Synergy testing of multidrug resistant Acinetobacter baumanii against tigecycline and polymyxin using an E-test methodology

Abstract

The emergence of multi drug resistant nosocomial Acinetobacter baumanii infection has prompted the evaluation of newer antimicrobial agents and older warehoused agents as possible therapeutic candidates. In addition to the carbapenems, two other agents have emerged in clinical use and the in-vitro literature as potential therapeutic agents. In a number of studies tigecycline, a new agent in the glycylcycline class, related to minocycline, and polymyxin B, a polypeptide antibiotic used parenterally in the past as a gram negative rod antimicrobial, have show promising activity against multidrug resistant A. baumanii isolates, including those isolates resistant to the carbapenems [1–3]. Tigecycline appears to be bacteriostatic in vitro and in one published clinical study there has been breakthrough bacteremia. Very little in vitro data exists on the activity of the combination of polymyxin plus tigecycline against multidrug resistant A. baumanii [4, 5]. The E-test methodology has proven reliable for in vitro antimicrobial sensitivity testing [6]. A modified E-test methodology for performing antimicrobial synergy tested has been preliminarily evaluated in the literature and although not extensively validated, has been found to correlate well with traditional checkerboard and time-kill methodologies in several studies [7, 8]. E-test synergy testing is performed by crossing the E-test strips for the 2 test antimicrobials at a 90° angle at their predetermined MIC value on an inoculated Mueller Hinton agar plate. The plates are then incubated for 24 h at 35°C. The MIC of each antimicrobial in combination is interpreted as the value at which the zone of inhibition intersects the E test strip. The fractional inhibitory concentration index (FICI) is calculated and interpreted for synergy, indifference or antagonism. The FICI=MICtigecycline in combination/MICtigecyclinealone+ MICpolymixin in combination/MICpolymixin alone. The FICI is interpreted as follows: 4.0 indicates antagonism. We tested 19 recent clinical isolates of multidrug resistant A. baumanii from a large university teaching hospital for susceptibility to tigecycline and polymyxin B and performed synergy testing on the combination using E-test methodology. Clinical sources of the isolates were as follows: respiratory 9/19 (47.4%), urine 6/19 (31.6%), blood 2/19 (10.5%), wound 2/19 (10.5%). E-test synergy testing, revealed FICI values for tigecycline plus polymyxin between 0.88 and 2.0 for all 19 isolates, with 11/19 isolates having an FICI of 2.0. The FICI distribution was as follows: