Assessment of two selective agar media to isolate colistin-resistant bovine Escherichia coli: Correlation with minimal inhibitory concentration and presence of mcr genes

The identification of colistin-resistant enterobacteria in veterinary medicine is impaired by the absence of first line reliable phenotypic assay. The purpose of this study was to assess two selective agar media for the detection of colistin-resistant bovine pathogenic Escherichia coli. A total of 158 E. coli (46 R < resistant >, 96 I < intermediate > and 16 S < sensitive > at the disk diffusion assay) isolated between 2013 and 2018 from < 3 month-old calves suffering enteritis or septicaemia, were (i) tested by the broth dilution assay to determine colistin Minimal Inhibitory Concentrations (MIC); (ii) streaked on CHROMID (R) Colistin_R and CHROMagar (TM) COL-APSE agar plates; (iii) submitted to a pentaplex PCR to identify the presence of mcr-1 to mcr-5 genes. Of the 92 E. coli growing on both agar media, 90 had a MIC > 2.0 mu g/ml as had the 3 E. coli that grew only on the CHROMID (R) Colistin_R agar medium and one E. coil that grew on neither agar media. Therefore, the positive predictive values of the CHROMID Colistin_R and CHROMagarr (TM) COL-APSE agar media were both 0.98 whereas their negative predictive values were 0.98 and 0.94, respectively. Also noteworthy 43 of the 46 R isolates had a MIC > 2.0 mu g/ml and grew on both selective media as did half of the 96 I isolates and only 1 of the S isolates. Conversely, only 30 of the 90 isolates that grew on both agar media and with a MIC > 2.0 mu g/ml tested positive for the mcr-1 or mcr-2 genes with the pentaplex PCR. These two selective agar media can be used to reliably detect colistin-resistant E. coli. Positive growth was highly correlated with R results at the disk diffusion assay, but not with the presence of mcr genes.