Pseudomonas aeruginosa resistant to aminoglycosides: an assessment of the currently used methods of disk diffusion and dilution susceptibility.

In recent years, a number of clinical microbiology laboratories have isolated Pseudomonas aeruginosa with the unusual aminoglycoside disk diffusion result of resistance to both amikacin and gentamicin but susceptibility to tobramycin (ArGrTs). A total of 27 isolates of P. aeruginosa reported to have this resistance pattern was retested by the standard National Committee for Clinical Laboratory Standards disk diffusion procedure; twenty strains (74%) were confirmed to be ArGrTs. These twenty isolates were further examined for susceptibility to those aminoglycosides by agar dilution and both micro- and macrodilution methods. Only 25, 25 and 24% of the isolates appeared to be ArGrTs by agar, broth microdilution, and broth macrodilution testing respectively. Most of the remaining isolates were resistant to all three aminoglycosides when tested by broth dilution and resistant only to gentamicin when tested by agar dilution. The percentages of strains resistant to any particular aminoglycoside by agar dilution, broth microdilution and broth macrodilution, respectively were 44, 79 and 69 for amikacin, 95, 91 and 100 for gentamicin, 100, 100 and 100 for netilmicin, 28, 85 and 91 for sisomicin, and 11, 54 and 46 for tobramycin. These results indicate that strains showing the unusual aminoglycoside antibiogram are less susceptible to aminoglycosides in general and should probably be considered borderline resistant to all aminoglycosides. The efficacy of aminoglycosides in the treatment of infections produced by these strains is unknown.