Proposed Epidemiological Cutoff Values for Ceftriaxone, Cefepime, and Colistin in Salmonella

Abstract

We tested a diverse set of 500 isolates of nontyphoidal Salmonella enterica subsp. enterica from various animal, food, and human clinical sources for susceptibility to antimicrobials currently lacking epidemiological cutoff values (ECOFFs) set by the European Committee on Antimicrobial Susceptibility Testing. A consortium of five different laboratories each tested 100 isolates, using broth microdilution panels containing twofold dilutions of ceftriaxone, cefepime, and colistin to determine the minimum inhibitory concentrations of each drug when tested against the Salmonella isolates. Based on the resulting data, new ECOFFs of 0.25 μg/mL for ceftriaxone, 0.12 μg/mL for cefepime, and 2 μg/mL for colistin have been proposed. These thresholds will aid in the identification of Salmonella that have phenotypically detectable resistance mechanisms to these important antimicrobials.

Keywords: ECOFF; Salmonella; antimicrobial resistance; cefepime; ceftriaxone; colistin.

FIG. 1.

Nontyphoidal Salmonella enterica subsp. enterica MIC distributions for isolates from food animal, retail food, and human sources for (A) ceftriaxone, (B) cefepime, and (C) colistin. Raw counts depict the observed number of isolates at each MIC, with the fitted line of the MIC distribution modeled by ECOFFinder to include 99.0% of the wild-type isolates below the ECOFF. Vertical black lines indicate the proposed ECOFF dividing wild-type strains from nonwild-type strains presumed to have acquired resistance mechanisms. ECOFF, epidemiological cutoff values; MIC, minimum inhibitory concentration.

FIG. 2.

MIC distributions for each of five laboratories for (A) ceftriaxone, (B) cefepime, and (C) colistin. The height of each of the bars refers to the number of isolates with particular MIC values (depicted in the legend). All units are in μg/mL.