Antimicrobial resistance and molecular epidemiological analysis of Escherichia fergusonii harboring the mcr gene in pigs and broiler chickens in Okinawa, Japan

The dissemination of mcr-harboring Enterobacteriaceae, e.g., Escherichia fergusonii, with resistance to colistin via animal products is a public health concern. In our previous study, E. fergusonii harboring the mcr gene were isolated from 11 pigs and 43 chickens. To understand the spread of mcr-harboring E. fergusonii in Okinawa, Japan, and to gain further insights into how they can be controlled, an antimicrobial susceptibility testing, pulsed-field gel electrophoresis (PFGE), a conjugation test for the transferability of mcr-harboring plasmids, and PCR-based replicon typing (PBRT) were performed using the 54 strains. According to the disk-diffusion and broth microdilution methods, 9 of the 11 strains from pigs and 9 of the 43 strains from chickens had multidrug resistance (MDR). The broth microdilution method showed that all strains were resistant to colistin, and the minimum inhibitory concentration of colistin was 4–16 μg/mL. PFGE suggested identical PFGE types were being transmitted within one pig farm, within one chicken farm, and among several chicken farms. These findings showed that some mcr-harboring E. fergusonii in Okinawa exhibited MDR, and these had spread within farms and between farms. In the mcr gene conjugation test and PBRT, a type IncI2 plasmid replicon was detected in all mcr-1-harboring transconjugants. Therefore, evidence suggests that the IncI2 plasmid is probably involved in the transmission of the mcr-1 gene. It is important to monitor the antimicrobial resistance profile and dissemination of the IncI2 plasmid in mcr-harboring E. fergusonii.