F18:A-:B1 Plasmids Carrying bla(CTX-M-55) Are Prevalent among Escherichia coli Isolated from Duck–Fish Polyculture Farms

We determined the prevalence and molecular characteristics of bla(CTX-M-55)-positive Escherichia coli (E. coli) isolated from duck–fish polyculture farms in Guangzhou, China. A total of 914 E. coli strains were isolated from 2008 duck and environmental samples (water, soil and plants) collected from four duck fish polyculture farms between 2017 and 2019. Among them, 196 strains were CTX-M-1G-positive strains by PCR, and 177 (90%) bla(CTX-M-1G)-producing strains were bla(CTX-M-55)-positive. MIC results showed that the 177 bla(CTX-M-55)-positive strains were highly resistant to ciprofloxacin, ceftiofur and florfenicol, with antibiotic resistance rates above 95%. Among the 177 strains, 37 strains carrying the F18:A-:B1 plasmid and 10 strains carrying the F33:A-:B- plasmid were selected for further study. Pulse field gel electrophoresis (PFGE) combined with S1-PFGE, Southern hybridization and whole-genome sequencing (WGS) analysis showed that both horizontal transfer and clonal spread contributed to dissemination of the bla(CTX-M-55) gene among the E. coli. bla(CTX-M-55) was located on different F18:A-:B1 plasmids with sizes between ~76 and ~173 kb. In addition, the presence of bla(CTX-M-55) with other resistance genes (e.g., tetA, floR, fosA3, bla(TEM), aadA5 CmlA and InuF) on the same F18:A-:B1 plasmid may result in co-selection of resistance determinants and accelerate the dissemination of bla(CTX-M-55) in E. coli. In summary, the F18:A-:B1 plasmid may play an important role in the transmission of bla(CTX-M-55) in E. coli, and the continuous monitoring of the prevalence and transmission mechanism of bla(CTX-M-55) in duck–fish polyculture farms remains important.