A nationwide study of mechanisms conferring reduced susceptibility to extended-spectrum cephalosporins in clinical Escherichia coli and Klebsiella spp. isolates

Scand J Infect Dis. 2012 Dec;44(12):927-33. doi: 10.3109/00365548.2012.707330. Epub 2012 Sep 19.


Background: Enterobacteriaceae exerting a high level of extended-spectrum cephalosporin (ESC) resistance have increased significantly in Norway in the last decade. Various mechanisms acting alone or in concert mediate variable levels of ESC resistance and pose great challenges in the implementation of screening strategies and treatment. This study was undertaken to document the prevalence of underlying mechanisms conferring resistance to ESCs in a nationwide collection of clinical isolates of Escherichia coli, Klebsiella pneumoniae, and Klebsiella oxytoca, before the increase in extended-spectrum β-lactamase (ESBL)-producing strains.

Methods: Consecutive E. coli (n = 2213), K. pneumoniae (n = 303), and K. oxytoca (n = 66) isolates from 23 Norwegian diagnostic laboratories were collected and examined for reduced susceptibility to ESCs. Isolates displaying minimum inhibitory concentrations (MICs) of > 2 mg/l by Etest to cefpodoxime and/or MICs > 1 mg/l to any other ESCs were included (n = 54; 35 E. coli, 11 K. pneumoniae, and 8 K. oxytoca). Isoelectric focusing for the detection of β-lactamases, and polymerase chain reactions (PCRs) with subsequent sequencing for detection of ESBLs CTX-M, TEM, and SHV, plasmid-mediated AmpC, OXA subtypes, and alterations of porin genes ompC and ompF, and quantitative reverse transcriptase (RT)-PCR for investigation of enhanced expression of chromosomal ampC were performed.

Results: Eight E. coli isolates (0.4%) were ESBL producers and 20 (1.0%) were hyperproducers of the chromosomal ampC. Three K. pneumoniae isolates (1.1%) were ESBL producers, and all K. oxytoca isolates (n = 8; 13.6%) were OXY-hyperproducers. No definite mechanisms for reduced susceptibility to ESCs could be inferred for 7 E. coli (0.4%) and 8 K. pneumoniae (3.0%) isolates.

Conclusions: This study identified chromosomal AmpC-hyperproducing E. coli and OXY-hyperproducing K. oxytoca in addition to ESBLs in Enterobacteriaceae as major mechanisms of resistance to ESC, and documented their rates of prevalence for the first time in Norway.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Anti-Bacterial Agents / pharmacology*
  • Cephalosporins / pharmacology*
  • DNA, Bacterial / genetics
  • Escherichia coli / drug effects*
  • Escherichia coli / isolation & purification
  • Escherichia coli Infections / microbiology
  • Genes, Bacterial
  • Humans
  • Isoelectric Focusing
  • Klebsiella / drug effects*
  • Klebsiella / isolation & purification
  • Klebsiella Infections / microbiology
  • Microbial Sensitivity Tests
  • Norway
  • Polymerase Chain Reaction
  • beta-Lactam Resistance*
  • beta-Lactamases / chemistry
  • beta-Lactamases / isolation & purification


  • Anti-Bacterial Agents
  • Cephalosporins
  • DNA, Bacterial
  • beta-Lactamases