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Abstract

Introduction: Administering daptomycin with β-lactams may prevent resistance or achieve synergy in enterococcal infections. Daptomycin-non-susceptibility (DAP-NS) phenotype is commonly associated with mutations in the liaFSR and YycFGHIJ systems. Cardiolipin synthase (cls) mutations have been found in conjunction with liaFSR mutations, associated with higher MICs, suggesting they are stepwise mutations. In vitro, ampicillin restores daptomycin bactericidal activity in Enterococcus faecium isolates with liaFSR mutations, but not when they occur in the cls gene; cls-only Enterococcus faecalis mutants have not yet been reported. Aim: The aim was to explore daptomycin synergy with β-lactams for high-MIC DAP-NS E. faecalis and E. faecium cls mutants. Methodology: Experimentally-induced DAP-NS E. faecalis (A7) and DAP-NS E. faecium (C_002) from a persistent infection on daptomycin-ampicillin therapy had whole-genome sequencing (AB SOLiD 5500, CLC Genomics Workbench 9). Time-kill studies were performed with DAP, ampicillin, and DAP-AMP; and ceftriaxone and DAP-ceftriaxone for E. faecalis. Results: Both isolates’ only DAP-NS-associated mutation was in cls. DAP-ceftriaxone demonstrated bactericidal activity and synergy for E. faecalis A7, whereas E. faecium C_002 grew in the presence of DAP and DAP-AMP. Conclusion: High-level DAP-NS arose in association with a cls mutation alone in E. faecalis A7, and in the absence of other documented DAP-NS-associated genetic mutations in E. faecium C_002. DAP-ceftriaxone combination therapy may be a useful salvage option in DAP-NS E. faecalis infections carrying the cls mutation identified in this study. There was no in vitro DAP-ampicillin synergy for cls-associated DAP-NS E. faecium C_002.