title: Detection of AmpC beta-lactamase in Escherichia coli: comparison of three phenotypic confirmation assays and genetic analysis.
abstract: Two mechanisms account for AmpC activity in Escherichia coli, namely, mutations in the ampC promoter and attenuator regions resulting in ampC overexpression and acquisition of plasmid-carried ampC genes. In this study, we analyzed 51 clinical E. coli isolates with reduced susceptibility to amoxicillin-clavulanic acid, piperacillin-tazobactam, or extended-spectrum cephalosporins for the presence of AmpC production. Three phenotypic AmpC confirmation assays (cefoxitin-cloxacillin disk diffusion test, cefoxitin-EDTA disk diffusion test, and AmpC Etest) were compared for the detection of AmpC activity. All 51 isolates were characterized genetically by mutational analysis of the chromosomal ampC promoter/attenuator region and by PCR detection of plasmid-carried 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, and plasmid-carried ampC genes were detected in 8/21 isolates. One strain contained both ampC promoter mutations and a plasmid-carried 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 extended-spectrum cephalosporins, e.g., ceftriaxone, ceftazidime, and cefotaxime, was found in 9 of the 21 AmpC-positive strains. Considering the elevated zone diameter breakpoints of the 2010 CLSI guidelines, 2/21 AmpC-positive strains were categorized as susceptible to extended-spectrum cephalosporins.
authors.affiliation: University of Zurich, Institute of Medical Microbiology, Gloriastrasse 30/32, CH 8006 Zurich, Switzerland.
authors.forename: S
authors.initials: S
authors.lastname: Peter-Getzlaff
authors.forename: S
authors.initials: S
authors.lastname: Polsfuss
authors.forename: M
authors.initials: M
authors.lastname: Poledica
authors.forename: M
authors.initials: M
authors.lastname: Hombach
authors.forename: J
authors.initials: J
authors.lastname: Giger
authors.forename: E C
authors.initials: EC
authors.lastname: Böttger
authors.forename: R
authors.initials: R
authors.lastname: Zbinden
authors.forename: G V
authors.initials: GV
authors.lastname: Bloemberg
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology; D001426:Bacterial Proteins / Q000235:genetics* / Q000378:metabolism*; D004252:DNA Mutational Analysis; D004926:Escherichia coli / Q000187:drug effects / Q000201:enzymology* / Q000235:genetics / Q000302:isolation & purification; D004927:Escherichia coli Infections / Q000382:microbiology; D006801:Humans; D008826:Microbial Sensitivity Tests / Q000379:methods; D010957:Plasmids; D016133:Polymerase Chain Reaction; D011401:Promoter Regions, Genetic; D001618:beta-Lactamases / Q000235:genetics* / Q000378:metabolism*; D047090:beta-Lactams / Q000494:pharmacology
chemical_list: D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D047090:beta-Lactams; C048795:AmpC beta-lactamases; D001618:beta-Lactamases