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 β-lactam antimicrobials in A. baumannii were not clearly defined. In order to observe the resistance mechanisms against β-lactams and carbapenem, we characterized the production of β-lactamases and outermembrane protein (OMP) profiles for the 44 clinical isolates of A. baumannii. METHODS: The MICs of antimicrobials were determined by agar dilution test. The secondary β-lactamases were characterized by isoelectric focusing, polymerase chain reactions and nucleotide sequencing, and the production of chromosomal β-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 β-lactamase or PER-1 extended-spectrum β-lactamase, respectively. However, when we analyzed the MICs of several β-lactams with the β-lactamase production, the resistance level of β-lactam was mainly determined by the production of chromosomal β-lactamase, not by the secondary β-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.