title: Characterizing vancomycin-resistant Enterococcus strains with various mechanisms of daptomycin resistance developed in an in vitro pharmacokinetic/pharmacodynamic model.
abstract: Two daptomycin (DAP) regimens were evaluated in a pharmacokinetic/pharmacodynamic (PK/PD) model, and the mutants recovered were examined for changes in phenotypic characteristics. Three Enterococcus faecium strains (vancomycin-resistant Enterococcus [VRE] ATCC 51559, VRE 12311, and VRE SF 12047) were utilized in a 7-day, 1-compartment in vitro PK/PD model. The simulated dosing regimens were DAP at 6 mg/kg/day (free C(max) [fC(max)] = 7.9 μg/ml, half-life [t(1/2)] = 8 h) and DAP at 10 mg/kg/day (fC(max) = 13.17 μg/ml, t(1/2) = 8 h). Samples were plated daily on Mueller-Hinton agar containing DAP at 16 μg/ml and 50 mg/liter Ca(2+) to assess the emergence of DAP resistance. For each strain, the mutant with the highest DAP MIC was then evaluated for changes in relative surface charge, cell wall thickness, and cytoplasmic membrane depolarization induced by DAP. The initial DAP MICs were 4 μg/ml for all 3 strains. A dose-dependent response and regrowth were observed for DAP 6 mg/kg/day and DAP 10 mg/kg/day against all 3 strains. Mutants of VRE ATCC 51559 (MIC = 128 and 64 μg/ml) and VRE 12311 (MIC = 256 and 32 μg/ml) were recovered from the DAP 6 mg and DAP 10 mg regimen, respectively. For VRE SF 12047, a mutant (MIC = 64 μg/ml) was recovered from the DAP 6 mg model. All mutants displayed an increase in relative surface charge compared to those of their respective parent strains. The DAP-resistant mutants displayed a 43 to 58% increase in cell wall thickness (P < 0.0001), while DAP membrane depolarization decreased by 53 to 65% compared to that of the susceptible strains. VRE with DAP resistance displayed increased surface charge, increased cell wall thickness, and decreased depolarization induced by DAP, consistent with previous observations in Staphylococcus aureus with reduced DAP susceptibility. Further characterization of DAP-resistant VRE is warranted.
authors.affiliation: Anti-Infective Research Laboratory, Pharmacy Practice - 4148, Eugene Applebaum College of Pharmacy & Health Sciences, Wayne State University, 259 Mack Ave., Detroit, MI 48201, USA.
authors.forename: Molly E
authors.initials: ME
authors.lastname: Steed
authors.forename: Celine
authors.initials: C
authors.lastname: Vidaillac
authors.forename: Warren E
authors.initials: WE
authors.lastname: Rose
authors.forename: Patricia
authors.initials: P
authors.lastname: Winterfield
authors.forename: Glenn W
authors.initials: GW
authors.lastname: Kaatz
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Rybak
mesh_terms: D019881:Aminoacyltransferases / Q000235:genetics / Q000378:metabolism; D000900:Anti-Bacterial Agents / Q000493:pharmacokinetics / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D002462:Cell Membrane / Q000502:physiology; D002473:Cell Wall / Q000502:physiology; D017576:Daptomycin / Q000493:pharmacokinetics* / Q000494:pharmacology*; D024881:Drug Resistance, Bacterial; D016983:Enterococcus / Q000187:drug effects* / Q000302:isolation & purification / Q000502:physiology; D008564:Membrane Potentials; D008826:Microbial Sensitivity Tests; D010641:Phenotype; D013211:Staphylococcus aureus / Q000187:drug effects / Q000502:physiology; D014640:Vancomycin / Q000494:pharmacology; D020713:Vancomycin Resistance
chemical_list: D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D014640:Vancomycin; D019881:Aminoacyltransferases; C429146:mprF protein, Staphylococcus aureus; D017576:Daptomycin