A novel ABC family protein participates in the regulation of fitness cost caused by tet(X4)-bearing plasmids in Escherichia coli

Abstract

The worldwide prevalence of the plasmid-mediated tigecycline resistance gene tet(X4) in diverse bacterial species challenges the clinical application of tigecycline and poses an increasing threat to global health. However, the underlying mechanisms by which tet(X4)-positive plasmids adapt to different host bacteria and maintain resistance genes remain elusive. Herein, we performed a systematic investigation of the fitness costs of tet(X4)-bearing plasmids in different host strains. We found that the combination of tet(X4) gene and Escherichia coli (E. coli) showed a minimal fitness cost. Meanwhile, E. coli harboring tet(X4)-bearing IncFII plasmid pC41 displayed high lethality in a Galleria mellonella (G. mellonella) infection model, superior conjugation frequency and great competitiveness in murine models. Furthermore, we investigated the regulatory mechanisms for controlling the fitness cost of tet(X4)-bearing pC41 in E. coli. A putative ABC transport family protein, designated as AtlR, balanced the tet(X4) expression and bacterial fitness cost by suppressing the plasmid copy number. It inhibited the plasmid replication by downregulating the expression of repA. Overall, these findings uncover a novel adaptative mechanism accounting for the persistence of tet(X4)-bearing plasmids.