IS26-Flanked Composite Transposon Tn6539 Carrying the tet (M) Gene in IncHI2-Type Conjugative Plasmids From Escherichia coli Isolated From Ducks in China.

Tet(M)-type proteins confer resistance to tetracycline and related antibiotics by interacting with the ribosome. Genes encoding Tet(M) have been found in a range of bacteria, including Escherichia coli . In the current study, conjugation experiments were performed between seven different tetracycline-resistant, azide-susceptible E. coli strains isolated from ducks and tetracycline-sensitive, azide-resistant E.coli J53. Transconjugants were obtained from two of the strains at a frequency of 1.2 × 10-8 . PCR, southern blotting and sequencing demonstrated that tet (M) in the transconjugants was located on a ~50 kb IncHI2-type plasmid and was part of a composite transposon, designated Tn6539. This transposon is flanked by two IS26 elements in opposite orientation and contains the Tn3Δ tnpA +Δ orf13 - lp - tet (M)+gamma delta+ tnpX +Δ tnpR sequences. The Δ orf13 - lp - tet (M) sequence was a highly conserved genetic fragment in E. coli harboring tet (M) and mainly located in the composite transposons flanked by IS6-family elements. In summary, Tn6539 is a new composite transposon capable of horizontal transfer of tet (M) among E. coli isolates.