Multidrug-Resistant Escherichia coli Isolate of Chinese Bovine Origin Carrying the blaCTX-M-55 Gene Located in IS26-Mediated Composite Translocatable Units

Elevated detection rates of the blaCTX-M-55 gene in animals have been reported as a result of antibiotic misuse in clinics. To investigate the horizontal transfer mechanism of blaCTX-M-55 and its associated mobile genetic elements (MGEs), we isolated 318 nonrepetitive strains of Escherichia coli (E. coli) from bovine samples in Xinjiang and Gansu provinces, China. All E. coli strains were screened for the CTX-M-55 gene using PCR. The complete genomic data were sequenced using the PacBio triplet sequencing platform and corrected using the Illumina data platform. The genetic environment of the plasmids carrying the resistance blaCTX-M-55 gene was mapped using the software Easyfig2.2.3 for comparison. The results showed that all blaCTX-M-55-positive strains were resistant to multiple antibiotics. Five strains of Escherichia coli carry the blaCTX-M-55 gene, which is adjacent to other resistance genes and is located on the IncHI2-type plasmid. Four of the five blaCTX-M-55-harbor strains carried translocatable units (TUs). All the donor bacteria carrying the blaCTX-M-55 genes could transfer horizontally to the recipient (E. coli J53 Azr). This study demonstrates that the transmission of blaCTX-M-55 is localized on IS26-flanked composite transposons. The cotransmission and prevalence of blaCTX-M-55 with other MDR resistance genes on epidemic plasmids require enhanced monitoring and control.