Intracellular Transposition of Mobile Genetic Elements Associated with the Colistin Resistance Gene mcr-1 .

Mobile colistin resistance () genes are often located on conjugative plasmids, where their association with insertion sequences enables intercellular and intracellular dissemination throughout bacterial replicons and populations. Multiple genes have been discovered in every habitable continent, in many bacterial species, on both plasmids and integrated into the chromosome. Previously, we showed the intercellular transfer of on an IncI1 plasmid, pMCR-E2899, between strains of Escherichia coli. Characterizing the intracellular dynamics of transposition and recombination would further our understanding of how these important genes move through bacterial populations and whether interventions can be put in place to stop their spread. In this study, we aimed to characterize transfer events from the -containing transposon Tn (ISIS), located on plasmid pMCR-E2899, using the pBACpAK entrapment vector. Following the transformation of pBACpAK into our DH5α-Azi/pMCR-E2899 transconjugant, we captured IS in pBACpAK multiple times and, for the first time, observed the IS-mediated transfer of the transposon (Tn) into the chromosome of E. coli DH5α. Whole-genome sequencing allowed us to determine consensus insertion sites of IS and Tn in this strain, and comparison of these sites allowed us to explain the transposition events observed. These observations reveal the consequences of IS transposition within and between multiple replicons of the same cell and show transposition within the cell as part of the novel transposon Tn. By analyzing the intracellular transfer of clinically relevant transposons, we can understand the dissemination and evolution of drug resistance conferring mobile genetic elements (MGEs) once a plasmid enters a cell following conjugation. This knowledge will help further our understanding of how these important genes move through bacterial populations. Utilizing the pBACpAK entrapment vector has allowed us to determine the mobility of the novel -containing transposon Tn.