Mechanisms of aminoglycoside resistance in variants of Pseudomonas aeruginosa isolated during treatment of experimental endocarditis in rabbits.

We examined multi-aminoglycoside-resistant variants of Pseudomonas aeruginosa, which were isolated during unsuccessful therapy for experimental endocarditis, to determine their mechanism(s) of resistance. No aminoglycoside-modifying enzymes were observed, and outer membrane compositional analyses of the resistant variants revealed porin protein and lipopolysaccharide profiles similar to those of the aminoglycoside-susceptible parental strain (PA-96). PA-96 accumulated [3H]tobramycin in a normal triphasic manner, whereas the aminoglycoside-resistant variants were unable to take up measurable quantities of [3H]tobramycin even when long incubation times (120 min) and high external aminoglycoside concentrations (27 micrograms/mL) were used. We duplicated this uptake defect for [3H]tobramycin by pretreating parental cells with KCn, which is an energy-dependent transport inhibitor. Electrical potentials of cytoplasmic membranes of the aminoglycoside-resistant variants were significantly lower than that of the parental strain (P less than .01). A ribosomally resistant mutant of the parental strain (PA-96r) showed monophasic, energy-dependent uptake of [3H]tobramycin, without the accelerated, secondary aminoglycoside uptake phase.