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Access Type

WSU Access

Date of Award

January 2015

Degree Type


Degree Name



Pharmaceutical Sciences

First Advisor

Michael J. Rybak





August 2015

Advisor: Dr. Michael J. Rybak

Major: Pharmaceutical sciences

Degree: Master of Science

Objective: Enterococcus faecalis and Enterococcus faecium are generally resistant to daptomycin and β-lactam antibiotics. However, documented in vitro data suggests synergy between several β-lactam antibiotics and daptomycin against resistant organisms. Primary goal of the study was to check various combinations of β-lactam antibiotics with daptomycin (DAP) and assess the potential of synergy and conclude which could be the superior combination. Study also evaluated potential of β-lactam antibiotics to enhance activity of LL-37, which is a cationic peptide demonstrating bacteriostatic activity similar to DAP.

Methods: Fifteen E. faecalis and twenty E. faecium strains were evaluated DAP enhancement via combination MICs. DAP MICs were obtained by broth microdilution in the absence and presence of various β-lactam antibiotics. Time-kill studies were carried out on two E. faecalis (R6981 and R7808) and one isogenic DAP-susceptible/ DAP-non-susceptible E. faecium strain (8019/5938). DAP was tested at 0.5x MIC with β-lactam antibiotics at free peak concentrations. Fluorescent DAP (BoDipy) was used to evaluate DAP binding studies on an E. faecium strain (5938). Two E. faecium (R6370 and 8019) and one E. faecalis (R6981) strains were chosen for evaluating enhancement of killing by LL-37 in the presence and absence of β-lactam antibiotics.

Results: Highest reduction in DAP MIC value was observed when given in combination with ceftaroline. Ceftaroline (CPT), ertapenem (ERT), ampicillin (AMP), cefepime (FEP) and ceftriaxone (CRO) demonstrated synergy with DAP in time-kill studies. However, DAP synergy was observed with cefazolin (CFZ) and cefotaxime (CTX) only in one strain (R7808). In DAP binding studies, ceftaroline enhanced the DAP binding most as compared to other β-lactam antibiotics tested (P <0.001). In LL-37 assay study, CPT, AMP and ERT enhanced LL-37’s killing against strain 8019. For strain R6981 and R6370, LL-37 demonstrated better killing with CPT and ERT as compared to AMP (P <0.001).

Conclusion: Results of out study conclude that the use of DAP in combination with β-lactam antibiotics to treat resistant Enterococcus infections is lucrative. Combination regimens as compared to DAP alone demonstrate better killing. Combination also enhanced LL-37’s activity, which could possibly be a way to avoid DAP resistance. Overall, our data supports the evidence that combination therapy of daptomycin with β-lactams maybe be helpful in combating non-susceptible Enterococcus strains.

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