IS Reorganizes Plasmids in a Multidrug-Resistant Escherichia coli Strain.

The aims of this study were to elucidate the role of IS in plasmid reorganization and to analyze biological characteristics of cointegrates derived from different daughter plasmids. The genetic profiles of plasmids in Escherichia coli strain C21 and its transconjugants were characterized by conjugation, S1 nuclease pulsed-field gel electrophoresis (S1-PFGE), Southern hybridization, whole-genome sequencing (WGS) analysis, and PCR. The traits of cointegrates were characterized by conjugation and stability assays. -bearing IncI2 pC21-1 and nonresistant IncI1 pC21-3, as conjugative helper plasmids, were fused with nonconjugative -bearing IncN-X1 pC21-2, generating cointegrates pC21-F1 and pC21-F2. Similarly, pC21-1 and pC21-3 were fused with nonconjugative IncF33:A-:B- pHB37-2 from another E. coli strain to generate cointegrates pC21-F3 and pC21-F4 under experimental conditions. Four cointegrates were further conjugated into the E. coli strain J53 recipient at high conjugation frequencies, ranging from 2.8 × 10 to 3.2 × 10. The formation of pC21-F1 and pC21-F4 was the result of host- and ISmediated reactions and occurred at high fusion frequencies of 9.9 × 10 and 2.1 × 10, respectively. Knockout of RecA resulted in a 100-fold decrease in the frequency of plasmid reorganization. The phenomenon of cointegrate pC21-F2 and its daughter plasmids coexisting in transconjugants was detected for the first time in plasmid stability experiments. IS-- was excised from cointegrate pC21-F2 through a circular intermediate at a very low frequency, which was experimentally observed. To the best of our knowledge, this is the first report of IS-mediated fusion between plasmids with different replicons. This study provides insight into the formation and evolution of cointegrate plasmids under different drug selection pressures, which can promote the dissemination of MDR plasmids. The increasing resistance to β-lactams and aminoglycoside antibiotics, mainly due to extended-spectrum β-lactamases (ESBLs) and 16S rRNA methylase genes, is becoming a serious problem in Gram-negative bacteria. Plasmids, as the vehicles for resistance gene capture and horizontal gene transfer, serve a key role in terms of antibiotic resistance emergence and transmission. IS, present in many antibiotic-resistant plasmids from Gram-negative bacteria, plays a critical role in the spread, clustering, and reorganization of resistance determinant-encoding plasmids and in plasmid reorganization through replicative transposition mechanisms and homologous recombination. However, the role of IS, present in many MDR plasmids, in the formation of cointegrates remains unclear. Here, we investigated experimentally the intermolecular recombination of IS, which occurred with high frequencies and led to the formation of conjugative MDR cointegrates and facilitated the cotransfer of and , and we further uncovered the significance of IS in the formation of cointegrates and the common features of IS-driven cointegration of plasmids.