Silent dissemination of plasmid-borne tigecycline resistance gene tet(X6) in livestock-associated Acinetobacter towneri

A tigecycline-resistance gene, tet(X6), was detected on a 159-kb novel plasmid carried by a tigecycline-susceptible livestock-associated Acinetobacter towneri isolate. The genetic context of tet(X6) (ΔISVsa3-tet(X6)-abh-guaA-ISVsa3) is highly similar with that of the other plasmid-borne tet(X) variants. The 23-Ala residue of the first FAD binding site conferred higher activity to Tet(X6) than the 23-Gly reside conserved in the other plasmid-borne tet(X)s. To our knowledge, this is the first report of tet(X6) carried by the plasmid.