by transferable plasmids. This
novel family of plasmid-mediated ESBLs has been classified in
Ambler class A (
2) and in group 2be of the Bush, Jacoby, and
Medeiros classification (
12). They are capable of hydrolyzing
broad-spectrum cephalosporins and are inhibited by clavulanic
acid, sulbactam, and tazobactam. They confer a high level of
resistance to cefotaxime but have a low level of activity against
ceftazidime (
37). The first two CTX-M-type enzymes were reported
in Europe in 1989 (
7,
8). So far, more than 30 CTX-M-type ß-lactamases
have been identified in various clinical isolates, but mostly
in enterobacterial species such as
Escherichia coli,
Klebsiella pneumoniae, and
Salmonella enterica serovar Typhimurium. These
enzymes have been classified into four major phylogenetic branches
on the basis of their amino acid sequence homologies (
10). These
clusters are CTX-M-1, CTX-M-2, CTX-M-8, and CTX-M-9. Two of
them (the CTX-M-2 and CTX-M-8 types) were recently shown to
be similar (95 to 100%) to the chromosomally encoded ß-lactamase
of
Kluyvera ascorbata and
Kluyvera georgiana, respectively (
19,
29). Although CTX-M-producing strains were initially found in
western Europe, they have now been observed over a wide geographic
area, including Latin America (
9,
10), Asia (
14,
20,
23,
24,
27,
40), some parts of eastern Europe (
6,
17,
18), and, recently,
North America (
34). In some countries, CTX-M-type enzymes are
the ESBLs most frequently isolated from gram-negative strains
(
37). They have been involved in several outbreaks (
6,
40),
although the isolation of CTX-M-producing strains remains sporadic,
including in France (
16,
30,
32).
bla genes are often associated with transferable plasmids, and some of them are parts of transposons or constitute cassettes in integrons. The association of insertion sequences (e.g., ISEcp1) that mobilize ß-lactamase-encoding genes with these ß-lactamase genes may be involved in their dissemination and expression (13). Insertion sequences such as IS26 (32) and IS903 (13, 27, 32) have also been associated with blaCTX-M genes. Unusual class 1 integrons, similar to In6 and In7, have been reported to carry antibiotic resistance genes such as blaDHA-1 (39). These sul1-type integrons with similar genetic organizations contain a partial duplication of the 3' conserved segment (3'-CS) and have an open reading frame (ORF), ORF513, between the two sul1 genes. Part of a novel complex sul1-type integron (In60), including the blaCTX-M-9 gene and its downstream nucleotide sequence, was recently characterized (31). blaCTX-M-2 and the surrounding DNA, which includes ORF513, have been reported in the complex sul1-type integron, designated In35 (3) and InS21 (15). Here we report on the diffusion of various CTX-M-type ß-lactamases among clinical isolates of the family Enterobacteriaceae in Paris, France.

MATERIALS AND METHODS
Bacterial strains and plasmids.
The 19 strains of the family
Enterobacteriaceae (16
E. coli strains and 3
K. pneumoniae strains) studied here are listed
in Table
1; they were isolated in four different hospitals between
2000 and 2002 and were identified with API 20E systems (Bio-Mérieux
SA, Marcy l'Etoile, France).
E. coli J53-2 (
pro met Rif
r) and
E. coli DH10B (Invitrogen SARL, Cergy-Pontoise, France) were
used for resistance transfer assays (conjugation and electroporation).
Antibiotic susceptibility.
Susceptibilities to antimicrobial agents that are usually active
against the
Enterobactericaeae were determined by an antibiotic
disk (Bio-Rad, Marnes-la-Coquette, France) diffusion method
on Mueller-Hinton (MH) agar (Bio-Rad). The MICs of the antibiotics,
including penicillins and cephalosporins with and without ß-lactamase
inhibitors (clavulanic acid at 2 µg/ml or tazobactam at
4 µg/ml), were determined by a dilution method on MH agar.
An inoculum of 10
4 CFU per spot was delivered with a multipoint
inoculator. ESBLs were detected by using the standard disk synergy
test (
21).
ß-Lactam resistance transfer assays.
Mating experiments were performed with E. coli J53-2 (met pro Rifr). One-milliliter volumes of cultures of each donor and the rifampin-resistant E. coli recipient strain grown in Trypticase soy broth (Bio-Rad) were mixed and incubated for 18 h at 37°C. Transconjugants were then selected on Drigalski (Bio-Rad) agar plates containing rifampin (250 µg · ml-1) and cefotaxime (2.5 µg · ml-1).
E. coli strains liable to produce colicins were plated on MH agar that had been plated with E. coli J53-2 (32).
Plasmid DNA was isolated by the method of Takahashi and Nagano (35), and 2 µl was transformed into 20 µl of E. coli DH10B cells by electroporation according to the instructions of the manufacturer (Bio-Rad). Transformants were incubated for 1.5 h at 37°C and mated on Drigalski medium supplemented with cefotaxime (2.5 µg · ml-1) or cefoxitin (40 µg · ml-1).
Plasmid DNA analysis.
Plasmid DNA from the K. pneumoniae and E. coli isolates and their corresponding E. coli transconjugants and transformants was obtained as described by Takahashi and Nagano (35) and Kado and Liu (22). Plasmid DNA was detected by electrophoresis in a 0.8% agarose gel. The molecular sizes of plasmid DNAs were estimated by comparison with the following plasmids of known sizes from the Institute Pasteur Collection: pIP112 (100.5 kb), pCFF04 (85 kb), and pIP173 (125.8 kb).
IEF of ß-lactamases.
Bacteria growing exponentially at 37°C in 50 ml of Trypticase soy broth were pelleted and sonicated with a Vibracell apparatus (twice for 30 s each time, 60 W). Isoelectric focusing (IEF) was performed on a pH 3.5 to 10 ampholine polyacrylamide gel. ß-Lactamase activity was detected with the chromogen nitrocefin (Oxoid, Dardilly, France) (25). The pI values used as standards were those of TEM-1 (pI 5.4), TEM-2 (pI 5.6), SHV-4 (pI 7.8), and CMY-2b (pI 9.3).
Characterization of ß-lactamase-encoding (bla) genes.
Detection of gene sequences coding for the TEM, CMY, and CTX-M-type enzymes was performed by PCR with genomic DNA. The oligonucleotide primer sets specific for the ß-lactamase genes used in the PCR assays are listed in Table 2. To characterize the 3' end of blaCTX-M-15, we used primers ORF1 pol M3 and M3 int upp (Table 2); primer ORF1 pol M3 was designed by using the sequence of ORF1 described downstream of the CTX-M-3 gene of Citrobacter freundii (GenBank accession no. AF550415). The blaTEM genes were characterized by PCR-restriction fragment length polymorphism (RFLP) analysis (4).
The PCR products of
blaCMY and
blaCTX-M were subjected to direct
sequencing by the dideoxy chain termination method of Sanger
et al. (
33). Both strands of the PCR products were sequenced
twice with an Applied Biosystems sequencer (model ABI 377).
The nucleotide sequences and deduced protein sequences were analyzed with the BLAST and Clustal W programs (for preparation of multiple-sequence alignments, pairwise comparisons of sequences, and preparation of dendrograms) (1, 36).
Genetic environment of blaCTX-M genes.
The genetic organization of the blaCTX-M genes was investigated by PCR, cloning, and sequencing of the regions surrounding these genes. The internal IS26 and ISEcp1 forward primers and the CTX-M reverse consensus primer (MA1 reverse) were used to investigate the promoter regions of the blaCTX-M genes. PCR primers corresponding to sequences upstream of the blaCTX-M genes (ORF513) and downstream of the blaCTX-M genes (IS903, ORF1005, ORF1, and sul1) were also used.
ß-Lactamase gene cloning was performed with plasmid DNA digested and ligated in the EcoRI or HindIII (Ozyme; New England Biolabs Inc., Saint Quentin en Yvelines, France) site of phagemid pBK-CMV (Stratagene, La Jolla, Calif.). E. coli DH10B was transformed by electroporation. The transformants harboring the recombinant CTX-M-encoding plasmids were selected on MH agar supplemented with 2.5 µg of cefotaxime per ml and 25 µg of kanamycin per ml. The molecular sizes of the inserts were estimated from the results of restriction enzyme digestion and electrophoresis in 1% agarose gels. Finally, inserts were investigated by sequencing the ends and then by PCR.
Rep-PCR and ERIC-PCR.
DNA was extracted by using the Qiagen Mini kit (Qiagen, Courtaboeuf, France). Repetitive extragenic palindromic sequence PCR (Rep-PCR) was performed with primers rep-1R and rep-2T; enterobacterial repetitive intergenic repetitive consensus sequence PCR (ERIC-PCR) was performed with primer ERIC-2, as described previously (26). The resulting products were run in 1.5% agarose gels.
Plasmid DNA fingerprinting.
Plasmid DNA was purified from transformant cells with the Qiagen Plasmid Midi kit (Qiagen), according to the recommendations of the manufacturer. For fingerprinting analysis, plasmid DNA was digested with the BamHI restriction enzyme (New England Biolabs Inc.) and subjected to electrophoresis in a 1% agarose gel at 80 V for 4 h.

RESULTS
Description of clinical isolates.
Sixteen
E. coli strains and 3
K. pneumoniae strains were recovered
between June 2000 and August 2002. Most isolates were isolated
at Tenon Hospital. The strains were associated with urinary
tract infections (11 strains), blood infection (1 strain), a
wound infection (1 strain), and vaginal or gastrointestinal
colonization (6 strains) (Table
1).
ß-Lactam susceptibility profile and associated resistance (Table 3).
All the strains were resistant to penicillins at high concentrations (MICs,
128 µg/ml), but clavulanic acid and tazobactam partially restored the activities of amoxicillin (MICs, 4 to 16 µg/ml) and piperacillin (MICs, 2 to 8 µg/ml) against all except three strains (strains TN13, TN17, and TN18). One of these strains (strain TN13) was also resistant to cefoxitin (MIC, 128 g/ml), but its transconjugant was not. All but one of the strains (strain TN13) had a higher level of resistance to cefotaxime, cefepime, and aztreonam than to ceftazidime. Similar results were observed with the transconjugants or electroporants. The disk diffusion method showed synergy between ceftazidime, cefotaxime, aztreonam, cefepime, and clavulanic acid against all the strains and their transconjugants or electroporants: these results agreed with the MICs of the extended-spectrum cephalosporins and aztreonam combined with clavulanic acid. The non-ß-lactam antibiotic resistance markers are also listed in Table 3. Fourteen isolates were resistant to aminoglycosides, and this resistance was transferred for 12 of them.
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TABLE 3. Characteristics of ESBL-producing clinical isolates and their transformants, including ß-lactam MICs for the clinical strains, their E. coli J53-2 transconjugants, and E. coli DH10B transformants and non ß-lactam antibiotic resistance markers
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Transfer of resistance.
Cefotaxime resistance transfer by conjugation was obtained for
only seven
E. coli isolates and two
K. pneumoniae isolates.
The probable production of colicins by three
E. coli isolates
(isolates TN07, TN16, and TN19), suggested by observation of
a growth inhibition zone, possibly explained why the mating-out
assays failed to yield transconjugants of these three strains.
Electroporation of plasmid DNA from the other 10 strains into
E. coli DH10B successfully transferred cefotaxime resistance.
Cefoxitin resistance was not cotransferred with the ESBL in
E. coli TN13 but was independently transferred by electroporation
by selection on cefoxitin.
Plasmids encoding ESBLs.
Plasmid DNA was isolated from all the strains and their transconjugants or electroporants. All 19 wild-type isolates had one or more plasmids. Analysis of E. coli transconjugants or electroporants expressing ESBLs revealed the presence of large plasmids, with estimated molecular sizes of 80 to >130 kb (data not shown).
IEF.
All clinical strains and their transconjugants or electroporants exhibited a band of ß-lactamase activity with an alkaline pI (7.6 to 8.4). In addition to enzymes with these pIs, 11 isolates had another band of ß-lactamase activity with a pI of 5.4. One strain (strain TN13) also had an additional band of ß-lactamase activity with a pI of >9. IEF of sonic extracts of E. coli transconjugants or electroporants revealed that all but two strains (strains TN13 and TN17) transferred the ß-lactamase activity with a pI of 5.4 (Table 3).
Characterization of ß-lactamase-encoding (bla) genes.
The results of PCR and sequence analysis are summarized in Table 3. PCR experiments were positive for blaCTX-M for all isolates and their transconjugants or electroporants. These results corresponded to pI values of 7.6 to 8.4 for CTX-M-type enzymes.
Sequence analysis of the deduced amino acid sequences showed the presence of various CTX-M-type enzymes: CTX-M-1, -2, -3, -9, and -14 and particularly CTX-M-15 (with a glycine at position 240) (23), produced by TN03, TN08, TN09, TN12, TN14, TN15, TN17, TN18, LM-01, and KP-03.
The 11 isolates harboring the enzyme with a pI of 5.4 were found to carry TEM-1 by PCR-RFLP analysis. Amplification was obtained with the CMY-specific primers for E. coli TN13 producing a ß-lactamase of pI >9, and sequencing showed that this ß-lactamase was CMY-2.
Exploration of the regions surrounding blaCTX-M genes.
PCR identified the insertion sequence ISEcp1 upstream of the blaCTX-M gene in 15 strains (Table 4). The sizes of the PCR products were about 0.8 kb in all except one strain (strain TN13). For this strain, the PCR fragment was about 2.4 kb, suggesting the insertion of an additional DNA fragment. The upstream region of blaCTX-M-15 in E. coli strain TN17, analyzed by PCR, contained the transposase gene of the insertion sequence IS26. PCR with the IS903-specific primer produced no amplicons.
The genetic organization of the
blaCTX-M genes of
E. coli TN05,
TN06, and TN19 was investigated by cloning, partial sequencing
of the regions surrounding these genes, and PCR analysis. Upstream
of the
blaCTX-M gene, these strains harbored a region common
to the complex
sul1-type integron (
38,
39). This region includes
ORF513, which is present in In
6 and In
7. Analysis of the nucleotide
sequence of the region downstream of the
blaCTX-M-9 gene from
E. coli TN05 revealed the presence of ORF1005. In strains TN06
and TN19, we found part of the 3'-CS complex
sul1-type integron
downstream of the
blaCTX-M-2 stop codon.
Epidemiological analysis.
ERIC-PCR and Rep-PCR analyses were used to analyze the molecular epidemiology of the 19 clinical isolates. Rep-PCR showed that five E. coli clinical isolates were clonally related (Fig. 1A). Plasmids isolated from transconjugants or electroporants of these five E. coli strains yielded similar restriction patterns after digestion with BamHI (Fig. 2) These strains were isolated from five different patients with no apparent relationships in time or space. ERIC-PCR of the three K. pneumoniae strains gave different restriction patterns (Fig. 1B).

DISCUSSION
We studied 19 enterobacterial strains collected in four Paris
hospitals between June 2000 and August 2002. The antimicrobial
susceptibility patterns showed that the strains harbored ESBLs
responsible for resistance to penicillin, broad-spectrum cephalosporins,
and aztreonam. The ESBLs were inhibited by clavulanate and tazobactam.
Importantly, the isolates were more resistant to cefotaxime
and aztreonam than to ceftazidime, suggesting that they were
CTX-M producers. Some CTX-M ESBLs confer high-level resistance
to ceftazidime, and this was the case for 10 of our strains.
Cefotaxime resistance could be transferred by conjugation for
9 of the 19 isolates and by electroporation for 10 isolates.
The transconjugants or electroporants showed similar resistance
profiles.
We used primers specific to each cluster of CTX-M-type enzymes. We obtained amplification of all the strains, confirming the presence of such enzymes. Analysis of the deduced amino acid sequences showed the diversity of the CTX-M-type enzymes. Indeed, we found six different enzymes, including CTX-M-15 (23), produced by 10 isolates. This member of the CTX-M family showed increased activity against ceftazidime. Sequence analysis revealed an Asp-240 Gly substitution. This substitution has already been reported in CTX-M-16 and is known to confer high-level resistance to ceftazidime (9, 23). Another substitution known to increase the hydrolyzing activity of ceftazidime is due to the Pro-167 Ser substitution in CTX-M-19 (30).
Of the 19 strains producing cefotaximases and screened for the presence of TEM ß-lactamases genes, 11 were found to express TEM-1. This association is frequent and has already been described in the literature (9, 30, 32). Lastly, E. coli strain TN13, which had a high level of resistance to cefoxitin (MIC, 128 mg · liter-1), harbored both CTX-M-14 and CMY-2; to our knowledge this is only the third report of such a combination (5, 40). All the strains yielded an enzyme with an alkaline pI, as observed for the CTX-M-type enzymes. Some had an additional band of ß-lactamase activity with a pI of 5.4 (11 strains) and, in one case (strain TN13), a third band of pI >9; these bands correspond to TEM-1 and CMY-2, respectively.
Previously reported analyses of the surrounding regions have shown the frequent association of ß-lactamase genes with the insertion sequence ISEcp1. This element was first described upstream of blaCMY-4 (P. D. Stapleton, Abstr. 39th Intersci. Conf. Antimicrob. Agents Chemother., abstr. 1457, 1999), and then ISEcp1 was detected upstream of several blaCTX-M genes (13, 14, 16, 23, 32). Of the 19 clinical isolates studied here, 15 had this same insertion sequence upstream of the blaCTX-M gene. Another insertion sequence, IS26, was described by Saladin et al. (32) to be upstream of a blaCTX-M-1 gene. This structure was found upstream of the blaCTX-M gene in one of our strains (strain TN17). It is worth mentioning that none of the 19 strains harbored the IS903 insertion sequence. Interestingly, PCR amplification with primers specific for the tnpA genes of ISEcp1 or IS26 was negative for three strains. Analysis of the surrounding regions after cloning and end sequencing allowed us to draw some primers. PCR analysis showed the presence of a structure-type integron. The sizes of the fragments were compatible with the genetic organization described in the literature for two blaCTX-M-2 sequences and one blaCTX-M-9 sequence inserted in novel complex sul1-type integrons In35, InS21, and In60, respectively (3, 15, 31). Our three blaCTX-M genes (two blaCTX-M-2 genes and one blaCTX-M-9 gene) were located in unusual class 1 integrons. ORF1005 was found in the 3' end of blaCTX-M-9 (strain TN05) when PCR amplification with the sul1-specific primer was positive with strains TN06 and TN19. ORF513 was found at the 5' ends of these bla genes in these three strains and has already been reported; ORF513 may be a transposase (38). This is the first report of a blaCTX-M gene in a complex sul1-type integron in France. These findings confirm the diversity of transposable elements and integrons associated with blaCTX-M genes.
Ten of the 19 strains harbored a CTX-M-15 enzyme, enabling us to determine whether these clinical isolates were genetically related. The results of Rep-PCR strongly suggested that five E. coli isolates with no epidemiological relationship were identical. In addition, plasmids extracted from transconjugants or electroporants yielded similar restriction patterns, supporting evidence of the dissemination of these enterobacterial strains. The corresponding patients were from three long-term-care facilities near Tenon Hospital, suggesting the presence of this enzyme in these institutions (28).
CTX-M ß-lactamases constitute a novel and rapidly growing family of plasmid-mediated ESBLs. Outbreaks have been described in several countries (6, 40). These enzymes have been reported at a much lower frequency in France (16, 30, 32). However, CTX-M ß-lactamase producers represented 50% of the ESBL-harboring E. coli clinical isolates in Tenon Hospital in 2001 and 2002 (unpublished data). A previous study described 9 CTX-M-producing strains collected over 11 years (32), while we found 19 CTX-M-producing isolates of the Enterobacteriaceae during a 2-year survey of four Paris hospitals. This work confirms the emergence of CTX-M-type enzymes and their spread in the Paris area.

ACKNOWLEDGMENTS
This work was financed by grants from Ministère de la
Recherche (réseau bêta-lactamase) and Faculté
de Médecine Saint-Antoine, Université Paris VI.

FOOTNOTES
* Corresponding author. Mailing address: Service de Bactériologie-Hygiène, Hôpital Tenon, 4 rue de la Chine, 75970 Paris cedex 20, France. Phone: 33 1 56 01 70 18. Fax: 33 1 56 01 61 08. E-mail:
guillaume.arlet{at}tnn.ap-hop-paris.fr.


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Antimicrobial Agents and Chemotherapy, April 2004, p. 1249-1255, Vol. 48, No. 4
0066-4804/04/$08.00+0 DOI: 10.1128/AAC.48.4.1249-1255.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
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