Characterization of an rmtB-carrying IncI1 ST136 plasmid in avian Escherichia coli isolates from chickens.

The rmtB gene, one of the 16S rRNA methylase genes whose products confer high-level resistance to aminoglycosides, is most prevalent among Enterobacteriaceae strains. In this study, eight non-duplicate rmtB-carrying avian Escherichia coli strains from a farm in China were isolated and characterized, and further examined by phylogenetic grouping, conjugation experiments and PCR-based replicon typing. In addition, the genetic environment of rmtB was investigated by cloning and sequencing. Six rmtB-carrying E. coli were identified as phylogroup A, sequence type (ST) 156 (A-ST156), with two assigned to D-ST117; however, all of them carried the same IncI1 ST136 plasmid. The genetic environment of the rmtB gene in these eight plasmids was the same, as shown by PCR mapping. A multidrug-resistant region carrying blaTEM-1, rmtB, a class 1 integron cassette array (intI1-dfrA12-orfF-aadA2-qacEΔ1-sul1) and aacC2 was characterized on the conjugative IncI1 ST136 plasmid. Co-location of the rmtB gene with a class 1 integron cassette array and aacC2 on the conjugative plasmid will facilitate its maintenance and dissemination.