title: Daptomycin-reversible rifampicin resistance in vancomycin-resistant Enterococcus faecium.
abstract: OBJECTIVE
  In a previous study, we observed marked synergy between daptomycin and rifampicin against 73% of rifampicin-resistant, vancomycin-resistant Enterococcus faecium (VRE), with approximately 100-fold reductions in rifampicin MICs observed at one-eighth to one-fourth daptomycin MIC. The purpose of this study was to determine whether the synergy between daptomycin and rifampicin could be explained by enhanced entry of rifampicin into the cell or was related to amino acid substitutions in the rifampicin-binding site in the beta subunit (rpo beta) of the RNA polymerase.
  
  
  METHODS
  We developed a bioassay for rifampicin to measure cell-bound rifampicin levels as well as metabolic inactivation of rifampicin. In addition, we sequenced the rifampicin-binding site in the rpo beta of VRE strains with and without synergy between daptomycin and rifampicin.
  
  
  RESULTS
  Cell-bound rifampicin levels were the same in rifampicin-susceptible VRE as in rifampicin-resistant VRE showing daptomycin synergy and were not affected by the presence of daptomycin. In contrast, rifampicin-resistant VRE without daptomycin synergy had undetectable cell-bound rifampicin. Sequencing the rpo beta rifampicin-binding site revealed that the synergistic strains had the same sequence as rifampicin-susceptible wild-type E. faecium. The daptomycin synergy-resistant strains all had mutations in known rifampicin-binding sites.
  
  
  CONCLUSIONS
  Daptomycin is able to reverse rifampicin resistance in some strains of VRE, but the mechanism could not be explained by an effect of daptomycin on entry of rifampicin into or transport out of the cell, by inactivation of rifampicin or by mutation involving the rifampicin-binding site.
authors: Department of Pathology, Immunology and Laboratory Medicine, University of Florida, Gainesville, FL 32610, USA. rand@pathology.ufl.edu
authors: Kenneth H
authors: KH
authors: Rand
authors: Herbert J
authors: HJ
authors: Houck
authors: Jared A
authors: JA
authors: Silverman
mesh_terms: D000595:Amino Acid Sequence; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D012321:DNA-Directed RNA Polymerases / Q000378:metabolism; D017576:Daptomycin / Q000494:pharmacology*; D024901:Drug Resistance, Multiple, Bacterial; D016984:Enterococcus faecium / Q000187:drug effects* / Q000378:metabolism; D008969:Molecular Sequence Data; D012293:Rifampin / Q000378:metabolism / Q000494:pharmacology*; D014640:Vancomycin / Q000494:pharmacology; D020713:Vancomycin Resistance
chemical_list: D000900:Anti-Bacterial Agents; D014640:Vancomycin; D012321:DNA-Directed RNA Polymerases; D017576:Daptomycin; D012293:Rifampin