Characterization of impermeability variants of Pseudomonas aeruginosa isolated during unsuccessful therapy of experimental endocarditis

We characterized five amikacin-resistant variants of Pseudomonas aeruginosa isolated from aortic valve vegetations during unsuccessful therapy of experimental endocarditis. These organisms were cross resistant to other aminoglycosides. No aminoglycoside-modifying enzymes were produced by these strains. However, all five variants demonstrated significant defects in permeability and intracellular uptake of [³H]amikacin when compared with the amikacin-susceptible parental strain (0 to 26% of that of the parental strain; mean, ~15%). The permeability defects were unstable in vitro, with normalization after serial passage in antibiotic-free media. The variants grew as nonpigmented, small-colony types, with in vitro generation times ~1.5 to 2 times longer than that of the parental strain (30 to 40 versus 20 min, respectively). Two impermeability variants were compared with the parental strain for ability to induce experimental endocarditis in rabbits with aortic catheters. Both variants were virulent in vivo; however, mean bacterial densities in vegetations were ~2.5 log₁₀ CFU/g lower in animals challenged with the variants than in animals challenged with the parental strain, probably reflecting a slower in vivo growth rate.