Objectives: 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.
Keywords: Epidemiological cut-off value; Inhibition zone value; Monitoring antimicrobial resistance; Population distribution.
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