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 , 96 I and 16 S 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® Colistin_R and CHROMagar™ 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 µg/ml as had the 3 E. coli that grew only on the CHROMID® Colistin_R agar medium and one E. coli that grew on neither agar media. Therefore, the positive predictive values of the CHROMID® Colistin_R and CHROMagar™ 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 µg/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 µg/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.