Effects of Efflux Transporter Genes on Susceptibility of Escherichia coli to Tigecycline (GAR-936)

Abstract

ABSTRACT The activity of tigecycline, 9-( t -butylglycylamido)-minocycline, against Escherichia coli KAM3 ( acrB ) strains harboring plasmids encoding various tetracycline-specific efflux transporter genes, tet (B), tet (C), and tet (K), and multidrug transporter genes, acrAB , acrEF , and bcr , was examined. Tigecycline showed potent activity against all three Tet-expressing, tetracycline-resistant strains, with the MICs for the strains being equal to that for the host strain. In the Tet(B)-containing vesicle study, tigecycline did not significantly inhibit tetracycline efflux-coupled proton translocation and at 10 μM did not cause proton translocation. This suggests that tigecycline is not recognized by the Tet efflux transporter at a low concentration; therefore, it exhibits significant antibacterial activity. These properties can explain its potent activity against bacteria with a Tet efflux resistance determinant. Tigecycline induced the Tet(B) protein approximately four times more efficiently than tetracycline, as determined by Western blotting, indicating that it is at least recognized by a TetR repressor. The MICs for multidrug efflux proteins AcrAB and AcrEF were increased fourfold. Tigecycline inhibited active ethidium bromide efflux from intact E. coli cells overproducing AcrAB. Therefore, tigecycline is a possible substrate of AcrAB and its close homolog, AcrEF, which are resistance-modulation-division-type multicomponent efflux transporters.