Aminoglycoside Resistance Mechanisms in Clinical Isolates of Pseudomonas aeruginosa from the Canary Islands

Strains of Pseudomonas aeruginosa resistant to aminoglycoside antibiotics were selected from 152 clinical isolates. We identified two patterns of resistance correlating with the resistance mechanism characterized by changes in permeability, enzymatic modification due to the acetylating enzyme, AAC(6′)-II, or a combination of both. We detected enzymatic activity of the phosphorylase enzyme, APH(3′), in all the isolates. We compared the mechanisms of resistance detected by three methods i. e., radioenzymatic assay, phenotype of resistance and DNA probes. The phenotype of resistance was tested using a kit developed by Schering-Plough Corp. Hybridization was made with 18 DNA probes for the most frequent genes encoding for aminoglycoside-modifying enzymes. All isolates with AAC(6′) activity hybridized with the aac(6′)-Ib probes and to a minor degree, with the aac(6′)-IIb probe. None of the isolates showed hybridization with aph(3′)-I, aph(3′)-II, or aph(3′)-III DNA probes. Serotyping of the strains showed that the O:11 serotype was the most frequent one in strains whose resistance was due to the AAC(6′) enzyme. The O:6 serotype was associated with changes in permeability. Encoding of the resistance mechanism seemed to be chromosomal in all the strains.