A Novel Structure Harboring bla(CTX-M-27) on IncF Plasmids in Escherichia coli Isolated from Swine in China

The aim of this study was to elucidate the prevalence of bla(CTX-M-27)-producing Escherichia coli and transmission mechanisms of bla(CTX-M-27) from swine farms in China. A total of 333 E. coli isolates were collected from two farms from 2013 to 2016. Thirty-two CTX-M-27-positive E. coli were obtained, and all were multidrug-resistant. Pulsed field gel electrophoresis (PFGE) and multilocus sequence typing (MLST) profiles indicated a wide range of strain types that carried bla(CTX-M-27), and the sequence type ST10 predominated. Conjugation, replicon typing, S1-PFGE and hybridization experiments confirmed that 28 out of 32 CTX-M-27 positive isolates carried bla(CTX-M-27) genes on plasmids F18:A-:B10 (16) and F24:A-:B1 (12).The bla(CTX-M-27) genes for 24 isolates were transmitted by plasmids with sizes ranging from 40 to 155 kb. A comparative analysis with bla(CTX-M-27)-plasmids indicated that the tra-trb region of F24:A-:B1 plasmids was destroyed by insertion of a complex region (eight isolates) and a novel structure containing bla(CTX-M-27) in the F18:A-:B10 plasmids (12 isolates). The novel structure increased the stability of the bla(CTX-M-27) gene in E. coli. This study indicated that the predominant vehicle for bla(CTX-M-27) transmission has diversified over time and that control strategies to limit bla(CTX-M-27) transmission in farm animals are necessary.