In Vitro Activity of Antimicrobial Combination against Multidrug-resistant Strains of Acinetobacter baumannii

Lee, Mi Ae; Moon, Hee Won

Abstract

BACKGROUND
Acinetobacter baumannii has been reported as a major cause of nosocomial infections with increasing frequency. Recently, the emergence of carbapenem-resistant strains has become a major problem in treatment. The use of nontraditional agents such as colistin and a combination therapy have been tried. The purpose of this study was to evaluate the activity of antimicrobial combinations against multidrug-resistant (MDR) A. baumannii. METHODS: Twenty-nine strains of MDR A. baumannii, either resistant or intermediate to imipenem, were collected from February 2003 to February 2004. Minimum inhibitory concentrations (MICs) were determined by the agar dilution method. The checkerboard method was used to assess the activity of ampicillin-sulbactam in combination with amikacin, tobramycin or meropenem and colistin in combination with ceftazidime, meropenem, or rifampin. RESULTS: The MIC90 of ceftazidime and cefepime were 2, 048 g/mL and 512 g/mL, respectively, while the MIC90 of colistin was 0.5 g/mL. The antimicrobial combinations that showed an additive effect for one or two strains were colistin with rifampin or ceftazidime and ampicillin-sulbactam with tobramycin or meropenem. Other antimicrobial combinations showed indifferent effects against most strains. There were no synergistic or antagonistic combinations. CONCLUSIONS: These data suggested that colistin may be an alternative drug for MDR A. baumannii. For the effective treatment of patients infected with these resistant strains, further studies are needed to evaluate antimicrobial combinations against a large number of heterogeneous isolates, and these studies must be followed by clinical trials.