Aminoglycoside resistance mechanisms in clinical isolates of Pseudomonas aeruginosa from the Canary Islands

Strains of Pseudomonas aeruginosa resistant to aminoglycoside antibiotics w ere selected from 152 clinical isolates. We identified two patterns of resi stance correlating with the resistance mechanism characterized by changes i n permeability, enzymatic modification due to the acetylating enzyme, AAC(6 ')-II, or a combination of both. We detected enzymatic activity of the phos phorylase enzyme, APH(3'), in all the isolates. We compared the mechanisms of resistance detected by three methods i.e., radioenzymatic assay, phenoty pe of resistance and DNA probes. The phenotype of resistance was tested usi ng a kit developed by Schering-Plough Corp. Hybridization was made with 18 DNA probes for the most frequent genes encoding for aminoglycoside-modifyin g enzymes. All isolates with AAC(6') activity hybridized with the aac(6')-I b probes and to a minor degree, with the aac(6')-IIb probe. None of the iso lates showed hybridization with aph(3')-I, aph(3')-II, or aph(3')-III DNA p robes. 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. Th e O:6 serotype was associated with changes in permeability. Encoding of the resistance mechanism seemed to be chromosomal in all the strains.