In vitro effect of imipenem on Acinetobacter baumannii.

Imipenem at suprainhibitory concentrations (2x, 4x or 8x MIC) induced postantibiotic effects (PAEs) (suppression of bacterial growth after a short time exposure of bacteria to antimicrobials) against two of three Acinetobacter baumannii strains. The highest concentration tested demonstrated the longest delay of bacterial regrowth (1.7 h (strain 5570) or 3.9 h (strain 6070)). All. A. baumannii strains showed changes in surface hydrophobicity and serum sensitivity after treatment with imipenem. The antibiotic at 8x MIC reduced hydrophobicity of the strains most significantly (from 42.3%-72.0%) as compared to controls (without antibiotic). Susceptibility of the treated bacteria to serum bactericidal activity has also been lowered. Though imipenem suppressed bacterial growth and decreased surface hydrophobicity of the bacteria, it increased survival of bacteria after incubation with serum. These different alterations observed in the studied strains should be taken into account when evaluating the effects of imipenem.