Characterization of the IncHI2 plasmid pTW4 harboring tet(M) from an isolate of Escherichia coli ST162.

To investigate the genetic features and biological costs of the plasmid pTW4 harboring tet(M) in an isolate of Escherichia coli ST162 from a duck. The complete nucleotide sequence of plasmid pTW4 was determined. The characteristics of plasmid pTW4 in E. coli were investigated by stability and direct competition assays. pTW4 is an IncHI2-type plasmid that contained the resistant genes tet(M), floR, strAB, sul2, rmtB, and blaCMY-2. Tet(M) is located in the composite transposon Tn6539 within the multidrug resistant (MDR) region on this plasmid. Furthermore, the resistance gene rmtB and blaCMY-2 were found outside the MDR region. The plasmid pTW4 remained stable in the host strain E. coli J53 after passage under an antibiotic-free environment for 7 days. However, the strain E. coli J53/pTW4 showed a fitness disadvantage of 6% per ten generations in the process of growth competition with E. coli J53. In conclusion, the plasmid pTW4, a mobile MDR vehicle, may promote the dissemination of tet(M), floR, rmtB, strAB, sul2, and blaCMY-2 among bacteria and then, but it appears to confer growth disadvantage to the host.