Molecular characterization of the tet (M)-carrying transposon Tn7124 and plasmids in Escherichia coli isolates recovered from swine

Here, we report the genetic features and evolutionary mechanisms of two <i>tet</i> (M)-bearing plasmids (pTA2 and pTA7) recovered from swine <i>Escherichia coli</i> isolates. The genetic profiles of pTA2 and pTA7 and corresponding transconjugants were accessed by S1 nuclease pulsed-field gel electrophoresis and Southern hybridization, followed by whole genome sequencing and bioinformatics analysis. The biological influences of pTA2 and pTA7 were determined by stability and direct competition assays. Both pTA7 and pTA2 had the IncR backbone sequences but differed in the multidrug resistance region (MDR). The MDR of pTA2 consisted of <i>sul3</i>, <i>tet</i> (M), <i>qnr</i>S1, <i>bleO</i>, <i>oqxAB</i>, <i>floR</i>, <i>aadA1</i>, <i>cmlA1</i>, <i>aadA2</i>, and <i>tet</i> (A)-<i>tetR</i> (A) in addition to 22 insertion sequences. Notably, pTA2 carried the novel complex Tn<i>7124</i> (IS<i>26</i>-ctp-lp-<i>tet</i> (M)-hp-IS<i>406</i>tnp-IntI4-IS<i>26</i>) harboring <i>tet</i> (M). The fragment carrying <i>tet</i> (M) (IS<i>26-ctp-</i>lp-<i>tet</i> (M)-IS<i>406 tnp-ctp-aadA1</i>-<i>cmlA1</i>-<i>aadA2</i>-<i>dfrA12-</i>Int<i>I1</i>), named Tn<i>6942-like</i>, and the two resistance modules IS<i>Vsa3</i>-<i>VirD2</i>-<i>floR</i>-<i>lysR</i> and <i>tet</i> (A)-<i>tetR</i> (A) were located in the MDR of pTA7. Both pTA2 and pTA7 were highly stable in <i>E. coli</i> DH5α cells with no fitness cost to the host or disadvantage in growth competition. These results indicate that transposons carrying <i>tet</i> (M) continuously integrate via mediation with an insertion sequence, which accelerates the transmission of <i>tet</i> (M) in <i>E. coli</i> isolates through integration of other drug-resistant genes, thereby posing a potential serious threat to the efficacy of clinical treatment.