Carbapenem resistance in Pseudomonas aeruginosa from cystic fibrosis patients

The evolution of imipenem resistance was evaluated in Pseudomonas aeruginosa sequentially isolated from 42 patients with cystic fibrosis. Susceptibility was determined using a commercial microdilution system and imipenem resistance was confirmed by the agar dilution technique. Resistance to imipenem increased during the years from 1988 to 1992. A total of 12 patients (28.5%) carried resistant strains (11.6% of the total P. aeruginosa isolates) but only two of them were treated with the carbapenem The other patient under imipenem treatment did not harbour resistant isolates. Sixty-four per cent of the imipenem resistant isolates were also meropenem resistant and showed low susceptibility to the other beta-lactams and tobramycin and amikacin. Twenty-one strains were selected for biochemical study. Imipenem susceptible strains showed normal OprD in two strains and diminished OprD in two more. Five strains with MIC of imipenem of 4-8 mg/L lacked OprD while another two had a band with decreased density. All strains with MIC higher than 8 completely lacked this band in western-blot analysis. Imipenem MICs of 0.5-2 mg/L only slightly increased to 1-4 mg/L when a pattern of beta-lactamase derepression was observed. While those with imipenem MICs between 8-16 mg/L increased the imipenem MIC to 16-64 mg/L in the population with a beta-lactamase derepression phenotype.