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.