Extended spectrum cephalosporin resistance among clinical isolates of Enterobacteriaceae in West Norway during 2006–2013; a prospective surveillance study

Routine surveillance of resistance to broad‐spectrum cephalosporins in Enterobacteriaceae and phenotypic identification of underlying mechanisms using a simple strategy was commenced in 2006 at our laboratory, serving West Norway. This report focuses on the results until 2013. The classical plasmid‐mediated extended spectrum beta‐lactamase (ESBL_A) among clinically relevant Escherichia coli isolates showed an increase from 0.6% to 4.3% during the surveillance period, while prevalence for other mechanisms remained stable, below 0.7%. ESBL_A in Klebsiella pneumoniae had similar prevalence in 2006 (0.6%) and 2013 (4.4%), but in between it peaked to 3.9% in 2008 and to 9.3% in 2011. Within the other species, the numbers of clinically relevant isolates and isolates‐producing ESBL_A were much lower. An increasing resistance due to hyperproduction of AmpC enzymes was seen in Enterobacter and Citrobacter, with prevalence increasing from 18% and 12.2% in 2006 to 27.5% and 26.1% in 2013, respectively. Hyperproduction of KOXY enzyme in Klebsiella oxytoca remained below 9.5% and did not show an increasing trend. The overall increase in the proportions of isolates‐producing ESBL_A in E. coli/K. pneumoniae and hyperproduction of AmpC in Enterobacter/Citrobacter necessitates measures to hinder the spread of resistant bacteria and vigilant antibiotic stewardship.