A New tet(X6) Tigecycline Resistance Determinant Co-carried with mcr-1 by A Single Plasmid

Tigecycline and colistin are few of last-resort defenses used in anti-infection therapies against carbapenem-resistant bacterial pathogens. The successive emergence of plasmid-borne tet(X) tigecycline resistance mechanism and mobile colistin resistance (mcr) determinant, renders them clinically ineffective, posing a risky challenge to global public health. Here, we report that co-carriage of tet(X6) and mcr-1 gives co-resistance to both classes of antibiotics by a single plasmid in E. coli. Genomic analysis suggested that transposal transfer of mcr-1 proceeds into the plasmid pMS8345A, in which a new variant tet(X6) is neighbored with Class I integron. The structure-guided mutagenesis finely revealed the genetic determinants of Tet(X6) in the context of phenotypic tigecycline resistance. The combined evidence in vitro and in vivo demonstrated its enzymatic action of Tet(X6) in the destruction of tigecycline. The presence of Tet(X6) (and/or MCR-1) robustly prevents the accumulation of reactive oxygen species (ROS) induced by tigecycline (and/or colistin). Unlike that mcr-1 exerts fitness cost in E. coli, tet(X6) does not. In the tet(X6)-positive strain that co-harbors mcr-1, tigecycline resistance is independently of colistin resistance caused by MCR-1-mediated lipid A remodeling, and vice versa. Co-production of Tet(X6) and MCR-1 gives no synergistic delayed growth of the recipient E. coli. Similar to that MCR-1 behaves in the infection model of G. mellonella, Tet(X6) renders the treatment of tigecycline ineffective. Therefore, co-transfer of such two AMR genes is of great concern in the context of "one health" comprising environmental/animal/human sectors, and heightened efforts are required to monitor its dissemination. Author summaryWe report that tet(X6), a new tigecycline resistance gene, is co-carried with the other resistance gene mcr-1 by a single plasmid. Not only have we finely mapped genetic determinants of tet(X6), but also revealed its biochemical action of tigecycline destruction. Crosstalk of Tet(X6) with MCR-1 is addressed. Tet(X6) tigecycline resistance is independently of MCR-1 colistin resistance, and vice versa. Similar to MCR-1 that renders colistin clinically ineffective, Tet(X6) leads to the failure of tigecycline treatment in the infection model of G. mellonella. This study extends mechanistic understanding mechanism and interplay of Tet(X6) and MCR-1, coproduced by a single plasmid. It also heightens the need to prevent rapid and large-scaled spread of AMR.