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JOURNAL OF CLINICAL MICROBIOLOGY, Jan. 2007, p. 199–205 0095-1137/07/$08.00ϩ0 doi:10.1128/JCM.01319-06Copyright 2007, American Society for Microbiology. All Rights Reserved.
Effects of Phenotype and Genotype on Methods for Detection of Extended-Spectrum-␤-Lactamase-Producing Clinical Isolates of Escherichia coli and Klebsiella pneumoniae in Norwayᰔ Ståle Tofteland,1,2* Bjørg Haldorsen,1 Kristin H. Dahl,1 Gunnar S. Simonsen,1,5 Martin Steinbakk,3 Timothy R. Walsh,4 Arnfinn Sundsfjord,1,5* and the Norwegian ESBL Study Group Reference Centre for Detection of Antimicrobial Resistance (K-res), Department of Microbiology and Infection Control, University Hospital of North Norway, and Department of Microbiology and Virology, Institute of Medical Biology, Faculty of Medicine, University of Tromsø, Tromsø, Norway1; Department of Microbiology, Sørlandet Hospital, Kristiansand, Norway2; Department of Microbiology, Akershus University Hospital, Lørenskog, Norway3; Department of Medical Microbiology, Cardiff University, Cardiff, United Kingdom4; and Norwegian Institute of Public Health, Oslo, Norway5 Received 27 June 2006/Returned for modification 11 August 2006/Accepted 14 October 2006 Consecutive clinical isolates of Escherichia coli (n ؍ 87) and Klebsiella pneumoniae (n ؍ 25) with reduced
susceptibilities to oxyimino-cephalosporins (MICs > 1 mg/liter) from 18 Norwegian laboratories during March
through October 2003 were examined for bla
extended-spectrum-␤-lactamase (ESBL) genes,
TEM/SHV/CTX-M
oxyimino-cephalosporin MIC profiles, ESBL phenotypes (determined by the ESBL Etest and the combined
disk and double-disk synergy [DDS] methods), and susceptibility to non-␤-lactam antibiotics. Multidrug-
resistant CTX-M-15-like (n ؍ 23) and CTX-M-9-like (n ؍ 15) ESBLs dominated among the 50 ESBL-positive
E. coli isolates. SHV-5-like (n ؍ 9) and SHV-2-like (n ؍ 4) ESBLs were the most prevalent in 19 ESBL-positive
K. pneumoniae isolates. Discrepant ESBL phenotype test results were observed for one major (CTX-M-9) and
several minor (TEM-128 and SHV-2/-28) ESBL groups and in SHV-1/-11-hyperproducing isolates. Negative or
borderline ESBL results were observed when low-MIC oxyimino-cephalosporin substrates were used to detect
clavulanic acid (CLA) synergy. CLA synergy was detected by the ESBL Etest and the DDS method but not by
the combined disk method in SHV-1/-11-hyperproducing strains. The DDS method revealed unexplained CLA
synergy in combination with aztreonam and cefpirome in three E. coli strains. The relatively high proportion
of ESBL-producing E. coli organisms with a low ceftazidime MIC in Norway emphasizes that cefpodoxime
alone or both cefotaxime and ceftazidime should be used as substrates for ESBL detection.
Systemic infections with extended-spectrum-␤-lactamase alosporins and different bla groups, and (iii) the occurrence of (ESBL)-producing Enterobacteriaceae are associated with se- vere adverse clinical outcomes (7, 12, 25). It is thus essential (The results of this study were presented in part at the for a diagnostic microbiology laboratory to have updated European Congress of Clinical Microbiology and Infectious methods for the detection of ESBL-producing strains, taking into account the local epidemiology of ESBL genotypes andtheir various expression profiles. As very little is known about MATERIALS AND METHODS
ESBL genotypes in Norway, we designed a study for the de- Study design. Consecutive nonduplicate isolates of E. coli and K. pneumoniae
tection and characterization of ESBL production in clinical with reduced susceptibilities to oxyimino-cephalosporins (MIC Ͼ 1 mg/liter) isolates of Escherichia coli and Klebsiella pneumoniae with re- were collected in 18 of 24 Norwegian diagnostic microbiology laboratories cov- duced susceptibilities to oxyimino-cephalosporins from routine ering Ͼ90% of the Norwegian population from March through October 2003.
diagnostic samples. More specifically, we examined (i) the abil- Initial antimicrobial susceptibility testing was performed in each laboratory using ities of different phenotypic methods to detect ESBL-produc- agar disk diffusion systems from AB Biodisk (Solna, Sweden) or Rosco tablets(Taastrup, Denmark) on paper disk method agar (AB Biodisk) and/or the au- ing strains in relation to MICs of oxyimino-cephalosporins, (ii) ´rieux, Marcy l’Etoile, France) and the MAST the molecular basis for ESBL production by typing of the most multipoint system (Mast Diagnostics, Merseyside, United Kingdom), in agree- prevalent ␤-lactamase genes (bla ment with breakpoints from the Norwegian Working Group on Antibiotics and the relationships between MIC profiles for oxyimino-ceph- (NWGA) (16). All laboratories used either cefotaxime (CTX), ceftazidime(CAZ), or cefpodoxime (CPD) alone or two of these substrates in variouscombinations to screen for reduced susceptibility to oxyimino-cephalosporins.
Isolates expressing reduced susceptibilities to oxyimino-cephalosporins were sub- * Corresponding author. Mailing address for Ståle Tofteland: De- mitted to the Reference Centre for Detection of Antimicrobial Resistance, partment of Microbiology, Sørlandet Hospital, Serviceboks 416, 4604 Tromsø, Norway, with a registration form containing information on sex, age, Kristiansand, Norway. Phone: 47 38073309. Fax: 47 38074173. E-mail: inpatient and outpatient status, hospital department, and specimen type. Final staale.tofteland@sshf.no. Mailing address for Arnfinn Sundsfjord: bacterial identification was performed at the Reference Centre using the Vitek2 Department of Microbiology and Virology, Institute of Medical Biology, Faculty of Medicine, University of Tromsø, 9038 Tromsø, gene sequence typing in cases of low discrimination. Strains confirmed as E. coli Norway. Phone: 47 906161118. Fax: 47 77645350. E-mail: arnfinn or K. pneumoniae were included in the study.
Antimicrobial susceptibility testing. E. coli (n ϭ 89) and Klebsiella pneumoniae
ᰔ Published ahead of print on 1 November 2006.
(n ϭ 27) isolates with reduced susceptibilities to oxyimino-cephalosporins in the initial testing were examined at the Reference Centre using the following panel Isoelectric focusing of ␤-lactamases. Analytic isoelectric focusing (IEF) of
of Etest ␤-lactams (AB Biodisk) according to the manufacturer’s instructions: sonicated crude cell extracts was performed in precast Ampholine PAGplate ampicillin, amoxicillin-clavulanic acid (CLA), piperacillin, piperacillin-tazobac- polyacrylamide gels with a pH range of 3.5 to 9.5 (GE Healthcare, St. Giles, tam, cefoxitin, CPD, CTX, CAZ, cefepime (FEP), aztreonam, imipenem, and United Kingdom) using a Multiphor II apparatus (GE Healthcare). ␤-Lactamase meropenem. Vitek2 ASTN023 was used to determine susceptibility to non-␤- activity was detected with nitrocefin (0.5 g/liter). The ␤-lactamases TEM-1 (iso- lactam antibiotics. Interpretations were in accordance with NWGA guidelines.
electric point [pI] 5.4) and SHV-1 (pI 7.6) and IEF protein standards of pI 4.45 Breakpoints for cefpodoxime have not been established by the NWGA, and a to 9.6 (Bio-Rad Laboratories) were used for pI comparisons.
MIC of Ͼ1 mg/liter was thus defined as indicative of reduced susceptibility.
Quality control strains. E. coli J62 (blaTEM-3), E. coli (blaCTX-M-3), Kluyvera
Phenotypic detection of ESBL production. Phenotypic tests were performed
georgiana (blaKLUG-1), K. pneumoniae ILT-2 (blaCTX-M-14), and K. pneumoniae on the same day from the same subculture with ampicillin selection (100 mg/ ILT-3 (blaCTX-M-19), kindly provided by David Livermore and Laurent Poirel, as liter). ESBL production on isolates expressing a reduced susceptibility (MIC Ͼ well as K. pneumoniae ATCC 700603 (blaSHV-18) and E. coli ATCC 25922, were 1 mg/liter) to an oxyimino-cephalosporin (cefpodoxime and/or cefotaxime and/or ceftazidime) was examined using (i) CTX-CLA, CAZ-CLA, and FEP-CLAESBL Etests and (ii) disks containing cefpodoxime, ceftazidime, or cefotaximewith and without CLA (called the combined disk method) (Oxoid, Basingstoke, United Kingdom). An ESBL phenotype was defined by reduced susceptibility(MIC Ͼ 1 mg/liter) to an oxyimino-cephalosporin (cefpodoxime and/or cefo- ESBL detection and bla genotyping in E. coli. An ESBL
taxime and/or ceftazidime) and a significant increase in susceptibility to oxy-imino-cephalosporins tested in combination with CLA by the Etest and/or the phenotype was recognized in 52/87 (60%) isolates expressing combined disk method. For Etest analyses, ESBL production was defined as a reduced susceptibility to an expanded-spectrum cephalosporin Ն8-fold decrease in the MIC of cefotaxime, ceftazidime, or cefepime in the by Etest. Fifty isolates were ESBL positive by the combined presence of CLA or the presence of so-called phantom or deformity zones. In the disk method. Fifty-two isolates were positive by the ESBL combined disk method, ESBL production was defined as an increase of Ն5 mm Etests and included those positive by the combined disk in the zone around CLA disks compared to the zones of corresponding diskswithout CLA. In comparison, a modified version of the Jarlier double-disk synergy (DDS) method (10) for detecting CLA synergy was used. Aztreonam (30 Fifty of the 52 (96%) ESBL phenotype-positive E. coli iso- ␮g), cefpodoxime (10 ␮g), ceftazidime (30 ␮g), cefotaxime (5 ␮g), and cefpirome (30 ␮g) disks (Oxoid) were placed around an amoxicillin (20 ␮g)-clavulanic acid PCR results are summarized in Table 2. bla (10 ␮g) disk at a distance of 25 to 30 mm center to center. A clearly visible extension of the edge of the inhibition zone of any disk towards the amoxicillin- in 45 isolates (90%). CTX-M sequence grouping and typing clavulanic acid disk was interpreted as positive for CLA synergy.
performed according to the method of Bonnet (3) revealed the DNA analyses. Bacterial DNA extraction was performed in a QIAGEN model
CTX-M-1 group (n ϭ 29), the CTX-M-9 group (n ϭ 15), and M48 BioRobot (QIAGEN, Hilden, Germany) using a MagAttract DNA mini the CTX-M-2 group (n ϭ 1). Within the CTX-M-1 group, M48 kit (QIAGEN). 16S rRNA gene PCR-positive DNA extracts were screened (n ϭ 23) was the most prevalent genotype. Se- TEM, blaSHV, and blaCTX-M by consensus PCRs in a GeneAmp PCR system 9700 (Applied Biosystems, Foster City, CA) using Ap- quence typing within the CTX-M-9 group (n ϭ 15) revealed plied Biosystems standard PCR mixtures with GeneAmp PCR buffer and Taq DNA polymerase. PCR information is given in Table 1. Bidirectional sequencing was performed using a BigDye v. 3.1 cycle sequencing kit and a model 3100 21/24) types (n ϭ 10). bla genetic analyzer (Applied Biosystems). Editing and alignment of DNA se- quences were performed using the SeqMan II software package (DNAStar, Inc., TABLE 2. Distribution of ESBL genes in 50 E. coli and 19 K. pneumoniae isolates a Includes one CTX-M-16 isolate.
ESBL-PRODUCING E. COLI AND K. PNEUMONIAE IN NORWAY TABLE 3. Evaluation of ESBL Etest performance in relation to ESBL genotypes in 50 E. coli and 19 K. pneumoniae strains a ϩ, positive ESBL Etest; Ϫ, negative ESBL Etest.
b blaCTX-M-1 (CAZ/CAZ-CLA ratio, 5:3) (K4-55).
f Includes blaCTX-M-16 (K4-70).
A total of 19 ESBL genes, namely, bla negative strains. Sequence typing of the latter showed bla (n ϭ 1), were detected in 19 isolates -positive isolates. Two E. coli iso- lates were negative for CLA synergy in the combined disk method but positive by CAZ-CLA and FEP-CLA but not CTX-CLA ESBL Etests. Both isolates contained bla had phenotypic profiles consistent with hyperproduction of have previously been reported to express a non-ESBL pheno- SHV-1: moderate increases in ceftazidime MICs (2 to 4 mg/ type in K. pneumoniae (15, 30). The bla liter), wild-type cefotaxime MICs (0.125 to 0.25 mg/liter), and (n ϭ 8) genes were detected in nine strains. Discor- piperacillin-tazobactam MICs of Ͼ256 mg/liter. IEF analysis dant ESBL phenotype results were observed with three K. revealed single ␤-lactamase pI bands of approximately 7.5, pneumoniae strains. Interestingly, one bla consistent with an SHV-like enzyme in both strains (data not pressed significant CLA synergy in FEP-CLA and CAZ-CLA shown). In summary, the overall prevalence of ESBLs in E. coli ESBL Etests but was negative in the combined disk method.
isolates was 50 of 87 isolates (58%).
The difference in inhibition zones of ceftazidime disks with and The accuracy of the DDS method was evaluated in compar- without CLA was 3 mm (24 versus 21 mm).
ison to the results obtained by ESBL Etests, the combined disk A similar phenotypic profile was observed for one blaSHV-1 method, and bla typing. The DDS method revealed CLA syn- ergy with at least one substrate with 55/87 isolates, including all ogous to those of the two SHV-1-hyperproducing E. coli strains strains that scored positive in the ESBL Etest analysis. The two described above. They were consequently regarded as SHV-1 SHV-1-hyperproducing strains also expressed CLA synergy by and SHV-11 hyperproducers. In summary, the overall preva- the DDS method. Interestingly, three isolates with negative lence of ESBL-positive strains in the K. pneumoniae collection results by both the ESBL Etest and the combined disk method was 19/25 (76%). The lack of chromosomally encoded AmpC displayed reproducible CLA synergy to aztreonam and cefpi- ␤-lactamases in the genus Klebsiella may explain the relatively rome in the DDS method. The isolates shared common fea- higher occurrence of ESBL production in K. pneumoniae than tures, including CLA synergy to aztreonam and cefpirome and in E. coli isolates (50/87; 58%) with reduced susceptibilities to gle ␤-lactamase band for which the pI was 9.0, consistent with All 25 K. pneumoniae isolates with reduced susceptibilities to their AmpC profile of moderately elevated cefoxitin MICs (32 oxyimino-cephalosporins were examined by the DDS test.
to 48 mg/liter) and increased MICs of oxyimino-cephalospo- CLA synergy was observed in 21 strains that were identical to rins and aztreonam (cefpodoxime, 24 to 48 mg/liter; cefo- the ESBL Etest-positive strains. The putative SHV-1- and taxime, 2 to 4 mg/liter; ceftazidime, 2 to 6 mg/liter; and aztreo- SHV-11-hyperproducing strains also expressed CLA synergy in nam, 2 to 4 mg/liter). In summary, we have no explanation for the CLA synergy observed in 3 out of 55 DDS-positive isolates.
Performance of ESBL tests. The performance of ESBL
These strains were not defined as ESBL positive.
Etests and the combined disk method in the detection of ESBL ESBL detection and bla genotyping in K. pneumoniae. An
production in E. coli and K. pneumoniae is summarized in ESBL phenotype was recognized in 21/25 (84%) K. pneumoniae Tables 3 and 4. Significant CLA synergy was easily detected by isolates expressing reduced susceptibility to an expanded-spec- all substrates in both methods for the most prevalent ESBL trum cephalosporin by the Etest. Eighteen isolates were ESBL positive by the combined disk method. Twenty-one isolates notypes in E. coli and K. pneumoniae, respectively. However, were positive by the ESBL Etests and included those positive discordant ESBL test results were observed in one major and TABLE 4. Evaluation of the performance of the combined disk method in relation to ESBL genotype in 50 E. coli and Combined disk method results witha: a ϩ, positive combined disk test; Ϫ, negative combined disk test.
b blaTEM-128 (n ϭ 2; K4-71 and K5-25), blaCTX-3/32/22 (K5-58), blaCTX-3/22 (K8-8) (n ϭ 2), and blaCTX-M-1 (n ϭ 2).
c blaSHV-2 (K5-30) and blaSHV-2a (K2-36).
d blaSHV-28 (K2-79). The difference between the ceftazidime disk diameter with clavulanate and the one without was 3 mm (24 versus 21 mm).
genogroup E. coli strains, except for a single were positive by deformity in the ellipse only, whereas the two CTX-M-16 isolate, scored negative in the ceftazidime com- strains had the marginally positive CAZ/CAZ- bined disk method (14/15; 93%), in contrast to the results of CLA MIC ratios 12 (K5-58) and 11 (K8-8), respectively. More- the CAZ-CLA ESBL Etest, by which most of these strains (14/15; 93%) scored positive. However, a CAZ/CAZ-CLA strains scored negative in one or two of the ESBL Etests MIC ratio of Ն8 was observed only for the CTX-M-16 strain, (Table 3). Corresponding results were obtained by the com- whereas the other 13 CTX-M-9-positive strains scored positive bined disk method, except with one ESBL bla by the CAZ-CLA ESBL Etest by the appearance of deformity moniae strain (K2-79) that scored negative for all substrates in the ellipse only (the “eagle effect”). The single bla genogroup K. pneumoniae strain scored positive for ESBL pro- MIC profiles for ESBL-producing strains. The MIC means
duction by both methods using all three oxyimino-cephalospo- and ranges for oxyimino-cephalosporins and aztreonam within the different ESBL genogroups are presented in Table 5.
The minor ESBL types with aberrant ESBL test results in- CTX-M and SHV ESBL-producing strains generally expressed cluded six E. coli strains within the bla cefotaximase and ceftazidimase profiles, respectively. The MICs of all substrates for the strains with the most-prevalent were negative by the ceftazidime combined disk test (Table 4).
In contrast, all these strains were positive by the CAZ-CLA pneumoniae, were high to moderate.
genogroup E. coli strains expressed low MICs of TABLE 5. MIC means and ranges for oxyimino-cephalosporins and aztreonam in relation to ESBL genogroups of E. coli and K. pneumoniae a The value 256 ␮g/ml has been used to calculate the mean for strains displaying a MIC of Ͼ256 mg/liter. For simplicity, mean values are also given for genotypes b The blaCTX-M-9 genogroup but without the single blaCTX-M-16 strain.
c One blaSHV-5 isolate (K4-49) displayed an aberrant phenotype. For this isolate, the CPD, CAZ, and aztreonam MICs were 256 mg/liter; the CTX MIC was 64 mg/liter; and the cefoxitin MIC was 12 mg/liter.
ESBL-PRODUCING E. COLI AND K. PNEUMONIAE IN NORWAY ceftazidime (mean, 0.55 mg/liter) and aztreonam (mean, 1.8 tent with the emergence of CTX-M-producing E. coli strains mg/liter), as described previously (4, 24), except for the CTX- M-16 isolate. Interestingly, some of the minor bla genotypes The various genotypes and phenotypic expression patterns showed clinically significant differences in MIC levels for dif- detected in this study challenge the sensitivities of ESBL de- tection methods. The predominant CTX-M-15-like enzymes mean MICs for cefpodoxime (48 mg/liter) and cefotaxime (9 were easily detected by all methods due to their broad oxy- mg/liter) than for ceftazidime (0.63 mg/liter) and aztreonam (1 imino-cephalosporin substrate profile (11, 20). However, the ceftazidime combined disk method failed to detect 14 out of 15 group E. coli strains were intermediately susceptible to ceftaz- CTX-M-9-like enzymes in E. coli isolates, in contrast to the idime (MICs, 1 to 8 mg/liter). Interestingly, the two bla CAZ-CLA ESBL Etest results showing the presence of de- positive K. pneumoniae strains expressed comparatively low formed inhibition ellipses. Interestingly, all CTX-M-9-produc- MICs for cefotaxime (0.19 and 1 mg/liter) and cefpodoxime ing E. coli strains showed reduced susceptibility to cefpo- (0.75 and 12 mg/liter). The K2-79 strain, with a cefpodoxime doxime (mean, 55 mg/liter) and expressed significant CLA MIC of 0.75 mg/liter, scored negative in the combined disk synergy in the combined disk test using cefpodoxime as the Coresistance. The majority of ESBL-positive E. coli strains
Discrepancies between different detection methods were (36/50; 70%) expressed resistance to two or more non-␤-lac- also observed for some of the minor E. coli ESBL genotypes.
tam antibiotics (aminoglycosides [AG], fluoroquinolones [FQ], The two TEM-128 strains had a CTX-M-9-like phenotype and nitrofurantoin [NIT], and/or trimethoprim-sulfamethoxazole scored negative in the ceftazidime combined disk method.
[SXT]) and were defined as multidrug resistant (MDR). Six- has a T-to-G mutation, causing an Asp1573Glu substitution (Ambler numbering). ESBL activity was not ob- tant to three or more non-␤-lactam antibiotics, in contrast to served in a TEM-128 E. coli strain recently isolated from food animals in Denmark (17), in contrast to the TEM-128-positive (67%), 18 (40%), and 33 (73%) out of 45 CTX-M-positive strains in this study expressing cefotaximase activity.
strains were resistant to AG, FQ, NIT, and SXT, respectively.
SHV-5-like enzymes expressing a ceftazidimase profile pre- dominated among Norwegian clinical K. pneumoniae strains strains (n ϭ 5) was resistant to more than two non-␤-lactam (79%). Six isolates containing SHV-28-like (n ϭ 2) and SHV- antibiotics, and those strains were all susceptible to FQ.
2-like (n ϭ 4) enzymes expressed low MICs of cefotaxime and Twelve (63%) of the ESBL-producing K. pneumoniae strains ceftazidime. Accordingly, these strains scored negative in the expressed MDR. Nine (45%), 3 (15%), 18 (90%), and 4 (20%) ESBL confirmation tests when low-MIC substrates were used K. pneumoniae strains were resistant to AG, FQ, NIT, and SHV-1 hyperproduction in E. coli and K. pneumoniae has Epidemiological data. ESBL-producing strains were de-
previously been reported to mediate an increased MIC of tected in 16 laboratories. There were no indications of noso- ceftazidime (MIC Ͼ 1 mg/liter) and CLA synergy, suggesting comial outbreaks during the study period. Urinary tract iso- ESBL production (13, 21, 29). The three putative SHV-1- lates (n ϭ 42), specifically, 34 (68%) E. coli isolates and 8 hyperproducing E. coli and K. pneumoniae strains, as well as (42%) K. pneumoniae isolates, were dominant. ESBL-positive the SHV-11-hyperproducing K. pneumoniae strain, showed in- blood culture isolates were not detected. Twenty-two (44%) of consistent results by the ESBL detection methods. They scored the E. coli isolates were from outpatients; however, hospital positive for ESBL production in FEP-CLA and CAZ-CLA contact cannot be ruled out as the means of ESBL-positive ESBL Etests, in contrast to results by the combined disk strain acquisition. We did not detect any specific bla method, which were negative for CLA synergy using all three genogroups in hospitalized or nonhospitalized patients, and substrates. The basis for this discrepancy is not known, but it MDR phenotypes were detected in both groups. Only two could be due to differences in CLA content. The concordance (10%) K. pneumoniae isolates were from outpatients.
between the ESBL Etest and combined disk method resultswas otherwise excellent.
The overall high sensitivities of the ESBL Etests, as well as DISCUSSION
the sensitivity of the combined disk method, for this collectionof clinical E. coli and K. pneumoniae strains were based on the We have examined ESBL genotypes and phenotypes of Nor- combined use of cefotaxime and ceftazidime or cefpodoxime wegian clinical E. coli and K. pneumoniae isolates collected in alone as screening substrates for CLA synergy. All ESBL a prospective multicenter study during an 8-month period in strains were detected by using both cefotaxime and ceftazidime 2003. The relatively low breakpoint (MIC Ͻ 1 mg/liter) for in combination with CLA. The single SHV-28 K. pneumoniae susceptibility to oxyimino-cephalosporins in Norway ensures strain that failed in the detection of CLA synergy with cefpo- the detection of clinically relevant ESBL expression. The study doxime was associated with a low cefpodoxime MIC (0.75 design did not allow for any estimate of the prevalence of ESBLs. However, the Norwegian surveillance system for anti- The detected ESBL genotypes seem to be representative of microbial resistance showed a prevalence of ESBL production those circulating in Norway. The dominance of CTX-M ESBLs below 1% in clinical isolates of E. coli and K. pneumoniae in in E. coli and SHV ESBLs in K. pneumoniae has been verified 2003 (1). We observed that CTX-M was the most common in the 2 years following this study. CTX-M-15- and CTX-M- ESBL type in Norwegian E. coli (90%) isolates. This is consis- 9-like phenotypic patterns confirmed by positive consensus CTX-M PCRs were observed in 232/282 (82%) clinical ESBL- Iveland and Anne-Elise Johansen (Buskerud Hospital), Trond Jacobsen positive E. coli strains submitted to the Reference Centre dur- (St. Olavs UH), Pål Jenum (Asker and Bærum Hospital), Anne LiseBru and Astrid Lia (Vestfold Hospital), Einar H. Aandahl and Ingunn ing 2004 to 2005 (unpublished results). Thus, cefpodoxime Haavemoen (Innlandet Hospital), Eivind Ragnhildstveit and Anne alone or the combined use of cefotaxime and ceftazidime could Cath Hollekim (Østfold Hospital), Yngvar Tveten (AS Telelab), Ove be recommended as screening substrates for ESBL-mediated Pedersen (Innhered Sykehus), Dag Hvidsten (UH of North Norway), reduced susceptibility to oxyimino-cephalosporins.
and Sølvi Noraas and Torill S. Larsen (Sørlandet Hospital).
The DDS test is an inexpensive and easy-to-use method for We thank Merete Birkely, Aase-Mari Kaspersen, Manuela Kramer, and the medium production unit for excellent technical assistance.
the detection of CLA synergy with various substrates and dem- Also, thanks go to Inger Sperstad for creating the Access database.
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