Program Schedule

Lack of Synergy With Six Blood Isolates of Methicillin-Resistant Staphylococcus aureus (Vancomycin MIC of 2) Tested With Combinations of Vancomycin  + Gentamicin, Vancomycin  + Rifampin and Vancomycin + Cefazolin. 

Session: Poster Abstract Session: MRSA and VRE
Thursday, October 9, 2014
Room: The Pennsylvania Convention Center: IDExpo Hall BC
  • ID Week Poster rev1.pdf (409.2 kB)
  • Background: Vancomycin (V) is the mainstay for treatment of infections caused by MRSA. Yet failures of therapy with V are common. Current guidelines recommend V alone for treatment of most serious infections. Exceptions include prosthetic valve endocarditis and in some instances osteomyelitis and CNS infections where combinations of antibiotics are suggested.

    Methods: We used timed kill-curves to test  antibiotic combinations used in our center against six isolates of MRSA.  These six with V MICs of 2 confirmed by Microscan and E-test were chosen from  15 blood isolates of MRSA saved over three years. We used V and gentamicin (G), V and rifampin (R), two combinations used to treat MRSA, and V and cefazolin (C), a combination recommended in recent publications. Colonies were counted in duplicate at 0, 4, 8, 12 and 24 hour time points. Determinations of synergy, indifference and antagonism were made at the 24 hour time point. Standard definitions requiring 2 log differences in cfu/mL were used.


    Results are shown in the following table:





    V + G




    V + R




    V + C




    Antibiotic synergy was not demonstrated with any of the combinations tested. In all but one experiment, the combination was “indifferent”, however V + G was more active than V alone for three strains. Although “antagonistic” in only one, V+ R was less active than either drug alone for four of the isolates. Killing with V + C paralleled killing with V alone in all six strains.

    Conclusion: We have shown that combining G, R or C with V against six bloodstream isolates of MRSA with V MIC of 2 is not synergistic in vitro. These results did show enhanced killing with V+G in three strains and with V+R in one strain. Killing with V + R was slightly worse in three and significantly worse in one of the isolates. These results, although not showing synergy, may support the use of V + G, and, although not showing antagonism, do not support use of V + R.

    Virginia Long, BS, Infectious Diseases and Immunology Institute, Loyola University Chicago, Maywood, IL, J Paul O'keefe, MD, Loyola Univ. Med. Ctr., Maywood, IL and Paul Schreckenberger, PhD, Loyola University Medical Center, Maywood, IL


    V. Long, None

    J. P. O'keefe, None

    P. Schreckenberger, BioFire: Grant Investigator and Scientific Advisor, Consulting fee and Research grant
    bioMerieux: Research Contractor, Research grant
    Cepheid: Consultant and Grant Investigator, Consulting fee and Research grant
    Cubist: Speaker's Bureau, Speaker honorarium
    Merck: Speaker's Bureau, Speaker honorarium
    Quidel: Scientific Advisor, Consulting fee
    Siemens Healthcare: Consultant, Research Contractor and Speaker's Bureau, Consulting fee, Research grant and Speaker honorarium
    Becton-Dickinson: Speaker's Bureau, Speaker honorarium
    Theravance: Scientific Advisor, Consulting fee
    Thermo Fisher: Scientific Advisor, Consulting fee

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