Daptomycin synergy with rifampicin and ampicillin against vancomycin-resistant enterococci

Abstract

We used a novel screening method to look for synergy between daptomycin and 18 other antibiotics against 19 strains of high-level vancomycin-resistant enterococci (VRE) (vancomycin MIC ≥ 256 mg/L). In this approach, daptomycin was incorporated into Ca²⁺-supplemented Mueller–Hinton agar at subinhibitory concentrations, and synergy was screened by comparing test antibiotic Etest MICs on agar with and without daptomycin. A striking reduction in the rifampicin MIC was seen in 11/15 (73.3%) VRE that were resistant to rifampicin, from ≥12 mg/L to a mean ± s.d. of 0.22 ± 0.21 mg/L at daptomycin 0.25 × MIC and 0.85 ± 0.90 mg/L at daptomycin 0.125 × MIC. Synergy was also observed for 13/19 (68%) isolates with ampicillin (MIC ≥ 128 mg/L). There was no significant synergy between daptomycin and any other antibiotic by this screening method. If confirmed by further studies, daptomycin with either rifampicin or ampicillin may be useful in the management of infections caused by VRE.