Emergence of a Novel Plasmid-Mediated Tigecycline Resistance Gene Cluster, tmexCD4-toprJ4 , in Klebsiella quasipneumoniae and Enterobacter roggenkampii .

The occurrence of transferable tigecycline resistance determinants, tmexCD1-toprJ1 , tmexCD2-toprJ2 , tmexCD3-toprJ1b , and multiple tet (A) and tet (X) variants, presents an unprecedented challenge to clinical therapeutic options. tmexCD-toprJ -like gene clusters can mediate multidrug resistance and have been detected in a variety of bacteria. Here, we characterized the fourth tmexCD-toprJ -like gene cluster, tmexCD4-toprJ4 , identified on untypeable plasmids of Klebsiella quasipneumoniae and Enterobacter roggenkampii isolated from chicken meat and environmental samples from farm markets, respectively. TMexCD4-TOprJ4 was closely related (92 to 99% amino acid identity) to TMexCD1-TOprJ1, TMexCD2-TOprJ2, and TMexCD3-TOprJ1. Phylogenetic analysis revealed that tmexCD4-toprJ4 was not in the same branch as the other three variants. Expression of tmexCD4-toprJ4 increased tigecycline efflux in Escherichia coli and resulted in a 4- to 8-fold increase in MICs of tigecycline in E. coli and Klebsiella pneumoniae. Moreover, tmexCD4-toprJ4 can act synergistically with its upstream gene tet (A) to reduce the susceptibility of E. coli and K. pneumoniae strains to tigecycline. The tmexCD4-toprJ4- containing plasmid is a novel plasmid type and can be transferred to E. coli and K. pneumoniae only via electrotransformation. The increasing emergence of plasmid-mediated tigecycline resistance gene clusters suggests that the spread of tmexCD-toprJ -like gene clusters requires widespread attention. IMPORTANCE The plasmid-mediated tigecycline resistance gene cluster tmexCD1-toprJ1 and other variants have been detected in a variety of strains from multiple sources, including human-derived strains. In addition to tigecycline, these tmexCD-toprJ -like gene clusters reduce susceptibility of the host strain to many other antimicrobials. Here, we identified tmexCD4-toprJ4 in K. quasipneumoniae and E. roggenkampii , suggesting that this gene cluster is already present in the human-associated environment and the risk of transmission to humans is increased. Monitoring tigecycline-resistant Gram-negative bacteria is essential for understanding and addressing the spread of this gene cluster in agriculture and health care.