The Prevalence of Plasmid-Mediated Colistin Resistance Gene and Different Transferability and Fitness of -Bearing IncX4 Plasmids in Escherichia coli from Pigeons.

The prevalence of colistin-resistant bacteria limited the usage of colistin in the treatment of clinical multidrug-resistant Gram-negative bacterial infections. Here, we aimed to investigate the prevalence and molecular characterization of -carrying isolates from pigeons close to humans following the ban on the use of colistin as an animal feed additive in China. Methods, including PCR, antimicrobial susceptibility testing, conjugation experiments, plasmid replicon typing, genome sequencing, bioinformatics analysis, measurement of growth curves, competition experiments, and plasmid stability assays were used to identify and characterize -positive isolates. In total, 45 -positive E. coli isolates were acquired from 100 fecal samples, and MICs of colistin ranged from 4 to 8 mg/L. The prevalence of -positive E. coli isolates from pigeons was mainly mediated by IncX4 plasmids (39/45), including transferable -bearing IncX4 plasmids with fitness advantage in 21 isolates, and nontransferable -bearing IncX4 plasmids with fitness disadvantage in 18 isolates. There is a similar structure among the 6 -bearing nontransferable IncX4 plasmids and 10 -bearing transferable IncX4 plasmids in 16 E. coli isolates that have been sequenced. Plasmid transferability evaluation indicated that the same IncX4 plasmid has different transferability in different E. coli isolates. In conclusion, this study demonstrates that pigeons could act as potential reservoirs for the spread of -positive E. coli in China. Transferability of IncX4 plasmids may be influenced by host chromosome in the same bacterial species. Additional research on the factors influencing the transferability of IncX4 plasmids in different bacterial hosts is required to help combat antimicrobial resistance. The emergence of plasmid-mediated colistin resistance gene incurs great concerns. Since the close proximity of pigeons with humans, it is significant to understand the prevalence and molecular characterization of -positive isolates in pigeons, to provide a rationale for controlling its spread. Here, we found that the prevalence of -positive E. coli from pigeons was mainly mediated by IncX4 plasmids. However, different transferability and fitness of -bearing IncX4 plasmids in E. coli were observed, which demonstrated that transferability of IncX4 plasmids could be affected not only by genes on plasmids, but also by chromosomal factors in the same bacterial species. Our finding provided a new insight on studying the factors influencing the transferability of plasmids.