Comparative in vitro activity of carbapenems against clinical isolates of Acinetobacter baumannii.

ABSTRACT

AIMS: The aim of this multi-hospital study was to assess the in vitro activity of doripenem and its comparators, imipenem and meropenem, using the new CLSI breakpoints against a large population of a frequently isolated nosocomial pathogen, Acinetobacter baumannii. METHODS AND RESULTS: During a 2-year period, four referral or tertiary hospitals submitted 400 isolates of Ac. baumannii for susceptibility testing using imipenem, meropenem and doripenem via disc diffusion and E-test methods. A subset of 390 isolates was resistant to all three tested carbapenems. Doripenem and meropenem (MIC50 , 32 µg ml(-1) ) had comparable activity, albeit doripenem's activity was greater than imipenem (MIC50 , >32 µg ml(-1) ). A significantly higher proportion of the isolates were inhibited by doripenem than by imipenem at MIC values of 12, 16, 24 and 32 µg ml(-1) (P < 0·05). The cumulative percentage of imipenem MICs was lower compared to its comparators. The comparison of resistance rate to imipenem and meropenem based on old and new breakpoints showed <1% difference. The overall agreement between the two susceptibility testing methods was ≥95%. CONCLUSION: Doripenem has a slightly greater in vitro activity than imipenem in terms of zone breakpoints and MIC values, but its activity is comparable to meropenem. SIGNIFICANCE AND IMPACT OF THE STUDY Doripenem should be considered as a therapeutic option for monotherapy or combination therapy, particularly when the therapeutic options are limited.