Ampicillin enhances daptomycin- and cationic host defense peptide-mediated killing of ampicillin- and vancomycin-resistant Enterococcus faecium.

We studied an ampicillin- and vancomycin-resistant Enterococcus faecium (VRE) isolate from a patient with endocarditis and bacteremia refractory to treatment with daptomycin (6 mg/kg of body weight) plus linezolid. Blood cultures cleared within 24 h of changing therapy to daptomycin (12 mg/kg) plus ampicillin. We examined the effects of ampicillin on daptomycin-induced growth inhibition and killing, surface charge, and susceptibility to several prototypical host defense cationic antimicrobial peptides. MICs and time-kill curves with daptomycin were assessed in the presence and absence of ampicillin. The impact of ampicillin on surface charge was assessed by flow cytometry and a poly-l-lysine binding assay. The effects of ampicillin preexposures upon VRE killing by five distinct cationic peptides of different structure, charge, origin, and mechanism of action were analyzed using the epidermal cathelicidin LL-37, thrombin-induced platelet microbicidal proteins (tPMPs), and a synthetic congener modeled after tPMP microbicidal domains (RP-1), human neutrophil peptide-1 (hNP-1), and polymyxin B (bacteria derived). Fluoroscein-Bodipy-labeled daptomycin was used to evaluate daptomycin binding to VRE membranes in the presence or absence of ampicillin. In media containing ampicillin (25 to 100 mg/liter), daptomycin MICs decreased from 1.0 to 0.38 mg/liter. Based on time-kill analysis and an in vitro pharmacodynamic model, ampicillin enhanced daptomycin activity against the study VRE from a bacteriostatic to a bactericidal profile. VRE grown in ampicillin (25 to 150 mg/liter) demonstrated an incremental reduction in its relative net positive surface charge. When grown in the presence (versus absence) of ampicillin (25 and 100 mg/liter), the VRE strain (i) was more susceptible to killing by LL-37, tPMPs, hNP-1, and RP-1 but not to polymyxin B and (ii) exhibited greater binding to Bodipy-labeled daptomycin. We conclude that ampicillin induces reductions in net positive bacterial surface charge of VRE, correlating with enhanced bactericidal effects of cationic calcium-daptomycin and a diverse range of other cationic peptides in vitro. While the mechanism(s) of such β-lactam-mediated shifts in surface charge remains to be defined, these finding suggest a potential for β-lactam-mediated enhancement of activity of both daptomycin and innate host defense peptides against antibiotic-resistant bacteria.