Detection of the transferable tigecycline resistance gene tet(X4) in Escherichia coli from pigs in the United Kingdom

Tigecycline is one of the last-resort critically important antimicrobials and is used to treat MDR bacterial infections.1 The first report of transferable tigecycline resistance, mediated by newly reported tet(X3) and tet(X4) genes, in bacterial isolates (including Escherichia coli) from livestock occurred in May 2019 from China.2 It was suggested that, although tigecycline is not authorized for use in livestock, the extensive use of tetracyclines in livestock could have driven the spread of tet(X) variants,3 with subsequent potential for dissemination into human commensal and pathogenic bacterial species.2 Both tet(X3) and tet(X4) confer resistance to all tetracyclines, and can confer high levels of tigecycline resistance (MIC values of 16–32 mg/L),2,3 which exceed the EUCAST clinical breakpoint of >0.5 mg/L (http://www.eucast.org). Recently, an additional tet(X) variant, able to increase the MIC of tetracycline/tigecycline in E. coli [...