Aminoglycoside Resistance in Pseudomonas aeruginosa Isolated from Cystic Fibrosis Patients

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Abstract
The authors studied 30 gentamicin-resistant and 17 gentamicinsensitive strains of Pseudomonas aeruginosa isolated from respiratory cultures of patients with cystic fibrosis from five United States cities for the presence of plasmids, cross-resistance to other aminoglycosides, and the production of aminoglycosidemodifying enzymes. Four of 30 resistant strains and 3 of 17 sensitive strains contained one or more plasmids. Aminoglycoside cross-resistance to tobramycin, amikacin, and netilmicin was seen in 21 of 30 gentamicin-resistant strains. Seven strains that had low-level gentamicin resistance (minimum inhibitory concentrations [MIC] = 8–32 µg/mL) were sensitive to one or more of the other three aminoglycosides. Two strains with high-level gentamicin resistance (MIC ≥ 128 µg/mL) were sensitive to amikacin. These two strains, each containing three plasmids, were the only isolates of nine tested that produced an aminoglycoside-modifying enzyme with activity against gentamicin. None of the plasmids was transferable by conjugation. Four strains, three of which contained one or more plasmids, produced an aminoglycoside 3'-0-phosphotransferase II. The authors propose that the mechanism of gentamicin resistance in P. aeruginosa from patients with cystic fibrosis is not commonly plasmidmediated and likely is due to membrane impermeability to aminoglycosides.