The AmpC phenotype in Norwegian clinical isolates of Escherichia coli is associated with an acquired ISEcp1-like ampC element or hyperproduction of the endogenous AmpC.
OBJECTIVE
The aim of the study was to examine resistance mechanisms associated with an AmpC phenotype in Norwegian clinical isolates of Escherichia coli.


METHODS
Clinical E. coli isolates (n = 106) with reduced susceptibility to third-generation cephalosporins without clavulanic acid synergy were collected from 12 Norwegian laboratories from 2003 to 2005. Twenty-two isolates with an AmpC phenotype were selected for further characterization by PFGE, isoelectric focusing, different PCR-based techniques, DNA sequencing, AmpC qRT-PCR, transfer studies and plasmid analyses.


RESULTS
The 22 isolates were not clonally related by the PFGE analysis. All isolates expressed a beta-lactamase with a pI of 9.0-9.2. Ten isolates contained a bla(CMY) gene, which was linked to an ISEcp1-like element in all cases. Twelve isolates had mutations or insertions in the promoter or the attenuator regions, leading to increased expression of the chromosomal ampC gene. One of these isolates had an ISEc10 element inserted upstream of the chromosomal ampC gene.


CONCLUSIONS
This is the first molecular study of Norwegian clinical E. coli isolates with an AmpC phenotype. Resistance was mediated either by expression of bla(CMY) from acquired ISEcp1-like-bla(CMY) elements, or by mutations or insertions in the chromosomal ampC gene control region leading to hyperproduction of the endogenous AmpC enzyme. There was no correlation between the level of ampC mRNA and the MICs of cephalosporins.
Haldorsen
Bjørg
B
Reference Centre for Detection of Antimicrobial Resistance, Department of Microbiology and Infection Control, University Hospital of North-Norway, Tromsø, Norway.
Aasnaes
Bettina
B
Dahl
Kristin H
KH
Hanssen
Anne-Merethe
AM
Simonsen
Gunnar S
GS
Walsh
Timothy R
TR
Sundsfjord
Arnfinn
A
Lundblad
Eirik W
EW
D001426:Bacterial Proteins / Q000096:biosynthesis* / Q000737:chemistry / Q000235:genetics*; D001483:Base Sequence; D003227:Conjugation, Genetic; D016172:DNA Fingerprinting; D004251:DNA Transposable Elements; D004269:DNA, Bacterial / Q000737:chemistry / Q000235:genetics; D016521:Electrophoresis, Gel, Pulsed-Field; D004926:Escherichia coli / Q000187:drug effects* / Q000201:enzymology* / Q000235:genetics / Q000302:isolation & purification; D004927:Escherichia coli Infections / Q000382:microbiology*; D020869:Gene Expression Profiling; D006801:Humans; D007525:Isoelectric Focusing; D007526:Isoelectric Point; D008826:Microbial Sensitivity Tests; D008969:Molecular Sequence Data; D016254:Mutagenesis, Insertional; D009154:Mutation; D009664:Norway; D010957:Plasmids / Q000032:analysis; D016133:Polymerase Chain Reaction; D011401:Promoter Regions, Genetic; D020133:Reverse Transcriptase Polymerase Chain Reaction; D016415:Sequence Alignment; D017422:Sequence Analysis, DNA; D018440:beta-Lactam Resistance; D001618:beta-Lactamases / Q000096:biosynthesis* / Q000737:chemistry / Q000235:genetics*
D001426:Bacterial Proteins; D004251:DNA Transposable Elements; D004269:DNA, Bacterial; C048795:AmpC beta-lactamases; D001618:beta-Lactamases