246. Etest® Synergy Testing of Multi-drug Resistant Pseudomonas aeruginosa (MDR-PSAR) with Polymyxin B plus Fosfomycin, Meropenem or Rifampin
Session: Poster Abstract Session: Diagnostic Microbiology; Antimicrobial Sensitivities
Thursday, October 3, 2013
Room: The Moscone Center: Poster Hall C
  • pankey poster 9-18-13 rev 3.pdf (860.5 kB)
  • Background:

    Multi-drug resistant Pseudomonas aeruginosa infection is a major problem in burn patients.  Antimicrobial combinations are used empirically without supportive data.  The goal of this study was to evaluate synergy of polymyxin B (PO) plus fosfomycin (FM), meropenem (MP) or rifampin (RI) against MDR-PSAR using a rapid Etest method.


    Nine genetically-unique (by rep-PCR) multidrug-resistant Pseudomonas aeruginosa clinical isolates were collected from individual burn patients at Shriners Hospitals for Children-Galveston from 2004-2008.  Identification and susceptibility testing was performed using the MicroScan WalkAway®-96 and the Vitek ® 2 systems.  Isolates were resistant to all drugs tested using both systems.  In addition, Etest MICs (µg/ml) were determined for PO (7/9 isolates > 2 non-susceptible); FM (48 to >1024); MP (>32) resistant; and RI (>32).  There are no CLSI breakpoints for FM or RI.  Synergy testing with PO + FM, MP, or RI was performed in triplicate (mean value used) using the Etest MIC:MIC synergy method [AAC, 2005, 49(7):2959-64]. The summation fractional inhibitory concentration (∑FIC) was calculated for each organism/combination:  synergy ≤ 0.5; additivity > 0.5 – 1; indifference >1 to 4.




    (N = 9)



    N (%)



    N (%)



    N (%)


    5 (56%)

    4 (44%)

    0 (0%)


    2 (22%)

    6 (67%)

    1 (11%)


    0 (0%)

    8 (89%)

    1 (11%)

    a ∑FICs: 0.4-0.5; b ∑FICs: 0.6 – 1.0; c ∑FICs: 1.1, 1.2

    Conclusion: Using Etest methodology, the combination with polymyxin B plus rifampin was found to have the most synergistic/additive effect with ∑FICs ≤1 for all MDR P. aeruginosa isolates.  In vitro synergy/additive effect may or may not translate into in vivo effect. Further studies using this combination with more isolates are needed.  


    Cheryll N. Cash, MD1, Natalie Williams-Bouyer, PhD2, Deborah S. Ashcraft, BS, MT1, Abdulrahim Ismail, MD1 and George A. Pankey, MD1, (1)Infectious Diseases, Ochsner Clinic Foundation, New Orleans, LA, (2)Department of Pathology, UTMB Health and Clinical Laboratory Services, Shriners Hospitals for Children, Galveston, TX


    C. N. Cash, None

    N. Williams-Bouyer, None

    D. S. Ashcraft, None

    A. Ismail, None

    G. A. Pankey, None

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