title: Antimicrobial resistance surveillance in Escherichia coli by using normalized resistance interpretation.
abstract: OBJECTIVE
  To improve antimicrobial surveillance accuracy for results obtained by disk diffusion for porcine Escherichia coli, by comparing traditional clinical breakpoint interpretation with the Normalized Resistance Interpretation (NRI) method.
  
  
  METHODS
  The susceptibilities of 921 E. coli isolates from clinically healthy pigs at slaughter age was determined for 15 antimicrobials by the Kirby Bauer disk diffusion technique. NRI with previously established optimal controlled parameters for E. coli ATCC25922 was used to reconstruct the fully susceptible population of the tested E. coli isolates. Based on a lower limit for susceptibility, set at 2.5 standard deviations below the mean of the reconstructed susceptible population, the non-wild type percentage isolates was compared with the clinical resistance percentage.
  
  
  RESULTS
  The NRI method was applicable for 11 out of the 15 antimicrobials tested. Antimicrobials for which no normal distribution of inhibition zones for the population of susceptible isolates was seen, could not be used to reconstruct the susceptible population. Clinical breakpoints much lower than the epidemiological cut-off values resulted into presumptively identifying isolates as clinically susceptible, but likely carrying acquired resistance determinants. Otherwise, clinical breakpoints did cut through the WT population for several antibiotics tested, categorizing isolates from the WT population as not susceptible.
  
  
  CONCLUSIONS
  NRI was shown to be a valid method to define the WT population for disk diffusion outcomes, provided a normal distribution of the susceptible bacterial species population is present. Until international harmonization of breakpoints is achieved, it might give rise to a wide application in monitoring antimicrobial resistance in veterinary medicine.
authors.affiliation: Veterinary Epidemiology Unit, Department of Obstetrics, Reproduction and Herd Health, Faculty of Veterinary Medicine, Gent University, Salisburylaan 133, 9820 Merelbeke, Belgium. Electronic address: benedicte.callens@amcra.be.
authors.forename: Bénédicte
authors.initials: B
authors.lastname: Callens
authors.affiliation: Veterinary Epidemiology Unit, Department of Obstetrics, Reproduction and Herd Health, Faculty of Veterinary Medicine, Gent University, Salisburylaan 133, 9820 Merelbeke, Belgium. Electronic address: jeroen.dewulf@ugent.be.
authors.forename: Jeroen
authors.initials: J
authors.lastname: Dewulf
authors.affiliation: Department of Microbiology, Karolinska Institute, Tumor and Cell Biology, Nobels väg 16, 171 65 Solna, Sweden. Electronic address: goran.kronvall@telia.com.
authors.forename: Göran
authors.initials: G
authors.lastname: Kronvall
authors.affiliation: Scientific Institute of Public Health, Engelandstraat 642, 1180 Brussel, Belgium. Electronic address: Boudewijn.Catry@wiv-isp.be.
authors.forename: Boudewijn
authors.initials: B
authors.lastname: Catry
authors.affiliation: Department of Pathology, Bacteriology and Avian Diseases, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133, 9820 Merelbeke, Belgium. Electronic address: freddy.haesebrouck@ugent.be.
authors.forename: Freddy
authors.initials: F
authors.lastname: Haesebrouck
authors.affiliation: Department of Pathology, Bacteriology and Avian Diseases, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133, 9820 Merelbeke, Belgium. Electronic address: filip.boyen@ugent.be.
authors.forename: Filip
authors.initials: F
authors.lastname: Boyen
mesh_terms: D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D024881:Drug Resistance, Bacterial; D004926:Escherichia coli / Q000187:drug effects*; D004927:Escherichia coli Infections / Q000382:microbiology / Q000662:veterinary*; D008826:Microbial Sensitivity Tests / Q000379:methods*; D013552:Swine; D013553:Swine Diseases / Q000382:microbiology*
chemical_list: D000900:Anti-Bacterial Agents
keywords: Epidemiological cut-off value; Inhibition zone value; Monitoring antimicrobial resistance; Population distribution