Emergence of plasmid-mediated colistin-resistance in CMY-2-producing Escherichia coli of lineage ST2197 in a Tunisian poultry farm

Abstract Our study aimed to investigate colistin resistance and the mechanisms involved in a collection of 35 extended-spectrum beta-lactamase (ESBL) and 13 CMY-2-producing E . coli strains which were previously recovered from chicken gut microbiota in Tunisia, as well as to determine the genetic location of mcr genes. Forty-eight ESBL and CMY-2-producing E . coli strains were obtained from 137 fecal samples of healthy chickens during 2013. These strains were tested for colistin resistance by the broth microdilution method, and screened for mcr-1 and mcr-2 genes by PCR. Two of these strains were colistin-resistant (MIC = 8 mg/L). Both harbored the mcr-1 gene, were CMY-2 producers, and were additionally resistant to tetracycline, ciprofloxacin, chloramphenicol, gentamicin, tobramycin and trimethoprim-sulfamethoxazole. They shared phylogroup A, the same pulsed-field gel electrophoresis (PFGE)-pattern, and were typed as ST2197. In both strains, IS Apl1 and pap2 were detected upstream and downstream of mcr-1 gene, respectively. The analysis of the two mcr-1 -positive strains and their transconjugants by PCR-based replicon typing and S1-PFGE, demonstrated that mcr-1 gene is linked to an IncP plasmid (~242 kb), and bla CMY-2 to an IncI1 plasmid (97 kb). The occurrence of E . coli harboring mcr-1 gene among intestinal microbiota in poultry and its location on a conjugative plasmid could represent a risk for public health. The evolution of this type of resistant microorganisms should be evaluated in the future.