In vitro activity of cethromycin against Burkholderia pseudomallei and investigation of mechanism of resistance.
OBJECTIVE
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.
Department of Microbiology, Immunology and Pathology, Rocky Mountain Regional Center of Excellence for Biodefense and Emerging Infectious Diseases Research, Colorado State University, Fort Collins, CO 80523-2025, USA.
Takehiko
T
Mima
Herbert P
HP
Schweizer
Ze-Qi
ZQ
Xu
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D016957:Burkholderia pseudomallei / Q000187:drug effects*; D004252:DNA Mutational Analysis; D024881:Drug Resistance, Bacterial; D017353:Gene Deletion; D015964:Gene Expression Regulation, Bacterial; D005816:Genetic Complementation Test; D006801:Humans; D048628:Ketolides / Q000494:pharmacology*; D026901:Membrane Transport Proteins / Q000235:genetics; D008826:Microbial Sensitivity Tests; D050296:Microbial Viability / Q000187:drug effects; D009154:Mutation
D000900:Anti-Bacterial Agents; D048628:Ketolides; D026901:Membrane Transport Proteins; C405499:cethromycin