Comparative genomics of rmtB-carrying IncI1 ST136 plasmids in avian escherichia coli isolates from chickens in China

Eight rmtB-carrying avian Escherichia coli strains from a farm in China were characterised in our previous study, but little is known about the backbones and entire multiresistance regions (MRRs) of these plasmids. Here, three rmtB-carrying IncI1 ST136 plasmids were analysed by whole-plasmid sequencing and were compared. These plasmids were composed of an 83 470-bp IncI1 backbone carrying genes responsible for plasmid replication, transfer, maintenance and stability functions, as well as a 17 330-bp MRR for pEC006 and pEC007, and a 34 626-bp MRR for pEC008. Plasmid pEC006 was not transferable, thus truncation of the traI gene may explain the inability to conjugate. pEC008 harboured the blaTEM-1, rmtB, aacC2, tetA, floR and strAB genes as well as a class 1 integron cassette array (|dfrA12|orfF|aadA2|), which were interspersed with different mobile elements, including Tn2, Tn1721, Tn1722, Tn5393, ISCfr1, IS5057, ISCR1 and ISCR2, and three copies of IS26. The MRR of pEC008 may have resulted from transposition of Tn1722 into the plasmid backbone. Acquisition and rearrangement of MRRs demonstrated the accumulation of different resistance determinants.