title: In vitro evaluation of dual carbapenem combinations against carbapenemase-producing Enterobacteriaceae.
abstract: OBJECTIVE
  This study aimed to analyse the in vitro activity of dual combinations of carbapenems against Klebsiella pneumoniae producing the main carbapenemase types.
  
  
  METHODS
  MIC values of the carbapenems, imipenem, meropenem, ertapenem and doripenem were determined alone and in dual combinations for 20 clinical K. pneumoniae isolates producing representative carbapenemases, i.e. OXA-48 (n = 6), NDM-1 (n = 4), NDM-1 + OXA-48 (n = 2) and KPC-2 (n = 8). MICs were also determined for Escherichia coli recombinant strains with or without permeability defects producing NDM-1, OXA-48 or KPC-2. In vitro synergy combination testing was performed using the microdilution and chequerboard techniques. Fractional inhibitory concentration indexes were calculated to determine whether the combinations were synergistic, indifferent or antagonistic.
  
  
  RESULTS
  All carbapenemase producers were resistant to the tested carbapenems, with most isolates showing MICs of carbapenems >32 mg/L. None of the combinations was antagonistic. For KPC producers, synergistic combinations were observed with imipenem/ertapenem (5/8 isolates), imipenem/doripenem (4/8), imipenem/doripenem (4/8), meropenem/doripenem (3/8) and ertapenem/doripenem (3/8), while no synergy was observed with meropenem/ertapenem. For OXA-48 producers, synergies were observed with imipenem/ertapenem and with imipenem/meropenem for both isolates tested. Notably, combining imipenem with a non-carbapenem β-lactam (cefalotin) did not give any synergistic result. No synergy was observed for all NDM-1 and NDM-1+OXA-48 producers. Time-kill assays confirmed most of the data obtained by chequerboard testing.
  
  
  CONCLUSIONS
  The data strongly support the hypothesis that dual carbapenem combinations might be effective against serine-β-lactamase producers (KPC, OXA-48). The imipenem-containing combinations appeared to be the most efficient.
authors.affiliation: Medical and Molecular Microbiology 'Emerging Antibiotic Resistance' Unit, Department of Medicine, Faculty of Science, University of Fribourg, Fribourg, Switzerland laurent.poirel@unifr.ch.
authors.forename: Laurent
authors.initials: L
authors.lastname: Poirel
authors.affiliation: Medical and Molecular Microbiology 'Emerging Antibiotic Resistance' Unit, Department of Medicine, Faculty of Science, University of Fribourg, Fribourg, Switzerland.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Kieffer
authors.affiliation: Medical and Molecular Microbiology 'Emerging Antibiotic Resistance' Unit, Department of Medicine, Faculty of Science, University of Fribourg, Fribourg, Switzerland Hôpital Fribourgeois-Hôpital Cantonal, Fribourg, Switzerland.
authors.forename: Patrice
authors.initials: P
authors.lastname: Nordmann
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000378:metabolism*; D015780:Carbapenems / Q000494:pharmacology*; D004347:Drug Interactions; D004926:Escherichia coli / Q000187:drug effects / Q000201:enzymology; D006801:Humans; D007710:Klebsiella Infections / Q000382:microbiology; D007711:Klebsiella pneumoniae / Q000187:drug effects* / Q000201:enzymology* / Q000302:isolation & purification; D008826:Microbial Sensitivity Tests; D001618:beta-Lactamases / Q000378:metabolism*
chemical_list: D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D015780:Carbapenems; D001618:beta-Lactamases; C063912:carbapenemase