Abstract
Two mechanisms account for AmpC activity in E. coli: mutations in the ampC promoter and attenuator regions resulting in ampC overexpression and/or acquisition of plasmid-encoded ampC genes. In this study we analyzed 51 clinical E. coli isolates with reduced susceptibility to amoxicillin/clavulanic acid, piperacillin/tazobactam or third generation cephalosporins for the presence of AmpC production. Three phenotypic AmpC confirmation assays (cefoxitin/cloxacillin disk diffusion test, cefoxitin/EDTA disk diffusion test, AmpC ETest) were compared for the detection of AmpC activity. All 51 isolates were genetically characterized by mutation analysis of the chromosomal ampC promoter/attenuator region and by PCR detection of plasmid-encoded ampC genes. Altogether, 21/51 (41 %) E. coli isolates were considered true AmpC producers. AmpC activity due to chromosomal ampC promoter/attenuator mutations was found in 12/21 strains, plasmid-encoded ampC genes were detected in 8/21 isolates. 1/21 strains contained both, ampC promoter mutations and a plasmid-encoded ampC gene. All three phenotypic tests were able to detect the majority (>90%) of AmpC positive strains correctly. Cefoxitin resistance was found to be a discriminative parameter, detecting 20/21 AmpC producing strains. Susceptibility to third generation cephalosporines, e.g. ceftriaxone, cetazidime and cefotaxime, was found in 9 of the 21 AmpC positive strains. When considering the elevated zone diameter breakpoints of the 2010 CLSI guidelines 2/21 AmpC positive strains were categorized susceptible to third generation cephalosporines.
Peter-Getzlaff, S