The resistance mechanisms of b-lactam antimicrobials in clinical isolates of Acinetobacter baumannii

Background : Despite increasing importance of Acinetobacter baumannii in nosocomial infections and rapid development of multi-antimicrobial resistance in this strain, the resistance mechanisms of -lactam antimicrobials in A. baumannii were not clearly defined. In order to observe the resistance mech-anisms against -lactams and carbapenem, we characterized the production of -lactamases and outermembrane protein (OMP) profiles for the 44 clinical isolates of A. baumannii. Methods : The MICs of antimicrobials were determined by agardilution test. The secondary -lactamases were characterized by isoelectric focusing, polymerase chain reactions and nucleotide sequencing, and the production of chromosomal -lactamases was quantitated by spectrophotometric method. For two strains with an elevated MIC of carbapenem, outermembrane protein (OMP) profile was analyzed by ultracentrifugation of the sonicated bacteral cells and SDS-PAGE. Results and conclusion : Twenty two or 4 of 44 strains produced TEM-1-like -lactamase or PER-1 extended-spectrum -lactamase, respectively. However, when we analyzed the MICs of several -lactams with the -lactamase production, the resistance level of -lactam was mainly determined by the production of chromosomal -lactamase, not by the secondary -lactamases in the clinical isolates of A. baumannii. In two strains with an elevated MIC of imipenem, a decrease or loss of about 35 kDa and 22 kDa proteins in OMP was observed, which suggested that the change of OMP played a role in carbapenem resistance.