Frequencies of variants resistant to different aminoglycosides in Pseudomonas aeruginosa.

The MICs of aminoglycosides for Pseudomonas aeruginosa are higher than those for Enterobacteriaceae and the number of variants resistant to high concentrations of aminoglycosides is greater in P. aeruginosa than in Escherichia coli. However, when the frequencies of resistant variants at different multiples of the MIC were calculated, these frequencies were similar in P. aeruginosa and E. coli. When large inocula of strains of P. aeruginosa, which were classified as sensitive in conventional MIC determinations, were incubated with amikacin, gentamicin, netilmicin or tobramycin at the break-point concentrations between sensitivity and resistance, 82%, 90%, 90% and 15%, respectively, of the strains regrew. The corresponding percentages for Enterobacteriaceae were much lower. The clinical relevance of this pronounced regrowth of P. aeruginosa is discussed.