Genetic characterization of tigecycline resistance in clinical isolates of Enterobacter cloacae and Enterobacter aerogenes

Mark Veleba, Shyamasree De majumdar, Michael Hornsey, Neil Woodford, Thamarai Schneiders*

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    43 Citations (Scopus)


    Objectives: The intrinsically encoded ramA gene has been linked to tigecycline resistance through the up-regulation of efflux pump AcrAB in Enterobacter cloacae. The molecular basis for increased ramA expression in E. cloacae and Enterobacter aerogenes, as well as the role of AraC regulator rarA, has not yet been shown. To ascertain the intrinsic molecular mechanism(s) involved in tigecycline resistance in Enterobacter spp., we analysed the expression levels of ramA and rarA and corresponding efflux pump genes acrAB and oqxAB in Enterobacter spp. clinical isolates. Methods: The expression levels of ramA, rarA, oqxA and acrA were tested by quantitative real-time RT-PCR. The ramR open reading frames of the ramA-overexpressing strains were sequenced; strains harbouring mutations were transformed with wild-type ramR to study altered ramA expression and tigecycline susceptibility. Results: Tigecycline resistance was mediated primarily by increased ramA expression in E. cloacae and E. aerogenes. Only the ramA-overexpressing E. cloacae isolates showed increased rarA and oqxA expression. Upon complementation with wild-type ramR, all Enterobacter spp. containing ramR mutations exhibited decreased ramA and acrA expression and increased tigecycline susceptibility. Exceptions were one E. cloacae strain and one E. aerogenes strain, where a decrease in ramA levels was not accompanied by lower acrA expression. Conclusions: Increased ramA expression due to ramR deregulation is the primary mediator of tigecycline resistance in clinical isolates of E. cloacae and E. aerogenes. However, some ramA-overexpressing isolates do not show changes in ramR, suggesting alternate pathways of ramA regulation; the rarA regulator and the oqxAB efflux pump may also play a role in tigecycline resistance in E. cloacae.

    Original languageEnglish
    Article numberdks530
    Pages (from-to)1011-1018
    Number of pages8
    JournalJournal of Antimicrobial Chemotherapy
    Issue number5
    Publication statusPublished - May 2013

    Bibliographical note

    Funding Information:
    This work was funded by an MRC New Investigator grant (G0601199) and a Pfizer grant (R5708CII) to T. S. Student support for M. V. was provided by the Department for Employment and Learning (Northern Ireland). S. D. M. was supported by a Pfizer grant (R5708CII).


    • Antibiotic resistance
    • Gram-negative
    • OqxAB
    • RamA
    • RarA


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