In vitro activity of cethromycin against Burkholderia pseudomallei and investigation of mechanism of resistance

Takehiko Mima, Herbert P. Schweizer, Ze Qi Xu

Research output: Contribution to journalArticlepeer-review

14 Scopus citations


Objectives: Most Burkholderia pseudomallei strains are intrinsically resistant to macrolides, mainly due to AmrAB-OprA- and/or BpeAB-OprB-mediated efflux. We assessed the in vitro anti-B. pseudomallei efficacy of cethromycin, a novel ketolide with broad-spectrum activity against Gram-negative and Gram-positive pathogens. Methods: The 2-fold broth microdilution technique was used to assess the in vitro cethromycin susceptibility of a prototype strain, efflux mutants, and a panel of 60 clinical and environmental strains. Time-kill curves were used to assess the mode of action. Spontaneous resistant mutants were isolated and AmrAB-OprA efflux pump expression assessed by quantitative real-time PCR. Deletion and complementation analyses were performed to demonstrate AmrAB-OprA efflux pump mutant involvement in high-level cethromycin resistance. Results: In contrast to macrolides, cethromycin was a weak substrate of AmrAB-OprA and BpeAB-OprB. Cethromycin was bactericidal at high concentrations and bacteriostatic at MIC levels. The ketolide showed efficacy against clinical and environmental strains of B. pseudomallei, with MIC values ranging from 4 to 64 mg/L. Environmental isolates were consistently more susceptible than clinical isolates. High-level cethromycin resistance (MIC 128 mg/L) was due to constitutive AmrAB-OprA efflux pump overexpression, but other mechanisms also seem to contribute. Conclusions: In contrast to macrolides, which are readily effluxed, cethromycin is weakly extruded in wild-type strains and thus demonstrates significant in vitro anti-B. pseudomallei activity against diverse strains. Acquired high-level cethromycin resistance is caused by constitutive AmrAB-OprA efflux pump overexpression and other, probably non-efflux, mechanisms may also contribute to lower-level acquired resistance.

Original languageEnglish (US)
Article numberdkq391
Pages (from-to)73-78
Number of pages6
JournalJournal of Antimicrobial Chemotherapy
Issue number1
StatePublished - Jan 2011
Externally publishedYes


  • Efflux
  • Ketolides
  • Melioidosis
  • Therapy

ASJC Scopus subject areas

  • Pharmacology
  • Microbiology (medical)
  • Infectious Diseases
  • Pharmacology (medical)


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