title: RamA, a transcriptional regulator, and AcrAB, an RND-type efflux pump, are associated with decreased susceptibility to tigecycline in Enterobacter cloacae.
abstract: Tigecycline, a novel broad-spectrum glycylcycline antibiotic, is active against many gram-positive and gram-negative bacterial pathogens including most strains of Enterobacter cloacae. Recently, however, a few clinical strains of E. cloacae with decreased susceptibility to tigecycline were isolated. In this study, two tigecycline-susceptible mutants of E. cloacae, GC7696 and GC7697, were obtained by transposon mutagenesis of a tigecycline-resistant clinical isolate G946. Transposon insertions were mapped to either the acrA or acrB genes. Restoration of the original resistant phenotype occurred when GC7696 and GC7697 were transcomplemented with a plasmid harboring the intact acrAB region amplified from G946. Northern blot analysis of G946 and several other E. cloacae clinical strains that exhibited decreased susceptibility to tigecycline, revealed increased levels of the acrAB transcript. In addition, overexpression of acrAB correlated with increased expression of the ramA gene, whereas the expression of another transcriptional activator, marA, was not changed. These results suggest that decreased susceptibility to tigecycline in E. cloacae is the result of RamA-mediated overexpression of the AcrAB efflux pump.
authors.affiliation: Department of Infectious Disease, Wyeth Research, 401 North Middletown Road, Pearl River, NY 10965, USA. keeneyd@wyeth.com
authors.forename: David
authors.initials: D
authors.lastname: Keeney
authors.forename: Alexey
authors.initials: A
authors.lastname: Ruzin
authors.forename: Patricia A
authors.initials: PA
authors.lastname: Bradford
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D001483:Base Sequence; D004251:DNA Transposable Elements; D024881:Drug Resistance, Bacterial; D016972:Enterobacter cloacae / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D008826:Microbial Sensitivity Tests; D008911:Minocycline / Q000031:analogs & derivatives* / Q000494:pharmacology; D008969:Molecular Sequence Data; D016296:Mutagenesis; D000078304:Tigecycline; D015534:Trans-Activators / Q000235:genetics / Q000378:metabolism*; D015533:Transcriptional Activation
chemical_list: D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D004251:DNA Transposable Elements; D015534:Trans-Activators; D000078304:Tigecycline; D008911:Minocycline