Mechanistic Insights into the Differential Efficacy of Daptomycin plus β-Lactam Combinations against Daptomycin-Resistant Enterococcus faecium.

BACKGROUND Daptomycin (DAP) plus ampicillin (AMP), ertapenem (ERT) or ceftaroline (CPT) have demonstrated efficacy against a DAP-tolerant E. faecium (Efm) strain (HOU503). However, the mechanism for the efficacy of these combinations against DAP-resistant E. faecium strains is unknown. METHODS We investigated the efficacy of DAP in combination with AMP, ERT, CPT, ceftriaxone (CRO) and amoxicillin (AMX) against DAP-R Efm R497 using established in vitro and in vivo models. We evaluated pbp expression, PBP5 protein levels and β-lactam binding affinity in HOU503 vs R497. RESULTS DAP plus AMP was the only efficacious regimen against DAP-R R497 and prevented emergence of resistance. DAP at 8, 6 and 4 mg/kg in combination with AMP was efficacious but showed delayed killing compared to 10 mg/kg. PBP5 of HOU503 exhibited amino acid substitutions in the penicillin-binding domain relative to R497. No difference in pbp mRNA or PBP5 protein levels was detected between HOU503 vs. R497. Bocillin labeling of PBPs showed increased β-lactam binding affinity of PBP5 of HOU503 compared to that of R497. CONCLUSIONS Only DAP (10 mg/kg) plus AMP or AMX was efficacious against a DAP-R Efm strain. pbp5 alleles may be important contributors to efficacy of DAP plus β-lactam therapy.