Escherichia coli strain 22a1303, carrying both mcr-1.1/IncX4 and mcr-3.5/IncP1 plasmids, was isolated from an ongoing antimicrobial resistance monitoring program. We were concerned that the dual presence of the mcr genes could mediate more complex transmission of resistance, so we further analyzed 116 known E. coli strains carrying both genes in the public database. We found that E. coli 22a1303 carrying the mcr-3/dgkA combination and mcr-1 gene, which should mediate high levels of resistance, was sensitive to polymyxin with a MIC value of only 1mg/L. Conjugation experiment demonstrated that the two plasmids can be transferred from the donor strain 22a1303 to the recipient strain E. coli J53 at the same time, and the colistin resistance of transconjugants was increased by 8-fold compared with E. coli 22a1303.

To Cite This Article: Li H, Lin H, Fu Q, Fu Y, Tan J, Wang Y, Zhou R, Sun Z and Li J 2025. Co-transfer of mcr-1 and mcr-3 variant in Escherichia coli ST1632 isolate in China: silence is not always golden. Pak Vet J, 45(2): 913-9169. http://dx.doi.org/10.29261/pakvetj/2025.200