CARBAPENEM RESISTANCE IN PSEUDOMONAS-AERUGINOSA FROM CYSTIC-FIBROSIS PATIENTS

The evolution of imipenem resistance was evaluated in Pseudomonas aeru ginosa sequentially isolated from 42 patients with cystic fibrosis. Su sceptibility was determined using a commercial microdilution system an d imipenem resistance was confirmed by the agar dilution technique. Re sistance to imipenem increased during the years from 1988 to 1992. A t otal of 12 patients (28.5%) carried resistant strains (11.6% of the to tal 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 iso lates were also meropenem resistant and showed low susceptibility to t he other beta-lactams and tobramycin and amikacin. Twenty-one strains were selected for biochemical study. Imipenem susceptible strains show ed normal OprD in two strains and diminished OprD in two more. Five st rains with MIC of imipenem of 4-8 mg/L lacked OprD while another two h ad a band with decreased density. All strains with MIC higher than 8 c ompletely 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 be tween 8-16 mg/L increased the imipenem MIC to 16-64 mg/L in the popula tion with a beta-lactamase derepression phenotype.