Broad-host-range IncW plasmid harbouring tet(X) in Escherichia coli isolated from pigs in Japan

Objectives: Tetracyclines are used in veterinary medicine for livestock. Tigecycline, a semisynthetic tetra- cycline derivative, is a last-resort antimicrobial used to treat multidrug-resistant Gram-negative bacterial infections. The prevalence of variants of the mobile tigecycline resistance gene tet (X) in livestock is in- creasing worldwide. However, the prevalence of Tet(X) among livestock in Japan is unclear. This study was conducted to clarify the prevalence of Tet(X) in pigs in Japan, focusing on isolation and molecular characterisation of plasmid-mediated tet (X)-positive Escherichia coli through retrospective analysis. Methods: We retrospectively screened for tigecycline-resistant E. coli strains isolated from pigs. The tigecycline-resistant strain and tet (X)-harbouring plasmid were characterised. Results: The IncW plasmid harbouring the tet (X) variant [previously named as tet (X6)] was detected in one E. coli isolate from pigs (0.8%; 1/120) in 2012. The tet (X) plasmid was transferable by conjugation to the E. coli ML4909 recipient strain. Some mobile genetic elements (Tn As3 and IS Vsa3 ) were observed in the region surrounding tet (X). The tet (X)-harbouring plasmid shared a conserved backbone with IncW plasmid R388, which is a broad-host-range plasmid. Conclusion: The emergence and spread of tet (X) variants in Enterobacterales poses a public-health con- cern. To the best of our knowledge, this is the first report of the emergence of an IncW plasmid har- bouring tet (X). Using tetracyclines in livestock exerts selective pressure on the tet (X) plasmid; therefore, prudent use of tetracyclines is required.